The 16th Femtochemistry Conference (FEMTO16)

Jun 22, 2025, 4:00 PM → Jun 27, 2025, 1:00 PM Europe/Rome

Savoia Hotel, Trieste, Italy

We are pleased to announce the 16th Femtochemistry Conference (FEMTO 16) - Dynamics of Complex Molecular Processes in Chemistry, Biology, and Physics. 

This conference will bring together scientists from across the globe to present and discuss the latest advancements in the understanding of ultrafast molecular and chemical dynamics. 
The conference will encompass complex processes in chemistry, biology, materials science, and physics, from both theoretical and experimental perspectives. Notably, it will highlight the most advanced methods in ultrafast spectroscopy and structural research. 

We invite you to join us for this distinguished event, where pioneering ideas and insights will be shared.

Scientific topics at FEMTO16

• Attosecond science
• Biological dynamics
• Chiral dynamics
• Gas phase dynamics
• Novel experimental methods
• Novel theoretical methods
• Reaction dynamics in liquids
• Solid materials
• Solvation dynamics
• Structural dynamics

History:

The series of biennial Femtochemistry Conferences started in 1993 and builds on the rich history of previous meetings:
 

• 1993 Berlin, Germany  (organized by Jörn Manz) • 1995 Lausanne, Switzerland  (Majed Chergui) • 1997 Lund, Sweden  (Villy Sundström) • 1999 Leuven, Belgium  (Frans De Schryver) • 2001 Toledo, Spain  (Abderrazzak Douhal) • 2003 Paris, France  (Monique Martin and James Hynes) • 2005 Washington DC, USA  (A. Welford Castleman) • 2007 Oxford, UK  (David Clary)

• 2009 Beijing, China  (Dongping Zhong and Qihuang Gong) • 2011 Madrid, Spain  (Luis Bañares and Jesus Santamaria) • 2013 Copenhagen, Denmark  (Niels E. Henriksen) • 2015 Hamburg, Germany  (Jochen Küpper) • 2017 Cancun, Mexico  (Marcos Dantus and Jorge Peón) • 2019 Shanghai, China  (Dongping Zhong and Jian Wu) • 2023 Berlin, Germany  (Thomas Elsaesser and Marc J. J. Vrakking)

 

FEMTO16_BOOK of ABSTRACTS.pdf

FEMTO16_Programme.pdf

Documents FEMTO16 Sponsorship Prospectus_03.pdf.pdf

Sun, June 22

4:00 PM → 7:00 PM Registration

6:00 PM → 7:30 PM Welcome Reception

Mon, June 23

8:00 AM → 9:00 AM Registration

8:30 AM → 9:00 AM Opening Session

8:30 AM A. Franciosi, President and CEO Elettra Sinctrotrone Trieste

8:40 AM C. Masciovecchio, Manager for the time resolved experimental techniques at Elettra Sincrotrone Trieste

8:50 AM M. Chergui, Project leader at Elettra Sinctrotrone Trieste

9:00 AM → 10:20 AM Session 1 - Attosecond Science I: Attosecond Science I

9:00 AM Femtosecond and attosecond studies of ionization in aqueous systems

Understanding the elementary steps following ionization in aqueous systems provides a framework for radiation-matter interactions in chemistry and biology. Radiation chemistry has been largely explored on picosecond timescales through pulse radiolysis - a timescale which precludes mechanistic understanding of the birth of reactive species. Synchronized, two-color sub-femtosecond x-ray pulses from x-ray free-electron lasers [1] provide a qualitatively new approach to track the electronic and nuclear dynamics following ionization. We studied radiation-induced processes in pure liquid water via x-ray transient absorption in the water window, first following outer-valence ionization [2] and then following full-valence ionization [3]. The latter represents the first attosecond pump/attosecond probe experiment on a condensed phase sample and introduced the technique of all x-ray attosecond transient absorption (AX-ATAS). In condensed phase AX-ATAS we find a strong influence of electron collisional ionization and weaker effects from electronic coherence . Interestingly, the latter study demonstrates the ability of attosecond pump/probe experiments to reveal information on equilibrium properties and resolves a controversy surrounding the interpretation of x-ray emission spectra as evidence for two structural motifs of liquid water. Beyond pure liquid water, studies of aqueous salt solutions are being studied and initial results will be presented.

[1] Z. Guo et al., Nat. Photonics 18, 691–697 (2024).
[2] Z.-H. Loh et al., Science 367, 179–182 (2020).
[3] S. Li et al., Science 383, 1118-1122 (2024).

Speaker: Linda Young (Argonne National Laboratory)

9:30 AM Asynchronous and Interferometric Nonlinear Spectroscopy (AI-NS)

Since the advent of time-resolved spectroscopy based on precision frequency technology of laser sources, it has been considered an alternative way to study dynamic processes in photochemical systems. Recently, we have developed asynchronous and interferometric nonlinear spectroscopy (AI-NS), a spectroscopic technique that combines asynchronously generated laser pulses and interferometric detection. This technique offers an unprecedented temporal dynamic range with high spectral resolution and rapid data acquisition capabilities. By eliminating the need for mechanical delay stages, AI-NS facilitates the rapid collection of time-resolved data on dynamics ranging from femtoseconds to nanoseconds, while simultaneously distinguishing frequency-dependent responses. Here, we detail the technical methodology of AI-NS and explore its applications to the studies of various systems, including semiconductors and biological systems. Additionally, we highlight prospective advancements, such as integration with multidimensional spectroscopy techniques. AI-NS not only expands the scope of spectroscopic analysis but also opens new avenues for the exploration of diverse materials and molecular systems.

Speaker: Minhaeng Cho (IBS Center for Molecular Spectroscopy and Dynamics)

10:00 AM Nonlinear science with multi-harmonic FEL at sub-femtosecond resolution

The technical ability to generate phase-locked high harmonics of an optical laser by the HHG technique, and to monitor them with the RABBIT scheme opened the way to sub-femtosecond pulses [1,2], and to the investigation of electronic motion in molecules on its natural timescale, decoupled from nuclear motion [3]. The advent of seeded Free Electron Lasers in the XUV range [4], and in particular the realization that they could be operated as a phase-locked multi-harmonic source [5], added valuable flexibility in the synthesis of more complex waveforms [6], as well as greater pulse energy. Let us note that all the experimental schemes implemented so far have been based on the mixing of two electric fields, i.e., they are intrinsically non linear, and that one of the fields is that of an external, typically IR, laser.

I will present here recent results obtained with a FEL-only scheme, thus involving at least two FEL photons. In one experiment, autoionizing doubly-excited states of neutral helium [7] were excited with two FEL photons of combined energy of $\approx 60$ eV in a three- and four-harmonic configuration. A similar experiment was performed in neutral argon, with two-photon ionization in the continuum via an intermediate autoionizing (3s 3p$^6$ 5p) resonance. In the latter experiment, non-resonant two-photon ionization of Ar$^+$ to Ar$^{2+}$ was also observed, implying the presence of processes involving the absorption of three or more photons. In all cases, the experiments showed a clear dependence on the relative phase of the harmonics.

The results presented originate from the joint effort of many international laboratories and of a large number of researchers, whose work is gratefully acknowledged.

[1] P. M. Paul et al., Observation of a train of attosecond pulses from high harmonic generation. Science 292, 1689 (2001).
[2] M. Hentschel et al., Attosecond metrology. Nature 414, 509 (2001).
[3] F. Calegari and F. Martin, Open questions in attochemistry. Commun Chem. 6, 184 (2023).
[4] E. Allaria et al., Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet. Nature Photon. 6, 699 (2012).
[5] K. C. Prince et al., Coherent control with a short-wavelength free-electron laser. Nature Photon. 10, 176 (2016).
[6] P. K. Maroju et al., Attosecond pulse shaping using a seeded free-electron laser. Nature 578, 386 (2020).
[7] M. Žitnik et al., High Resolution Multiphoton Spectroscopy by a Tunable Free-Electron-Laser Light. Phys. Rev. Lett. 113, 193201 (2014).

Speaker: Carlo Callegari (Elettra - Sincrotrone Trieste)

10:20 AM → 10:50 AM Coffee Break

10:50 AM → 12:30 PM Session 2 - Gas Phase Dynamics: Gas Phase Dynamics

10:50 AM Luis Banares (Chair)

11:00 AM Ultrafast Photoacid-Base Reactions in Aqueous Solution

Microscopic descriptions of solution phase photoacid-base reactions involve the concept of “tight” contact and “loose” solvent separated reaction pairs for the elementary steps of proton transfer. Typically one has to deal with multiple distributions of complexes with possible different configurations, that makes ultrafast experiments rather involved {1]. Even when only the dynamics on the fastest time scales are followed, one has to take this into account. In this contribution we will show to what information is obtained with both ultrafast infrared and sof-x-ray spectroscopies upon electronic excitation of a photoacid [2,3], 2-naphthol-6,8-disulfonate (2N-6,8-diS), and imidazole base (HIm) or azide anion (N$_3$$^-$) in aqueous solution.
Ultrafast UV/IR spectroscopy is more sensitive in probing IR-active transitions of 2N-6,8-diS, providing insight into the proton release of the photoacid, the first step in proton transfer of the bimolecular reaction. For molar concentrations of base (HIm/N$_3$$^-$) initial components in the transient IR-active transitions reveal not only the fraction of photoacid exhibiting reaction dynamics but also the subpicosecond time scale this occurs. Interestingly the transient spectra reveal a broadband continuum much akin to the Zundel continuum with dynamics that may hinting at a stronger hydrogen bond [2] with a possible double well potential of the proton transfer coordinate of the reactive 2N-6,8-diS – base reactive complex.
Ultrafast nitrogen K-edge spectroscopy on the other hand reveals when the base (HIm/N$_3$$^-$) is converted into the conjugate acid (HImH$^+$ -the imidazolium cation-, or HN$_3$ – hydrazoic acid). This will first and foremost provide information on the time scale of completion of the proton transfer reaction, yet spectral shifts observed for HIm resonances also are indicative of the changes in strength of the hydrogen bonds of imidazole before, during after the proton transfer reaction: a frequency upshift implies a stronger hydrogen bond, whereas the opposite frequency downshift means a weaker hydrogen bond.
We have observed the interplay of contributions by multiple imidazole molecules in the proximity of the photoacid 2N-6,8-diS, that can only be disentangled in a combined experimental and theoretical approach. From classical dynamics simulations we deduce the probability density distributions of HIm around 2N-6,8-diS and conclude that under our sample conditions hydrogen bonding as well pi-pi stacking interactions are important, hence the relative contributions are of similar magnitude. This finding has led us to conclude that IVR in and vibrational energy dissipation from 2N-6,8-diS will lead to a transient local heating in the first hydration shell, leading to changes in the hydrogen bonds of imidazole on a time scale of 1 ps.

[1] O. F. Mohammed, D. Pines, E. T. J. Nibbering and E. Pines, Angew. Chem. Int. Ed. 46, 1458–1461 (2007).
[2] B. T. Psciuk, M. Prémont-Schwarz et al., J. Phys. Chem. A 119, 4800-4812 (2015).
[3] S. Eckert et al., Angew. Chem. Int. Ed. 61, e202200709 (2022).
[4] S. K. Das, M.-O. Winghart et al., J. Phys. Chem. Lett. 15, 1264-1272 (2024).

Speaker: Dr Erik T.J. Nibbering (Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy)

11:30 AM Attosecond-resolved probing of recolliding electron wave packets in liquids and aqueous solutions

We establish high-harmonic generation (HHG) in liquids as a powerful ultrafast probe for tracking spatial and temporal electron dynamics on attosecond time scales. Beyond the traditional three-step model, we uncover new nonlinear features such as multi-plateau structures and bandgap modification. These findings open pathways to attosecond-scale exploration of solvation dynamics, light–matter interactions, and electronic structure in complex environments.

High-harmonic generation (HHG) in bulk liquids has been recently explained by the “scattering-limited three-step model" [1]. In liquids such as H₂O, D₂O, and alcohols, the harmonic cut-off energy remains fixed and independent of laser wavelength, intensity, and pulse duration [1,2]—an outcome of strong electron scattering and dominant on-site recombination. However, this gas-like picture fails to account for the emergence of higher-order nonlinearities.
Here, we report the discovery of a second plateau in the HHG spectra of multiple liquids [3], marking a new regime of electron dynamics driven by recombination with neighboring molecules—particularly from the second solvation shell—enabled by hole delocalization [4,5]. This plateau displays unique signatures, such as a weak cutoff scaling and distinct ellipticity dependence, supported by advanced experiments, ab-initio simulations, and semi-classical models. Additionally, two-color interferometric measurements [6,7] provide attosecond-resolved access to the recollision process, revealing a large linear atto-chirp and an effective, field-induced reduction of the electronic band gap by several electron volts. Aqueous salt solutions exhibit spectral minima whose positions and depths are sensitive to anion type and con-centration. These features are well-described by a two-emitter interference model and reflect modulations in the relative phase and band structure induced by chemical environment and laser field. Together, these findings establish HHG in liquids as a versatile tool for probing ultrafast phenomena—capturing both temporal recollision dynamics and spatial recombination pathways. By moving beyond the simplistic gas-phase analogy, we unlock the potential of HHG to explore complex light–matter interactions, electronic structure modifications, and solvation dynamics on attosecond time scales.

References
[1] A Mondal et al., Nature Physiscs 19, 1813-1820 (2023)
[2] A Mondal et al., Optics Express 31, 34348–343461 (2023)
[3] A Mondal et al., Nature Photonics, under review
[4] I Jordan et al. Science 369, 974-979 (2020).
[5] X Gong et al. Nature 609, 507–511 (2022).
[6] N Dudovich et al. Nature Physics 2, 781–786 (2006).
[7] G Vampa et al Nature 522, 462–464 (2015).

Speaker: Worner Hans Jakob (ETH Zürich)

11:50 AM The Excited State Dynamics of Ethylene: A new theoretical model

The excited state dynamics of ethylene following excitation into the energetically lowest lying absorption band, nominally understood as arising from the $\pi\pi^*$ and $\pi3s$ states, have been the focus of numerous theoretical and computational studies for many decades. Here, we present new quantum dynamics computations and spectroscopic simulations that indicate the non-adiabatic population dynamics can be better understood as arising from strong vibronic coupling between the $\sigma\pi^*$ and $\pi\pi^*$, and separately, between the $\sigma3s$ and $\pi3s$ electronic states. This model is shown to offer a clear explanation for previously unassigned bands in the UV absorption spectrum and is the basis for the interpretation of recent time-resolved photo-electron spectrosopic results.

Speaker: Michael Schuurman (National Research Council of Canada)

12:10 PM Identifying Key Factors in Open-Loop Control of Molecular Fragmentation with Shaped Strong Fields

Pulse shaping has long been used to tailor femtosecond laser pulses for studying and controlling the fragmentation of polyatomic molecules [1, 2]. We employed 80-bit binary spectral phase functions (BPs) to reduce the search space while maintaining the ability to generate essentially any arbitrary phase [3]. Analysis of the reduced search space allowed us to gain insight into which pulse parameters most influence the fragmentation of triethylamine $(N(C_2H_5)_3)$ [4]. We focused on influencing the relative yield of m/z 86, corresponding to the loss of a methyl group. Given that peak intensity is a confounding variable in strong-field control [5], we evaluated thousands of BPs that generated similar second harmonic intensity ($I_{SHG}$), and found some produced twice the product yield, a difference not attributable to noise. Analysis of these pulses, including their duration, instantaneous frequency, power spectra, pulse trains, and autocorrelation, revealed that pulses with features spaced by ~2 ps enhanced the desired product. This was further confirmed by pump-probe measurements.

We evaluated 200 additional 80-bit BPs with pulse structures spaced 1.25–2.5 ps and compared them to 200 randomly generated BPs, all with identical $I_{SHG}$ values to eliminate peak intensity as a variable. The selected BPs produced nearly twice the m/z 86 yield compared to the randomly shaped pulses, outperforming both transform-limited and pump-probe pulses. This enhancement is attributed to control via a dissociative Rydberg state with an estimated 2 ps lifetime in the neutral molecule [6]. The second feature in the shaped pulse likely ionizes the 3s Rydberg state, leading to m/z 86 formation. These findings suggest that open-loop control with BPs could uncover new control mechanisms and reveal key pulse parameters that influence laser-matter interactions.

[1] A. Assion, et al. Science 282, 919 (1998).

[2] M. Dantus, V. V. Lozovoy, Chem. Rev. 104, 1813 (2004).

[3] V. V. Lozovoy, M. Nairat, M. Dantus, J. Opt-UK 19, 105506 (2017).

[4] J. Stamm, S. Kwon, M. Dantus, Chem. Phys. Lett. 15, 12464 (2024).

[5] V. V. Lozovoy, et al., J. Phys. Chem. A. 112, 3789 (2008).

[6] T. I. Sølling, C. Kötting, and A. H. Zewail, J. Phys. Chem. A 107, 10872 (2003).

Speaker: Marcos Dantus (Michigan State University)

12:30 PM → 2:00 PM Lunch Break

2:00 PM → 3:40 PM Session 3 - Solvation and reaction dynamics I: Solvation and Reaction Dynamics I

2:00 PM Majed Chergui (Chair)

2:10 PM Caught in the act: Optical pump THz probe allows real-time observation of the solvent response subsequent to photoexcitation

We have set up a novel Optical Pump Terahertz Probe (OPTP) Spectrometer, which enables monitoring of the solvent response and the propagation of vibrational energy upon photoexcitation. We have reached a sensitivity that allows us to follow with ps time resolution the steps from photoexcitation to the formation of intermediates and the subsequent energy transfer into the bulk solvent (1). Here, we will present the results of monitoring two prototype photoreactions:

Real-time observation of the solvent response that promotes excited-state proton transfer: The solvent response of pyranine and two derivatives after photoexcitation has been observed in the time range of from sub-ps to 300 ps. We map the effect of the reaction on the surrounding solvent to reveal details that were partly or even fully hidden when focusing solely on observables tagged to the photoacid or its conjugate base (2). The rather surprising observation of an oscillation in the signal within the first 5 ps could be assigned, with the help of theory, to vibrational beatings, which, when undamped, correspond to energy transfer between the photoexcited chromophore and the water solvent. Fragment-localized modes were identified that make and break hydrogen bonds between the chromophore and solvent subsystems, which facilitates proton transfer. The time scales for this vibrational energy transfer could be deduced to be < 0.5 ps. We conclude that efficient energy transfer stops the oscillatory mode within the first cycle (0.5 ps) and corresponds to the onset of proton transfer.

The birth and evolution of solvated electrons in the water: The solvated electron is a fundamental reaction intermediate in physical, chemical, and biological processes. Using OPTP, we follow the birth and time evolution of the solvated electron via state-of-the-art, solvent-sensitive kinetic terahertz spectroscopy, and molecular simulations (3). In the first, we observe a spectroscopic signature attributed to the delocalized electron, followed by the onset of a solvent perturbation. While initially, the electron is delocalized, the solvated electron converges to an average value of within. Due to the high experimental sensitivity, we can observe the spectroscopic signature of the localized electron, which is long-lasting (> 250 ps). While the water network rearranges to accommodate an anion or cation, usually causing an entropic penalty for creating a cavity for the solvated electron, we observe a weakening of the hydrogen bond network by the localized electron, which correlates with an increase in entropy. This stabilizes the electron in the water network.

References
(1) C. Hoberg, P. Balzerowski, M. Havenith, Integration of a rapid scanning technique into THz time-domain spectrometers for nonlinear THz spectroscopy measurements, AIP Adv. 9, 035348 (2019).
(2) C. Hoberg, J.J. Talbot, J. Shee, T. Ockelmann, D. Das Mahanta, F. Novelli, M. Head-Gordon, M. Havenith, Caught in the act: Real-time observation of the solvent response that promotes excited-state proton transfer in pyranine, Chem. Sci. 14, 4048–4058 (2023).
(3) F. Novelli, K. Chen, A. Buchmann, T. Ockelmann, C. Hoberg, T. Head-Gordon, M. Havenith, The birth and evolution of solvated electrons in the water, PNAS 120, e2216480120 (2023).

Speaker: Martina Havenith (Department of Physical Chemistry II, Ruhr University Bochum, Germany)

2:40 PM Electronic Dynamics of Aqueous Nucleobases Studied by Ultrafast EUV Photoemission and IR Absorption Spectroscopy

Ultrafast electronic relaxation of nucleobases from the 1ππ* state to the ground electronic state is crucial for the photostability of DNA and RNA. However, it has been suggested that electronic relaxation of pyrimidine nucleobases, nucleosides, and nucleotides in an aqueous environment generates an electronically-excited intermediate state with a lifetime of tens to hundreds of picoseconds with a relatively high quantum yield (QY) of 0.2–0.5. The generation of such a long-lived excited state seem to be inconsistent with the photostability of these molecules. We performed extreme ultraviolet time-resolved photoelectron spectroscopy and reinvestigated this problem and revealed that the accurately determined QY for long-lived excited states is much too low to allow an electronically excited reaction intermediate to exist. We investigated further the nature of the reaction intermediate using ultraviolet and infrared transient absorption spectroscopy, along with quantum chemical calculations to find that the intermediate is generated in the S0 state, and identified its structure.

Speaker: Toshinori Suzuki (Kyoto University)

3:00 PM Distinguishing cavity and non-cavity solvation structures of the hydrated electron

Solvated electrons in aqueous solution are a prototypical low dimensional quantum system in interaction with a fluctuating many-body environment. Despite substantial theoretical and experimental effort, conflicting views prevail regarding the hydration structure of electrons in water, where cavity and non-cavity solvation structures have been suggested. We present first principles molecular dynamics simulations of the electron localization dynamics in liquid water, employing hybrid-meta-GGA and hybrid-GGA density functionals that both provide an excellent description of the liquid water structure. Nevertheless, characteristic differences occur regarding the localization dynamics and solvation structure of excess electrons. We identify perturbations of the local hydrogen bond structure of water due to the interaction with the excess charge that give rise to specific signatures in transient radial distribution functions. Respective signatures in simulated scattering patterns are compared to preliminary data obtained in a liquid phase UED early science campaign at SLAC. The results shine light on the coupling mechanism of the aqueous electron with its environment and provide microscopic insight into the dynamics of polaron formation in disordered condensed matter systems.

Speaker: Benjamin Fingerhut (Ludwig-Maximilians-Universität München)

3:20 PM Ultrafast electron delocalization in aqueous L-cysteine

Charge transfer (CT) processes play a fundamental role in chemistry and biomolecular interactions, particularly in aqueous environments where biological reactions occur. In this study, we utilize hard X-ray spectroscopy, specifically core-hole clock spectroscopy (CHCS), to probe ultrafast CT dynamics in L-cysteine solutions at different pH levels. The experimental setup involves high-resolution Auger electron spectroscopy at the SOLEIL and PETRA III synchrotrons, complemented by theoretical simulations to interpret the observed CT mechanisms.

Our results show that CT efficiency is strongly pH-dependent, with significant electron delocalization occurring in deprotonated L-cysteine at pH 12. This finding highlights the role of solvation, particularly the effects of the hydrogen-bonding network, in facilitating charge migration. The core-hole lifetime provides a natural timescale for electron transfer, enabling direct quantification of CT rates. Computational analysis further supports these trends, indicating that the electron transfer pathway is modulated by the protonation state of the thiol (-SH) and amino (-NH₂) functional groups.

Understanding CT in biomolecules is crucial for elucidating protein interactions, as well as for applications in fields such as radiation chemistry and environmental science. Our findings contribute to a deeper understanding of charge transport in solvated amino acids, paving the way for future research on complex biological molecules under X-ray irradiation.

Speaker: Dr Nicolas Velasquez (Fritz-Haber-Institut der Max-Planck-Gesellschaft)

3:40 PM → 4:10 PM Coffee Break

4:10 PM → 6:00 PM Session 4 - Theory: Theory

4:10 PM Simulating, Modeling, and Analyzing Multidimensional Vibrational Spectroscopies of Water

Spectral line shapes in the condensed phase contain information about various dynamical processes that modulate the transition energy, such as microscopic dynamics, inter- and intramolecular couplings. In this talk I will explore and describe the role of different physical phenomena that arise from the peculiarities of dissipative dynamics in multidimensional spectra. The methodology will be illustrated by calculating multidimensional signals for water in 2D infrared, 2D THz-Raman, and 2D IR-Raman spectra obtained from an equilibrium-non-equilibrium hybrid MD simulation algorithm. These signals are analyzed in terms of anharmonicity and nonlinear polarizability of vibrational modes using a Brownian oscillator (BO) model with linear-linear (LL) and square-linear (SL) system-bath interactions from the quantum hierarchical Fokker-Planck equations approach for non-Markovian noise. All characteristic 2D profiles of the signals obtained from MD are reproduced by the LL+SL BO model, indicating that this model captures the essential features of the inter- and intra-molecular motion.

Speaker: Yoshitaka Tanimura (Kyoto University, Department of Chemistry)

4:40 PM Simulating the ultrafast dynamics of multi-mode multi-state molecular systems coupled to a dissipative environment

Ultrafast processes in condensed phase photoexcited molecular systems involve the transition from a coherent dynamical regime – where a precise phase relation exists between different wave packet components – to incoherent "classical-like" dynamics. The decoherence process is driven the dissipation due to the surrounding molecular environment.

From the computational viewpoint, modelling the dynamics of nonadiabatic vibronic quantum systems interacting with fluctuating environments becomes especially challenging when the system’s high dimensionality precludes the calculation of its eigenstates. To overcome this limitation, a novel eigenstate-free formalism is introduced. This approach represents the open quantum system as a mixture of high-dimensional, time-dependent wave packets, governed by coupled Schrödinger equations [1], while the environment is modeled using a multi-component quantum master equation [2]. A computationally efficient implementation of this formalism employs a variational Gaussian/multiconfigurational time-dependent Hartree (G-MCTDH) ansatz for the wave packets and propagates the environment dynamics using hierarchical equations truncated at the first or second level.

The methodology is validated through simulations of multichromophoric aggregate dynamics, explicitly incorporating multiple vibrational modes, and through the study of vibrationally coherent symmetry-breaking charge transfer in a donor-acceptor-donor triad [3,4].

These results demonstrate the potential of the approach as a powerful quantum dynamical method for modeling complex system–bath interactions involving numerous degrees of freedom across multiple time scales.

[1] D. Picconi & I. Burghardt, J. Chem. Phys. 150, 224106 (2019)
[2] D. Picconi, J. Chem. Phys. 161, 164108 (2024)
[3] A. Aster, A.-B. Bornhof, N. Sakai, S. Matile, and E. Vauthey, J. Phys. Chem. Lett. 12, 1052 (2021)
[4] D. Picconi, J. Chem. Phys. 156, 184105 (2022)

Speaker: David Picconi (Heinrich Heine University Düsseldorf)

5:00 PM Nonlinear Femtosecond Signals at Finite Temperature including Static Disorder via a Thermo Field Dynamics-Tensor Train Method

We have developed a fully quantum, numerically accurate wave function-based approach for the calculation of third-order spectroscopic signals of polyatomic molecules and molecular aggregates at finite temperature including statiuc disorder effects. The systems are described by multimode nonadiabatic vibronic-coupling Hamiltonians, in which diagonal terms are treated in harmonic approximation, while off-diagonal interstate couplings are assumed to be coordinate independent. The approach is based on the Thermo Field Dynamics (TFD) representation of quantum mechanics and Tensor-Train (TT) representation of the vibronic wave function, providing a very efficient numerical simulation of quantum evolution of systems with many degrees of freedom. The effect of static disorder is included using a novel approach based on auxiliary harmonic oscillators variables. The developed TFD-TT approach is applied to the calculation of time- and frequency-resolved fluorescence spectra of the Fenna−Matthews−Olson (FMO) antenna complex at room temperature taking into account finite time-frequency resolution in fluorescence detection, orientational averaging, and static disorder.

Speaker: Raffaele Borrelli (University of Torino)

5:20 PM The Importance of Being a Conical Intersection in Ultrafast Photochemistry

It is well accepted that the most general case of photoinduced reaction dynamics occurs through non-adiabatic transitions. The complex panorama of potential energy surfaces describing the excited states of polyatomic molecules is characterized by non-adiabatic crossings and the presence of multiple conical intersections. A conical intersection (CI) is a 3N-8-dimension hypersurface of intersection between two electronic states. The two remaining internal coordinates, i.e., g as the difference gradient vector, and h as the non-adiabatic coupling vector, define the branching plane. Conical intersections can be considered as the transition states of electronic excited states and therefore the coupling between the different degrees of freedom, valence electrons and vibrations, and the timescales of these motions, are at the heart of the understanding of photochemistry. The main aim is to find an equivalent of the “Polanyi rules” for excited state polyatomic dynamics, in such a way that specific vibrational dynamics at CIs would be as important to dynamics as are the topographical features of the CIs themselves. In this contribution, we will highlight several cases of ultrafast non-adiabatic reaction dynamics in which CIs play a determining role for the photoinduced dynamics in ultrafast timescales. We will focus on the non-adiabatic dynamics in vinyl iodide [1] and the methyl iodide cation [2].

[1] M. L. Murillo-Sánchez, S. Marggi Poullain, P. Limão-Vieira, A. Zanchet, N. de Oliveira, J. González-Vázquez, Luis Bañares, Phys. Chem. Chem. Phys., submitted (2025).
[2] J. González-Vázquez, G. A. García, D. V. Chicharro, S. Marggi Poullain, L. Bañares, Chem. Sci. 15, 3203(2024).

Speaker: Prof. Luis Bañares (Universidad Complutense de Madrid)

5:40 PM A Mapping Approach to Surface Hopping

Fewest-switches surface hopping (FSSH) is one of the most popular methods for simulating photochemical experiments [1], even though it suffers from problems of inconsistency and overcoherence that significantly degrade the accuracy of its results. In particular, FSSH is unable to correctly describe the dynamics under strong electromagnetic pulses [2,3], such that a fully satisfactory approach for simulating the photoexcitation step of many experiments is currently lacking.

The mapping approach to surface hopping (MASH) [4] is a recently developed method that alleviates the problems of FSSH at no additional cost. Through application to experimentally relevant photochemical systems, I will demonstrate that the MASH algorithm can successfully treat the dynamics involving strong explicit laser pulses, including off-resonant excitation processes.

[1] J. E. Subotnik et al., Annu. Rev. Phys. Chem., 67, 387, (2016)
[2] T. Fiedlschuster, et al., Phys. Rev. A, 95, 063424 (2017)
[3] B. Mingolet, B. F. E. Curchod, J. Phys. Chem. A, 123, 3582-3591 (2019)
[4] J. R. Mannouch, J. O. Richardson, J. Chem. Phys. 158, 10411 (2023)

Speaker: Jonathan Mannouch (MPSD, Hamburg)

6:00 PM → 6:05 PM NEXT

6:05 PM → 7:35 PM Poster session 1: Poster Session 1

Tue, June 24

8:00 AM → 9:00 AM Registration

9:00 AM → 10:40 AM Session 5 - Chirality I: Chirality I

9:00 AM Carlo Callegari (Chair)

9:10 AM Laser-based sensing and driving of molecular chirality

Exploiting an electric dipole effect in ionization [1], photoelectron circular dichroism (PECD) is a highly sensitive enantioselective spectroscopy for studying chiral molecules in the gas phase using either single-photon ionization [2] or multiphoton ionization [3]. In the latter case, resonance-enhanced multiphoton ionization (REMPI) gives access to neutral electronic excited states. The PECD sensitivity opens the door to study control of enantiomers' coupled electron and nuclear motion. A prerequisite is a detailed understanding of PECD in REMPI schemes. In this contribution, I will report on our investigations on PECD with coherent light sources whose pulse durations span from femtoseconds to nanoseconds [4]. By this, we address impulsive excitation on the femtosecond time scale to highly vibrational state selective excitation in mixtures with the help of high-resolution nanosecond laser techniques [5]. Subcycle control of PECD in bichromatic fields will be discussed [6], as well as coherent control of the ion yield [7] and handedness.

[01] Ritchie, B. Theory of the angular distribution of photoelectrons ejected from optically active molecules and molecular negative ions. Phys. Rev. A 13, 1411–1415 (1976).
[02] Böwering, N., Lischke, T., Schmidtke, B., Müller, N., Khalil, T. & Heinzmann, U. Asymmetry in Photoelectron Emission from Chiral Molecules Induced by Circularly Polarized Light. Phys. Rev. Lett. 86, 1187 (2001).
[03] Lux, C., Wollenhaupt, M., Bolze, T., Liang, Q., Köhler, J., Sarpe, C. & Baumert, T. Circular dichroism in the photoelectron angular distributions of camphor and fenchone from multiphoton ionization with femtosecond laser pulses. Angew. Chem. Int. Ed. 51, 5001–5005 (2012).
[04] Lee, H., Ranecky, S. T., Vasudevan, S., Ladda, N., Rosen, T., Das, S., Ghosh, J., Braun, H., Reich, D. M., Senftleben, A. & Baumert, T. Pulse length dependence of photoelectron circular dichroism. Physical chemistry chemical physics : PCCP 24, 27483–27494 (2022).
[05] Ranecky, S. T., Park, G. B., Samartzis, P. C., Giannakidis, I. C., Schwarzer, D., Senftleben, A., Baumert, T. & Schäfer, T. Detecting chirality in mixtures using nanosecond photoelectron circular dichroism. Phys. Chem. Chem. Phys. 24, 2758–2761 (2022).
[06] Demekhin, P. V., Artemyev, A. N., Kastner, A. & Baumert, T. Photoelectron Circular Dichroism with Two Overlapping Laser Pulses of Carrier Frequencies ω and 2ω Linearly Polarized in Two Mutually Orthogonal Directions. Phys. Rev. Lett. 121, 253201 (2018).
[07] Das, S., Ghosh, J., Sudheendran, V., Ranecky, S., Rosen, T., Ladda, N., Lee, H., Stehling, T.-J., Westmeier, F., Mikosch, J., Senftleben, A., Baumert, T. & Braun, H. Control of circular dichroism in ion yield of 3-methyl cyclopentanone with femtosecond laser pulses. Physical chemistry chemical physics : PCCP, (2025).

Speaker: Thomas Baumert

9:40 AM Exciting and probing attosecond multielectron dynamics in chiral molecules at FERMI

Little is known about attosecond multielectron dynamics in photo-excited chiral molecules and their coupling to vibronic dynamics triggered by photoexcitation. Such photodynamics is of great importance for understanding photo-processes triggered in chiral molecules by electromagnetic radiation in outer space; this has important implications in bio-astrophysics and in the search for the origin of life and homochirality on Earth. The difficulty in studying such dynamics is due to a combination of factors. First, even localized excitations quickly redistribute among many degrees of freedom along the multidimensional landscape of the excited molecular states. Thus, it is very hard to follow the individual pathways. Second, the photoelectron spectrum of excited molecules corresponding to Rydberg excitations and valence holes of the final-state cation has a fundamentally many-body nature, and is challenging to resolve both computationally and experimentally due to broad featureless regions of overlapping lines.

We have discovered a strategy for contrastive detection of such states by exploiting several unique and powerful features of our spectroscopic scheme: it is (i) sub-cycle, (ii) interferometric and (iii) differential. The detection is enabled by the interference of two quantum pathways generated by the two phase-locked linearly polarized pulses in orthogonal configuration readily available at FERMI. The advantages are: (i) the phase-dependent signal arises only when the w and 2w pathways interfere, providing unique sensitivity to dynamics excited by the w-field; (ii) by tuning w, we can select a specific intermediate resonance for creating an excited many-body state.

This interferometric spectroscopy combines “the best of the two worlds”: high temporal resolution due to the sub-cycle phase lock, and high spectral resolution due to long duration of involved pulses. At FERMI, we can routinely achieve <40 meV spectral resolution and <40 as temporal resolution – the cutting edge of attosecond spectroscopy. The detection relies on measuring angle- and energy-resolved photoelectron distribution – one of the most direct spectroscopic tools. The interferometric detection of photoelectron signals offers an additional level of selectivity and robustness. Our observable comes from the interference of two quantum pathways: the reference – one-photon ionization by the 2w field, and the signal – the resonant photoexcitation by the w field into an excited state with an inner-valence hole that experiences Auger-Meitner decay assisted by absorption of the second w photon. Since the interference requires that both pathways lead to the same final state, i.e. the same energy/emission angle of the electron, the same state of the parent ion, and the same states of all fragments in case of fragmentation, it is an extremely selective probe of photodynamic pathways. The interferometric, i.e. two-color phase delay-dependent signal, is fundamentally sub-cycle and thus, for w~10 eV, only sensitive to multielectron dynamics occurring in less than 40 as after excitation with the first w photon. In fact, the majority of excitation pathways will yield fragmentation before the absorption of the second w photon, destroying the purity of the final quantum state and with it the interference with the reference 2w pathway. Thus, the two-color phase-dependent photoionization signal is a unique messenger of attosecond multielectron dynamics and subsequent chemical change.

Finally we use differential measurement to complete contrastive detection scheme. By subtracting photoelectron spectra recorded for the same two-color phase but for two slightly different photon frequencies w and w+dw we eliminate non-resonant background.

In our experiment we resolved the enantio-sensitive molecular phase associated with the laser-assisted Auger-Meitner decay and involving the contribution of 8 two-hole one-particle states in a chiral molecule propylene oxide. The multielectron states are associated with holes in Homo, Homo-1, Homo-2 and electron in Rydberg orbitals 3p,4p,3s in excellent agreement with calculations using ADC-family of methods. Understanding enantio-sensitive attosecond multielectron dynamics in excited chiral molecules is an important unsolved challenge in several fields: ultrafast spectroscopy, femtosecond mass-spectrometry, chemistry to name just a few. Addressing this challenge for randomly oriented chiral molecules brings our research well beyond the current state of the art in those fields.

Speaker: Davide Facciala' (CNR-IFN)

10:00 AM Ultra-fast nonlinear optical response of chiral molecules with a focus on conformer sensitivity

Chirality is a fundamental geometric property, present from molecular to macroscopic scales. Traditional chiroptical methods rely on weak magnetic interactions, limiting their efficiency. We aim to develop chiral recognition methods based solely on electric-dipole interactions, offering enhanced enantiosensitivity [1].

We investigate the carrier-envelope phase (CEP) dependence of enantio-sensitive observables within the electric dipole approximation by numerically analyzing the nonlinear response of randomly oriented chiral molecules in the gas phase. Using time-dependent density functional theory (TDDFT), we model their interaction with few-cycle, tightly focused, CEP-controlled linearly polarized laser pulses. Tight focusing induces a longitudinal field component, creating a forward-elliptically polarized field [2]. This drives a chiral response perpendicular to the polarization plane, leading to the emission of even-order chiral harmonics in addition to the odd-order achiral harmonics. Their CEP-dependent interference results in enantiosensitive non-linear optical rotation [2]. Here we explore the sensitivity of the CEP-dependent signal to chiral molecular conformations and the uniqueness of the CEP molecular markers.

We focus on the chiral dynamics of essential amino acids, using serine as a prototypical case. We analyze its three dominant conformers in the gas phase, with relative populations of $43.7\%$, $18.8\%$, and $14.8\%$ [3]. By placing a polarizer before the detector, one can convert enantio-sensitive polarization properties into an enantio-sensitive intensity distribution, which can be considered as chiral "QR codes" mapping the chiral dichroism of emitted harmonics as a function of the CEP of the incident light. We show that chiral dichroism (CD) vs. CEP has different patterns for different conformers (note that for the same harmonic order, the CD maximizes at different CEP values in serine I and serine II, see Fig. 1), reflecting different molecular phase accumulation due to ultrafast electron dynamics in two conformers and making chiral QR codes suitable for molecular fingerprinting.

We gratefully acknowledge ERC-2021-AdG project ULISSES, grant agreement No. 101054696.

Figure 1: Chiral "QR codes": Chiral Dichroism (CD) of the emitted harmonics as a function of the CEP of incident light for (a) serine I and (b) serine II conformers.

References:
[1] D. Ayuso et al., PCCP 24, 26962 (2022). DOI: 10.1039/D2CP01009G
[2] D. Ayuso et al., Optica 8, 1243 (2021). DOI: 10.1364/NLO.2021.NW2A.2
[3] K. He and W. D. Allen, J. Chem. Theory Comput. 12, 3571 (2016). DOI: 10.1021/acs.jctc.6b00314

Speaker: Elena Aethra Christou (Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy)

10:20 AM Light emission with a twist: Ultrafast evolution of chiral excited states determines the circularly-polarized luminescence of a chiral OLED complex

Circularly-polarized luminescence (CPL) has promising applications in the fields of optical data storage, biosensing and for the creation of more efficient OLED displays [1]. In this respect, chiral lanthanide complexes are particularly attractive CPL emitters due to their intense and long-lived emission lines, and their exceptional degree of circular polarization. However, despite the impressive progress in the synthesis of CPL complexes, the direct resolution and analysis of their chiral luminescent states has remained a formidable experimental challenge, due to a lack of ultrafast spectroscopic techniques with sufficient chiral sensitivity.
To address this gap, we have developed an ultrafast circular dichroism (CD) instrument that combines ultra-sensitive broadband detection with sub-picosecond time resolution to resolve the dynamics of chiral photoexcited states [2]. We now apply this technique to resolve the CPL mechanism of the prototypical CsEu((+)-hfbc)4 (hfbc = 3-heptafluoro-butylyrylcamphorate), which achieves record-breaking CPL emission purity by coupling the metal-centered (MC) luminescent states to a chiral ligand sphere [3]. In this mechanism, the ligands serve two roles. First, they act as photosensitizers providing efficient energy transfer (EnT) to the MC states. Second, the chiral ligand system breaks the spherical symmetry of the Eu(III) ion to induce its CPL activity. [4]
Combining ultrafast CD and transient absorption measurements with TDDFT calculations, we resolve the complete EnT mechanism and the associated chiral structural evolution that determine the CPL emission properties of the complex.We find that the initially excited ligand-centered singlet exciton states undergo a sub-picosecond intersystem crossing to populate a triplet state localized on a single ligand, from where EnT to the MC states proceeds in 150 ps. Quite remarkably, we observe an increase in optical activity of the ligand system upon EnT, which we assign to an ultrafast structural change to an achiral square antiprismatic geometry. Our findings thus support the predicted chirality transfer to the emissive states [4]: as the first coordination sphere of the excited Eu(III) is indeed achiral, its CPL is induced by the second coordination sphere, provided by the helical arrangement of hfbc-ligands. Our results demonstrate that it is now possible to determine the stereochemistry of electronically excited states, opening the path to directly capture the CPL mechanisms of chiral luminescent molecules and materials to further improve their designs.
[1] J. Crassous, M. Fuchter, D. Freedman, N. Kotov, J. Moon, M. Beard, S. Feldmann, Nat. Rev. Mat. 2023.
[2] M. Oppermann, B. Bauer, T. Rossi, F. Zinna, J. Helbing, J. Lacour, M. Chergui, Optica 2019, 6, 56-60.
[3] J. L. Lunkley, D. Shirotani, K. Yamanari, S. Kaizaki, G. Muller, J. Am. Chem. Soc. 2008, 130, 13814-13815.
[4] S. Di Pietro, L. Di Bari, Inorg. Chem. 2012, 51, 12007-12014.

Speaker: Ms Livia Müller (Department of Chemistry, University of Basel, Switzerland)

10:40 AM → 10:45 AM Light Conversion: Light conversion

10:45 AM → 11:15 AM Coffee Break

11:15 AM → 12:45 PM Session 6 - Chirality II: Chirality II

11:15 AM Ultrafast chiro-optical spectroscopy

Chirality is ubiquitous in nature, and understanding of chiral properties is critical to many applications in modern science and technology, ranging from protein function in biology and enantiomer differentiation in pharmaceutics, to light control in plasmonics and metamaterials all the way to probing magnetic properties in spintronic and superconducting devices. The chiro-optical response of a sample is manifested as the dependence of the complex refractive index on the handedness of circularly polarized light, which can be probed by its imaginary part (circular dichroism, CD) or its real part (optical rotatory dispersion, ORD). The ultrafast chiro-optical response of a sample is of broad interdisciplinary interest, ranging from structural dynamics of proteins to spin dynamics in semiconductors and magnetic materials to the nonlinear response of chiral plasmonic nanostructures. The measurement of time-resolved CD or ORD is however experimentally challenging, because it requires the detection of the small photoinduced change of an already small signal. Here we introduce two novel approaches to ultrafast chiro-optical spectroscopy and microscopy. We first present a broadband ultrafast chiroptical spectroscopy setup which combines time-domain Fourier transform detection and heterodyne amplification using a birefringent common-path interferometer. Our method allows the detection of transient CD and ORD spectra with sensitivity better than 1 millidegree. We then discuss an ultrafast widefield chiro-optical transient absorption microscope which uses a multiplexed off-axis holography scheme with two cross-polarized reference pulses. The holographic nature of the measurement enables retrieval of the electric field of the probe pulse and thus simultaneous detection of the transient CD and ORD signals, which can be reconstructed over a large field of view with high temporal (sub-100-fs) and spatial (sub-µm) resolution.

Speaker: Giulio Nicola Felice Cerullo (Politecnico di Milano)

11:45 AM Modeling plasmonic effects in photoinduced molecular processes

Plasmon is a collective electronic excitation that occurs in irradiated metallic nanoparticles (NPs) [1]. Plasmonic excitation is intense and sharp, and can be exploited to modify, even qualitatively, the properties of molecules interacting with an NP. In this presentation, the key ideas of molecular plasmonics [2], i.e., the study of the photoinduced interaction between molecule and NP, are given. A theoretical multiscale time-domain approach [3] was applied to study the selectivity toward methane in the photocatalytic hydrogenation of carbon dioxide in the presence of a rhodium nanocube [4,5], and to verify the plasmon enhancement of the circular dichroism signal in chiral molecules [6].

[1] L. Novotny and B. Hecht, Principles of Nano-Optics, Cambridge University Press (2012)
[2] A. Lauchner et al., Nano Lett., 15, 6208 (2015)
[3] E. Coccia, J. Fregoni, C. A. Guido, M. Marsili, S. Pipolo and S. Corni, J. Chem. Phys., 153, 200901 (2020)
[4] G. Dall’Osto, M. Marsili, M. Vanzan, D. Toffoli, M. Stener, S. Corni and E. Coccia, J. Am. Chem. Soc., 146, 2208 (2024)
[5] G. Dall’Osto, M. Vanzan, S. Corni, M. Marsili and E. Coccia, J. Chem. Phys., 161. 124103 (2024)
[6] L. Biancorosso, P. D’Antoni, S. Corni, M. Stener and E. Coccia, J. Chem. Phys., 161, 214104 (2024)

Speaker: Emanuele Coccia (University of Trieste)

12:05 PM Ultrafast photodynamics and detection of the elusive twist-wagged intramolecular charge transfer (TWICT) state of N$^6$,N$^6$-dimethyladenine (DMAde) by transient vibrational absorption spectroscopy

The DNA nucleobase derivative N$^6$,N$^6$-dimethyladenine (DMAde) stands out for its dual fluorescence, i.e. it shows short-lived emission in the near-ultraviolet from a $^1\pi\pi^*$ local excited (LE) state and longer-lived emission in the visible spectrum from a supposed twisted intramolecular charge transfer (TICT) $^1\pi\pi^*$ state. Experimental studies using time-resolved fluorescence up-conversion spectroscopy (TFLS) and time-resolved electronic absorption spectroscopy (TEAS) on the molecules in acetonitrile (ACN) solution confirmed corresponding previous works. Much deeper insight into the molecular dynamics of electronically excited DMAde has now been gained by the application of transient vibrational absorption spectroscopy (TVAS) as state- and structure-sensitive method, and interpreted by the aid of quantum chemical calculations. Vibrational marker bands have been observed in the fingerprint region of the IR spectrum that showcase the evolution of the excited state population from the initially accessed Franck–Condon region (FC) via the LE state with a lifetime of $\tau_1$ ~ 0.3 ps to subsequent (partially) twisted (pTICT) and twist-wagged (TWIST) conformations. En route, a number of vibrational bands show characteristic broadenings and wavenumber shifts indicating an extended, shallow and nearly barrierless region on the potential energy hypersurface (PEHS) during these transformations, which appear to take up to ~ 20 ps ($\tau_3$). The excited distorted molecules can return to their ground state (GS) in $\tau_2$ ~ 1.2 to 2.0 ps, or they can undergo an intersystem crossing (ISC) transition from the TWICT state to a much longer-lived ($\tau_4$ ~ 1.0 – 1.1 ns) state of $^3\pi\pi^*$ character. These data give first direct spectroscopic evidence for the elusive TICT/TWICT structures of DMAde and for the importance of a triplet state in the dynamics.

Speaker: Friedrich Temps (Institute of Physical Chemistry, Christian-Albrechts-University Kiel)

12:25 PM Excited state dynamics of azanaphthalenes

Azanaphthalenes are nitrogen containing heterocycles, which have systematic variations of the nitrogen heteroatom centres within a bicyclic aromatic structure. With relevance to biological and chemical systems, understanding the underlying ultrafast dynamics upon photoexcitation in these molecules may lead to opportunities for rational design of photoactive molecules. Here, we use quantum chemical calculations undertaken at the SCS-ADC(2) level of theory to explore the photorelaxation processes occurring in six azanaphthalenes in order to rationalise observations from ultrafast transient absorption spectroscopy (TAS) experiments [1]. Our results indicate substantial differences in the propensity for intersystem crossing vs. internal conversion across these molecules, significantly affecting the photorelaxation rates. We explain this behaviour in terms of spin-orbit coupling effects and barriers on the potential energy surfaces.

[1] M. Garrow, L. Bertram, A. Winter, A. W. Prentice, S. W. Crane, P. D. Lane, S. J. Greaves, M. J. Paterson, A. Kirrander, D. Townsend, Commun. Chem., 2025, 8, 1-11.

Speaker: Lauren Bertram (University of Oxford)

12:45 PM → 2:15 PM Lunch Break/IAB meeting

2:15 PM → 3:55 PM Session 7 - Biosystems: Biosystems I

2:15 PM Howe-Siang Tan (Chair)

2:25 PM Multidimensional Snapshots of Photosynthesis in Action

Coherent multidimensional spectroscopies (CMDS) have been applied to a wide range of condensed-phase systems, revealing the life-sustaining structural rearrangements of liquid water, ultrafast energy conversion in photosynthesis, protein folding pathways and many-body interactions in semiconductors. Both high temporal and spectral resolution can be achieved using Fourier transform CMDS. I will discuss recent advances in the field of CMDS, highlighting the development of high sensitivity approaches that are compatible with imaging. I will discuss our application of these approaches to gain insight into the ultrafast processes underlying photosynthesis.

Speaker: Prof. Jennifer Ogilvie (University of Ottawa)

2:55 PM A Coarse-Grained Simulation Approach for Two-Dimensional Electronic Spectroscopy: Dynamics in Photosynthetic Light-Harvesting Systems

Two-Dimensional Electronic Spectroscopy provides an excellent platform for studying the exciton and charge separation dynamics in photosynthetic systems. While applications to individual photosynthetic complexes does provide valuable insight, it has become increasingly clear that the interaction between individual complexes comprising the functional super-complexes affect the behaviour of the individual parts [1]. This calls for spectroscopic measurements of both full super complexes and carefully chosen sub-units. The resulting spectra are, however, challenging to interpret, and common simulation methods are limited to small systems. Here, we present a new efficient coarse grained spectral simulation protocol [2].

The new protocol uses a separation of chromophores into strongly coupled segments. The dynamics within segments may be coherent, while dynamics between segments must be incoherent. The dynamics in different segments is assumed to be uncorrelated. This allows the use of a kinetic model based on the time-dependent multi-chromophoric fluorescence resonant energy transfer method [3] to account for waiting time dynamics, while doorway-window picture response functions are applied to describe the coherence times. The resulting spectra are efficiently calculated with no additional cost for calculating spectra for numerous waiting times.

The developed method is demonstrated for the guinea pig photosynthetic system, LH2, of purple bacteria. Here, the strengths and weaknesses of the coarse-grained method are identified by comparison with other simulations and experiment. For example, an excellent description of the femto-to-picosecond anisotropy dynamics is demonstrated [2]. Finally, the application to the PSII photosynthetic super-complex of plants will be discussed. It will be demonstrated how the simulation technique can be used to aid experimental interpretation and reveal energy transfer pathways and exciton traps and sinks.

[1] Sci. Adv. 2024, 10, eadh0911
[2] J.Chem. Theory Comput. 2024, 20,6111−6124
[3] J.Chem. Theory Comput. 2025, 21,254−266

Speaker: Thomas Jansen (University of Groningen)

3:15 PM Two-dimensional electronic spectroscopy reveals ultrafast Energy Transfer processes in a low-Energy Chlorophylls-free organism: Posidonia Oceanica

Photosystem I (PSI-LHCI) is a multi-subunit pigment-protein complex responsible for light-driven electron transfer in oxygenic photosynthesis. It consists of a core reaction center (RC), where charge separation occurs at the primary electron donor, P700, and a peripheral light-harvesting complex (LHCI) that enhances light absorption capacity to drive photochemical reactions. P700 absorbs light around 700 nm, which is lower in energy than bulk chlorophylls (Chls), whose average absorption peaks around 680 nm. In general, PSI complexes contain red chlorophyll forms (Chls RF); in cyanobacteria, these are predominantly associated with the core, whereas in higher plants, they are localized in the outer antenna LHCI, extending absorption beyond 700 nm, reaching up to 750 nm. These Chls RF improve energy utilization under limiting light conditions enriched in near infrared (NIR) light (https://doi.org/10.1007/s11120-013-9838-x). However, in higher plants, Chls RF presents both advantages and limitations: while they enhance light-harvesting capacity in specific natural environments, their absorption at lower energy with respect to P700 (700 nm) can hinder efficient energy transfer to the RC, imposing limitations on energy trapping. (https://doi.org/10.1016/j.bbabio.2013.03.008, https://doi.org/10.1007/s11120-020-00717-y).
In this context, the seagrass Posidonia oceanica, a higher sea plant endemic to the Mediterranean Sea, evolved from terrestrial ancestors to thrive in underwater environments at depths of up to 40 meters. As an adaptive mechanism to support efficient photosynthesis in marine conditions where the solar radiation spectrum is firmly restricted to higher energy wavelength (NIR light above 700nm is absent) P. oceanica has lost Chls RF typical of higher land plants. This loss was evidenced by the blue-shifted emission spectrum of the PSI-LHCI complex, which could prevent any potential limitation on charge separation efficiency. While these adaptations have become known recently in biology, the P. oceanica PSI-LHCI supercomplex a comprehensive study of the ultrafast energy transfer (ET) mechanisms in this species is still completely lacking.
Specifically, this study required high temporal and spectral resolution since ET mechanisms are in the order of hundreds of femtoseconds up to few picoseconds. Two-dimensional electronic spectroscopy (2DES) is a prefect tool for this aim since it provides time-resolved excitation/detection maps allowing to gain high spectral excitation selectivity still preserving high temporal resolution (15fs) (https://doi.org/10.1063/1.4902938). In this work, we combine pump probe and 2DES to study the ET mechanisms that are taking place in P. oceanica PSI-LHCI by covering a spectral range from 580nm to 720nm.
Our results show a downhill ET from Chls bulk to the RC Chls (absorption peak at 680-690nm) and P700 in <500fs which is four time faster with respect the downhill process observed for higher plants (>2ps) (https://doi.org/10.1021/acs.jpcb.1c01498). This suggests that the absence of the Chls RF accelerate the downhill ET mechanism without imposing limitation on the energy trapping which we characterize around 6ps (twice faster with respect to other higher plants).
In conclusion, we provide for the first time, temporal and spectral characterization of the ET mechanisms in P. oceanica, which adapts their photosynthetic complexes to a natural environment where the NIR light cannot be absorbed. Furthermore, these results give the possibility to deeply understand how the presence of Chls RF impacts the photosynthetic efficiency, providing crucial insight for bioengineering enhanced light-harvesting complexes.

Speaker: Mattia Russo (Politecnico di Milano)

3:35 PM Microcavity mediated excitation dynamics of photosynthetic light harvesting complexes.

Strong light-matter interaction leads to the formation of hybrid polariton states and alters the photophysical dynamics of organic materials and biological systems without modifying their chemical structure. Here, we experimentally investigated a well-known photosynthetic protein, light harvesting 2 complexes (LH2) from purple bacteria under both strong and weak coupling with the light mode of a Fabry-Perot optical microcavity. Using femtosecond pump-probe spectroscopy, we analysed the polariton dynamics of the strongly coupled system. We observed a significant prolongation of the excited state lifetime compared with the bare exciton, which can be explained in terms of the exciton reservoir model. We also demonstrated cavity-mediated excitation transfer between different complexes even in case of weak effective light-mater interaction.

Wu, F.; Finkelstein-Shapiro, D.; Wang, M.; Rosenkampff, I.; Yartsev, A.; Pascher, T.; Nguyen-Phan, T. C.; Cogdell, R.; Börjesson, K.; Pullerits, T. Optical Cavity-Mediated Exciton Dynamics in Photosynthetic Light Harvesting 2 Complexes. Nat. Commun. 2022, 13, 6864.
https://doi.org/10.1038/s41467-022-34613-x.

Wu, F.; Nguyen-Phan, T. C.; Cogdell, R.; Pullerits, T. Efficient cavity-mediated energy transfer between photosynthetic light harvesting complexes from strong to weak coupling regime. arXiv2502.10144

Rosenkampff, I.; Pullerits, T. Microcavity-Enhanced Exciton Dynamics in Light-Harvesting Complexes: Insights from Redfield Theory. arXiv:250

Speaker: Tönu Pullerits (Lund University, Chemical Physics)

3:55 PM → 4:25 PM Coffee Break

4:25 PM → 6:15 PM Session 8 - Biosystems II: Biosystems II

4:25 PM Mechanism in Reversibly Switchable Fluorescent Proteins

Reversibly switchable fluorescent proteins (FPs) are critical to superesolution bioimaging. The widely used negative switching FPs are well characterized. The under-used positive and decoupled switching FPs much less so. Here we report complementary ultrafast transient optical and infra-red absorption measurements of photoswitching in Padron, Kohinoor (+FPs) and Dreiklag (dcFP). The two measurements allow independent study of chromophore and host protein matrix. It is well established that the protein matrix greatly influences chromophore photophysics. The present result suggest the matrix actively 'steers' the chromophore on the reactive surface.

Speaker: Steve Meech (University of East Anglia)

4:55 PM Multi-step 11-cis to all-trans retinal photoisomerization in bestrhodopsin, an unusual microbial rhodopsin

Rhodopsins constitute a broad class of sensory photoreceptors with retinal chromophores bound to the protein via a retinal Schiff base (RSB). Microbial rhodopsins are mostly activated through an all-trans to 13-cis photoisomerization reaction, whereas animal rhodopsins are invariably activated through an 11-cis to all-trans isomerization reaction. The recently discovered bestrhodopsins constitute a subfamily of very special bistable microbial rhodopsins. The P. antarctica bestrhodopsin photochemistry involves a very peculiar all-trans to 11-cis isomerization and vice versa, rather than the all-trans to 13-cis photoreaction of canonical microbial rhodopsins, and hence resemble animal rhodopsins in that regard. Here, we present the 11-cis to all-trans photoreaction as determined by femtosecond to sub-millisecond transient absorption (TA) and femtosecond stimulated Raman spectroscopy (FSRS). The primary photoreaction involves ultrafast isomerizations in 240 fs from the 11-cis RSB reactant to a mixture of highly distorted all-trans and 13-cis RSB isomeric photoproducts. The 13-cis RSB isomer fraction of the primary photoproduct then thermally isomerizes to a distorted all-trans RSB in 120 ps. To rationalize this highly unusual phenomenology, we propose bicycle pedal models for the branched photoisomerizations from the 11-cis reactant to all-trans and 13-cis RSB isomer products, with co-rotation of the C11=C12 and C13=C14 double bonds. The former fraction undergoes bicycle pedal motion aborted at the C13=C14 double bond, resulting in an all-trans RSB isomer. The latter fraction undergoes a full bicycle pedal motion of both C11=C12 and C13=C14 double bonds, resulting in a 13-cis RSB isomer. The primary products are trapped high up the ground state potential energy surfact (PES) owing to steric interactions with the protein binding pocket. Due to the resulting low energetic barrier on the ground state PES, thermal isomerization from 13-cis to all-trans RSB occurs in 120 ps. We suggest that the mechanism for simultaneous production of two different isomers may generally apply for rhodopsins, where the production of only one isomer as in most animal and microbial rhodopsins may be regarded as limiting cases.

Speaker: John T.M. Kennis (Vrije Universiteit Amsterdam)

5:15 PM Novel femtosecond photoreactions in flavo-enzymes

In exceptional cases, flavo-enzymes perform functional light-driven catalysis, such as in fatty acid photodecarboxylase (FAP) [1]. Yet most display light-independent functions, although in these photophysical processes also occur. Such processes can have photoprotective functions, and may also be exploited for photocatalysis or photoswitching applications [2]. This contribution highlights recent ultrafast spectroscopic studies on short-lived photoproducts in “nonphotoactive” flavoproteins exploring various redox and ligation states. They include the discovery of two hitherto unknown photoreactions that occur on the timescale of a few hundred femtoseconds or less.
First, we observed quasi-instantaneous (<100 fs) photo-oxidation of anionic flavin radicals in various flavoprotein oxidases, and subsequent charge re-separation in a few tens of picoseconds [3]. We will show that such a non-functional photoreaction also occurs in FAP, where it surprisingly involves hydrated electron intermediates.
Second, we studied the charge-transfer complex formed by the flavin ring system and the substrate-analog inhibitor methylthioacetate in monomeric sarcosine oxidase [4]. Here, upon population of the photo-excited CT state, with near-unity quantum yield a state spectroscopically identical to the non-complexed enzyme is formed in ~300 fs in a barrierless process. This implies that all CT interactions are vanished on this timescale. The initial CT complex is subsequently recovered in a strongly thermally activated way on the nanosecond timescale. These are properties of a highly efficient red-absorbing photoswitch. The possible ultrafast structural changes associated with this unprecedented process are discussed, as well as new in-depth characterizations of the process, and possible extensions of this system (the characteristics of which have recently been shown to be highly sensitive to the structural details of the system [5]), for practical applications [6].

References

1. A. Aleksandrov; A. Bonvalet; P. Müller; D. Sorigué; F. Beisson; L. Antonucci; X. Solinas; M. Joffre; M.H. Vos, Angew. Chem. Int. Ed. 2024, 63, e202401376.
2. B. Zhuang; U. Liebl; M.H. Vos, J. Phys. Chem. B 2022, 126, 3199.
3. B. Zhuang; R. Ramodiharilafy; U. Liebl; A. Aleksandrov; M.H. Vos, Proc. Natl. Acad. Sci. U.S.A. 2022, 119, e2118924119.
4. B. Zhuang; M.H. Vos, J. Am. Chem. Soc. 2022, 144, 11569.
5. B. Zhuang; G. Ran; W. Zhang; F. Gai, Chem. Sci. 2025, in press.
6. B. Zhuang; U. Liebl; M.H. Vos; M. Sliwa, ChemPhotoChem 2025, e202500012.

Speaker: Marten Vos (Ecole Polytechnique)

5:35 PM Optical coherent control of biological electron transfer

Optical control of dynamic processes has been challenging yet has been demonstrated in several chemical and biological systems. The control of a reaction passing the widely present conical intersection has not been realized. Here, we can modulate the phase of the excitation pulse to achieve control of an important chemical process, the dynamics of β-carotene to access the conical intersection (CI). We then report for the first time on the optical control of electron transfer (ET) processes in a protein flavodoxin. Such successful demonstration of optical coherent controlled CI and ET in the chemical and biological systems is significant to opening a new direction, especially to control a variety of ET processes in chemical and biological systems.

Speaker: Dongping Zhong (Shanghai Jiao Tong University)

5:55 PM Excitation dynamics in DNA-templated silver nanoclusters

DNA-templated silver nanoclusters have fascinating properties, including high absorption cross section and high fluorescence quantum yield. Importantly, these and other features can be readily tuned by changing the DNA sequence of strands, which stabilize the silver cluster inside. The combination of desirable properties and tunability makes them potentially suitable for a wide range of applications from biosensing to nanophotonics. We used two-dimensional electronic spectroscopy to investigate photo-induced dynamics in a few DNA-templated silver nanoclusters [1,2]. Rather surprisingly, some clusters feature similar behaviour, whereas others different, but in all of them we follow sub-100 fs energy relaxation between the absorbing and emitting states. We discuss electronic structure, nature of the observed transitions, as well as exceptional coherence and relaxation dynamics in these systems.

References
1. E. Thyrhaug et al. Ultrafast coherence transfer in DNA-templated silver nanoclusters. Nat. Commun., 8, 15577 (2017).
2. J. Chen et al. Excited-state dynamics in a DNA-stabilized Ag16 cluster with Near-Infrared emission. J. Phys. Chem. Lett., 14, 4078-4083 (2023).

Speaker: Donatas Zigmantas (Chemical Physics, Lund University)

6:15 PM → 7:45 PM Poster Session 2

Wed, June 25

8:00 AM → 9:00 AM Registration

9:00 AM → 10:40 AM Session 9 - Solvation and reaction dynamics II: Solvation and Reaction Dynamics III

9:00 AM Stefan Haacke (Chair)

9:10 AM Ultrafast Wolff rearrangement and solvent reactions of UV-photoexcited diazocarbonyl compounds

Following UV photoexcitation, organic diazo compounds can eliminate N$_{2}$ by C=N bond dissociation to form highly reactive singlet carbene intermediates. In solution, these carbenes rapidly react with nucleophilic solvents such as alcohols or ethers to form zwitterionic ylides. If the starting molecules are $\alpha$-diazocarbonyl compounds (with a carbonyl group adjacent to the diazo group), a second reactive pathway competes in which Wolff rearrangement makes a ketene containing a >C=C=O group. The N$_2$ dissociation and Wolff rearrangement are either concerted, or occur in a stepwise fashion via a carbene intermediate.
Here, outcomes will be presented from ultrafast time-resolved infrared (TRIR) spectroscopy studies of the competing rearrangement and reaction pathways for UV-photoexcited $\alpha$-diazocarbonyl compounds in various organic solvents. The TRIR measurements reveal both ketene production via Wolff rearrangement and carbene reactions with the solvent to form ylides [1]. Subsequent shifts in the ylide TRIR spectral bands show the response of the solvent to the reactions making these zwitterionic solute molecules [2]. These studies of $\alpha$-diazocarbonyl photochemistry in solution will be contrasted with the ultrafast structural dynamics observed for gas-phase molecules using time-resolved X-ray scattering at the LCLS-II X-ray free electron laser, and with computational simulations of the non-adiabatic photochemical dynamics using trajectory surface hopping methods.
Acknowledgements
We are grateful for the award of CXI beamtime L-10227 at LCLS-II in November 2023, and to our many collaborators who contributed to the collection of the ultrafast X-ray scattering data. The Bristol group thanks EPSRC for funding via Programme Grant EP/V026690/1.
References
[1] R. Phelps and A.J Orr-Ewing, J. Am. Chem. Soc. 2020, 142, 7836
[2] R. Phelps and A.J Orr-Ewing, J. Am. Chem. Soc. 2022, 144, 9330

Speaker: Andrew Orr-Ewing (University of Bristol)

9:40 AM Elucidating the Interplay between Ultrafast Internal Conversion, Intersystem Crossing, and Proton Transfer for Guiding New Photochemical Reactivities

The mechanism of newly discovered photochemical reactions of $\beta$-enaminones and maleimide derivatives is a topic of interest and ongoing discussion [1,2]. We have examined excited-state dynamics in these systems following femtosecond UV excitation by means of ultrafast transient absorption spectroscopy with dispersed, broadband probing, complemented by the tools of computational photochemistry.
Excited-state relaxation of the $\beta$-enaminones in protic and aprotic solvents has been found to be within the 500 fs range and involve a tautomerization process. A fast sub-50 fs molecular descent from the Franck-Condon region leads to a flatter portion of the S$_1$ potential which region is defined by three degenerate potential energy surfaces forming a conical intersection and a singlet-triplet crossing.
For the substituted maleimides, the maleimide structure is modified by adding the hydroxy (-OH) functionality to introduce the possibility of excited-state intramolecular proton transfer (ESIPT). Maleimides where a proton source for ESIPT is not present were also studied. We have shown that, following UV excitation of these molecules into the weakly allowed $\pi$ to $\pi$ transition, they follow a major cascade-like ultrafast radiationless relaxation pathway via two lower-lying singlet n$\pi$** excited states into the ground S$_0$ state. In addition, the hydroxy-substituted maleimide displays a minor pathway with spectral and kinetic signatures consistent with excited-state proton-transfer reaction [3]. The maleimide system lacking the hydroxyl substituent reveals an intersystem crossing channel. The dynamics in these systems is compared with excited-state dynamics of unsubstituted maleimide.
This work is funded by the National Science Foundation, under grant numbers CHE-1955524, CHE-2350308, and CHE-2102619.

[1] S. K. Kandappa, L. K. Valloli, S. Jockusch, J. Sivaguru, J. Am. Chem. Soc. 2021, 143, 3677.
[2] J. Parthiban, D. Garg, S. Ahuja, S. Jockusch, A. Ugrinov, J. Sivaguru, ACS Catal. 2024, 14, 8794.
[3] D. Garg, A. N. Tarnovsky, J. Sivaguru, J. Phys. Chem. A 2025 (accepted).

Speaker: Alexander Tarnovsky (Department of Chemistry and the Center for Photochemical Sciences, Bowling Green State University, USA)

10:00 AM Electronic structure and excited state reactions of molecules in aqueous solutions studied by time-resolved XUV photoelectron spectroscopy

We report on the state-of-the-art time-resolved photoelectron spectroscopy (LJ-TRPES) of molecular chromophores solvated in aqueous environment using wavelength-selected XUV pulses from high-order harmonic generation and micro-liquid jet (LJ) technology. LJ-TRPES is one of the most direct analytic methods to follow transient electronic structures of complex photoexcited molecules.

In one example, we investigate in combination with conventional time-resolved transient absorption spectroscopy (TAS) the relaxation timescales as well as absolute binding energies of the electronic states of Metanil Yellow (MY), an aminoazobenzene derivative. The excited-state dynamics obtained with both methods is compared to time-dependent density functional theory (TDDFT) calculations. As shown in previous work, the low-energy part of the absorption spectrum of MY consists of two overlapping bands, associated with the hydrated and non-hydrated forms of the dye, with maxima at 416 and 464 nm, respectively. Using TAS and TRPES with different excitation wavelengths (λ=370 nm and λ=490 nm), we reveal that both forms undergo similar dynamics characterized by ~1.5 ps time constant reflecting internal conversion to the trans ground state along the torsional coordinate.

In the other example we investigate with TRPES the ultrafast relaxation of NAIP, biomimetic molecular switch. In TAS experiments the switch demonstrates an almost ballistic approach to the conical intersection(CINT) and thus is a good candidate for observing passage of CINT by TRPES.

Speaker: Oleg Kornilov (Max Born Institute)

10:20 AM Pseudo-rotation versus rotational diffusion in the ligand exchange 2D-IR spectra of iron pentacarbonyl

We re-visit the ligand exchange dynamics in Fe(CO)$_5$ , a textbook example of fluxionality or Berry pseudo rotation, by high-resolution polarization-dependent 2D-IR spectroscopy. Coupling maps at short waiting times reveal detailed information about the anharmonic structure: a very small negative coupling between the IR-active CO stretch modes A2′′ and E′ and distinct diagonal and non-diagonal anharmonicities of the degenerate mode. Waiting-time dependent measurements in a series of alkanes of different chain lengths shows that Berry pseudo rotation takes place on a 10 ps timescale with very little dependence on solvent viscosity. In contrast, the anisotropy loss is much faster than pseudo rotation in short alkanes but longer in the most viscous ones. Both processes need to be considered to understand the redistribution of vibrational excitation during a pseudo-rotation step, and hence the actual exchange rate. We will discuss the implications for the determination of the rate of ligand exchange and the transition state geometry.

References
[1] J. F. Cahoon, K. R. Sawyer, J. P. Schlegel, and C. B. Harris, Science 319, 1820–1823 (2008).
[2] T.-T. Chen, M. Du, Z. Yang, J. Yuen-Zhou, and W. Xiong, Science 378, 790–794 (2022).
[3] A. Jouan et al. submitted (2025)

Speaker: Dr Jan Helbing (University of Zurich)

10:40 AM → 11:10 AM Coffee Break

11:10 AM → 12:40 PM Session 10 - Attosecond Science II: Attosecond Science II

11:10 AM New directions in Attosecond Chemistry

With the advent of attosecond light pulses at the dawn of the twenty first century, access to the time scale of electronic motion, i.e., the ultimate time scale responsible for chemical transformations, was finally at our reach. Since the first attosecond pump-probe experiments performed in molecules [1,2], the field has grown exponentially, leading to a discipline that we call attochemistry [3]. As a result, it is nowadays possible to follow in real time the motion of the “fast” electronic motion in molecules, mostly in the gas phase, and understand how this motion affects the “slower” motion of atomic nuclei and vice versa. There are, however, new scenarios [4] that will allow one to extend the range of applications to more complex molecular systems, including the condensed phase, and to overcome some of the limitations of current attosecond technologies [5-9], such as the low intensity of attosecond pulses produced by high harmonic generation, the impossibility to generate such pulses in the visible and UV spectral regions to avoid molecular ionization, or the difficulties to combine them with truly imaging methods for direct time-resolved observations of the electron density without the need for reconstruction from measured photoelectron, photoion or transient absorption spectra.

In this talk, I will describe current experimental and theoretical efforts aiming at overcoming the above-mentioned limitations, thus giving attochemistry the necessary push to investigate problems of real chemical interest.

[1] G. Sansone, F. Kelkensberg, J. F. Pérez-Torres, F. Morales, M. F. Kling, W. Siu, O. Ghafur, P. Johnsson, M. Swoboda, E. Benedetti, F. Ferrari, F. Lépine, J. L. Sanz-Vicario, S. Zherebtsov, I. Znakovskaya, A. L’Huillier, M. Yu. Ivanov, M. Nisoli, F. Martín, and M. J. J. Vrakking, Nature 465 763 (2010).
[2] F. Calegari, D. Ayuso, A. Trabattoni, L. Belshaw, S. De Camillis, S. Anumula, F. Frassetto, L. Poletto, A. Palacios, P. Decleva, J. B. Greenwood, F. Martín, and M. Nisoli, Science 346, 336 (2014).
[3] M. Nisoli, P. Decleva, F. Callegari, A. Palacios, and F. Martín, Chem. Rev. 117, 10760 (2017).
[4] F. Calegari and F. Martín, Commun. Chem. 6, 184 (2023).
[5] A. Palacios and F. Martín, WIREs Comput. Mol. Sci. e1430 (2020).
[6] G. Grell, Z. Guo, T. Driver, P. Decleva, E. Plésiat, A. Picón, J. González-Vázquez, P. Walter, J. P. Marangos, J. P. Cryan, A. Marinelli, A. Palacios, and F. Martín, Phys. Rev. Res. 5, 023092 (2023).
[7] M. Galli et al, Optics Letters 44, 1308 (2019).
[8] M. Reduzzi et al, Optics Express 31, 26854 (2023).
[9] M. Garg, A. Martín-Jiménez, M. Pisarra, Y. Luo, F. Martín and K. Kern, Nature Photonics 16, 196 (2022).
[10] F. Vismarra, F. Fernández-Villoria, D. Mocci, J. González-Vázquez, Y. Wu, L. Colaizzi, F. Holzmeier, J. Delgado, J. Santos, L. Bañares, L. Carlini, M. Castrovilli, P. Bolognesi, R. Richter, L. Avaldi, A. Palacios, M. Lucchini, M. Reduzzi, R. Borrego-Varillas, N. Martín, F. Martín and M. Nisoli, Nature Chemistry 16, 2017 (2024).

Speaker: Prof. Fernando Martin (Universidad Autonoma de Madrid and IMDEA Nanoscience)

11:40 AM Ultrafast dissociation dynamics of alkyl iodides induced by few-fs UV pulses

Alkyl iodides serve as key model systems for studying ultrafast nonadiabatic dynamics. UV radiation excites the A-band, with an absorption spectrum centered near 260 nm [1], leading to a rapid C–I bond cleavage. This neutral fragmentation is intrinsically governed by a conical intersection (CI) [2]. However, the limited temporal resolution of previous experiments has hindered direct observation of the early relaxation dynamics in the absence of the UV excitation pulse [3]. Here, we present measurements using few-femtosecond (fs) UV pulses [4,5] with a duration corresponding to <7% only of the vibrational period in methyl and ethyl iodide (CH$_3$I and C$_2$H$_5$I).

In CH$_3$I, the dynamics is probed by few-cycle near-infrared (NIR) pulses through multiphoton ionization and mass spectrometry. Trajectory surface hopping calculations are performed to identify the dissociation channels involved, relating to the number of absorbed NIR photons. The time resolution achieved with 4-fs UV pulses allowed us to benchmark our theoretical modelling, which can precisely determine the arrival to the CI of the neutral system. Furthermore, the results demonstrate that NIR ionization within a 5-fs window after UV excitation can prevent the otherwise inevitable cleavage of the C–I bond [6].

We also present UV pump–extreme ultraviolet (XUV) probe measurements of ethyl iodide [7]. Fitting of the time-dependent yields of the fragment ions (C$_2$H$_2^+$ and C$_2$H$_5^+$) reveals a relative time delay of 6.7 fs between these two fragments, indicating the presence of a few-fs relaxation channel following the UV-photoexcitation.

References
[1] C M Roehl et al., J Geophys Res. 102, 12819 (1997)
[2] A Eppink et al., J. Chem. Phys. 110, 832 (1999)
[3] K F Chang et al., J. Chem. Phys. 154, 234301 (2021)
[4] M Galli et al., Optics Letters 44, 1308 (2019)
[5] V Wanie et al., J Phys Photonics 6, 025005 (2024)
[6] L Colaizzi et al., Nat Commun 15, 9196 (2024)
[7] V Wanie et al., Rev. Sci. Instrum. 95, 083004 (2024)

Speaker: Erik Månsson (DESY CFEL)

12:00 PM Attosecond-resolved Ultrafast Electronic and Nuclear Wavepacket Dynamics in Furan at the C K-edge

The ultrafast relaxation mechanism of furan (C4H4O) is known to be prototypical of the Ring-Opening (RO) and Ring-Puckering (RP) dynamics of cyclic molecules [1-4]. Despite encouraging results obtained so far [2-4], experimentally identifying the main relaxation pathways with their electronic and vibrational coherences has been out of reach due to the ultrafast timescales and the involvement dark states.
In this combined experimental and theoretical work [5], we investigate the ultrafast nonadiabatic dynamics of furan and show that core-level x-ray absorption fine structure (XAFS) spectroscopy with attosecond soft x-ray pulses [6] is capable of meeting these challenges. We excite furan via multi-photon absorption and follow the subsequent relaxation dynamics measuring the time-dependent carbon K-edge absorption spectra with an isolated attosecond probe pulse. The data show rich and ultrafast dynamics that follows the photoinduced excitation. The extensive theoretical treatment and the comparison with the experiment identify initial pump excitation into ππ state, to which we can assign to the ππ a decay constant of 65 ± 10 fs. At this time delay, according to the theoretical investigation, the CI ($\pi\pi^{*}$)/($\pi\sigma^{*}$) passage transiently populates the dissociative dark state $\pi\sigma^{*}$, emphasized text which ultimately leads to bond breaking. This is confirmed by the experimental data, which registers a splitting of the SOMOs peak into 4 absorption peaks caused by the symmetry loss characteristic of the ring-opened geometry. This signature is an experimental evidence that the relaxation dynamics take the system along RO pathways and transiently populates the $\pi\sigma^{*}$. Finally, after 140 fs, the $\pi\sigma^{*}$ XAFS signature disappears and the emergence of a long-lasting component indicates the transition of the excited electronic state back into the electronic ground state. In addition, the analysis of the periodically modulated XAFS features provides information about the coherent wavepacket dynamics. Right after the pump-induced excitation, the data show intense modulations. The Fourier Transform (FT) shows a beating frequency of 63 ± 9 THz (16 ± 2 fs) originating from coherent electronic wavepacket dynamics as the nuclear wavepacket proceeds on almost parallelly lying excited electronic states. Moreover, the coherent electronic motion of charge density across the two distinct carbon atoms of furan exhibits a π-phase shift between the spatially separate nuclear sites [7], which is encoded in the XAFS time-resolved trace. At delays larger than 90 fs, the system is strongly stabilized along the RO trajectories, which causes a dephasing of the electronic coherence and the appearance of a new Fourier component at 36 ± 9 THz (28 ± 7 fs). This component is assigned to RO vibrational mode through theoretical analysis. Altogether, the excitation to the dissociative state, the activation of the vibrational mode, and the distinctive signature of the long-lasting signal allow us to identify the RO pathway as the dominant relaxation pathway, demonstrating the capability of core-level attosecond XAFS spectroscopy to disentangle the intricate pathways of coupled electronic and nuclear dynamics of a complex polyatomic system.

References:
[1] M. Stenrup and A. Larson, “A computational study of radiation-less deactivation mechanism of furan”, Chemical Physics 379, 6-12 (2011).
[2] E. Wang, et al., ” Time-Resolved Coulomb Explosion Imaging Unveils Ultrafast Ring Opening of Furan” arXiv:2311.05099v1 (2023).
[3] R. Uenishi et al., Signatures of Conical Intersections in Extreme Ultraviolet Photoelectron Spectra of Furan Measured with 15 fs Time Resolution”, J. Phys. Chem. Lett. 15(8), 2222-2227 (2024).
[4] S. Oesterling “Substituent effects on the relaxation dynamics of furan, furfural and b-furfural: a combined theoretical and experimental approach”, Phys.Chem.Chem.Phys. 19, 2025-2035 (2017).
[5] S. Severino, K. M. Ziems et al., “Attosecond Core-Level Absorption Spectroscopy Reveals the Electronic and Nuclear Dynamics of Molecular Ring-Opening” Nat. Photon. (2024).
[6] S. M. Teichmann et al., “0.5-keV Soft X-Ray Attosecond Continua” Nat. Commun. 7, 11493 (2016).
[7] Y. Kobayashi et al., "Theoretical analysis of the role of complex transition dipole phase in XUV transient-absorption probing of charge migration", Optics Express 30, 5673-5682 (2022).

Speaker: Stefano Severino (ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain)

12:20 PM Attosecond Coherent Electron Dynamics Triggered by XFEL Pulses

Two key concepts characterize the ultrafast many-electron dynamics triggered in atoms and molecules upon interaction with ultrashort X-ray laser pulses produced by an X-ray free electron laser (FEL) source: quantum coherence [1] and quantum entanglement: the former underpins the few-femtosecond charge dynamics in molecules and the ensuing photochemical transformation; the latter limits the coherence that can be observed within each subsystem when interrogated individually by probe measurements.

I will discuss these key concepts and present results from two exemplary, combined experimental-theoretical pump-probe studies of quantum electronic coherences in molecules. The first one concerns molecular glycine [2], where few-femtosecond X-ray pulses from the FLASH X-ray FEL were used to trigger coherent electron dynamics in the glycine cation and probe it by resonant x-ray absorption and sequential double photoionization. The results provide a direct support for the existence long-lived electronic coherence up to 25 femtoseconds [2]. The second one addresses electron motion resulting from photoionization of a prototypical aromatic system (para-aminophenol) [3]. In this work [3], a pair of attosecond X-ray pulses from the LCLS-II X-ray FEL was employed to both trigger coherent dynamics in the para-aminophenol cation and track it with atomic-site specificity via attosecond X-ray absorption.

References:
[1] M. Ruberti, “Onset of ionic coherence and ultrafast charge dynamics in attosecond molecular ionisation”, Physical Chemistry Chemical Physics, 21, 17584 (2019).
[2] D. Schwickert et al., “Electronic quantum coherence in glycine molecules probed with ultrashort x-ray pulses in real time”, Science Advances, 8 (22), eabn6848 (2022).
[3] T. Driver et al., “Attosecond Coherent Electron Motion in a Photoionized Aromatic Molecule”, arXiv:2411.01700 [physics.chem-ph] (2025).

Speaker: Marco Ruberti (Imperial College London)

12:40 PM → 12:50 PM Group photo

12:50 PM → 3:00 PM Lunch Break

2:30 PM → 5:30 PM Visit at Elettra Sincrotrone Trieste

3:00 PM → 6:00 PM City Tour

7:30 PM → 10:30 PM Conference Dinner

Thu, June 26

8:00 AM → 9:00 AM Registration

9:00 AM → 10:40 AM Session 11 - Structural Dynamics I: Structural Dynamics I

9:00 AM Claudio Masciovecchio

9:10 AM X-ray probing of photochemical dynamics

Ultrafast X-ray spectroscopic investigations and molecular dynamics are now achievable with short X-ray pulses produced by laboratory table-top high-order harmonics. Those X-rays probe transitions from localized inner shells on specific atomic sites in the molecules to valence orbitals, conveying new information about photochemical transformations. The interpretations of these spectra involve a new regime of core-to-valence X-ray probing that depends on energy shifts due to the surrounding electronic densities, spin-coupling effects, energy shifts due to bond elongation with vibrational excitation and bond angles. Coherent vibrational superpositions reveal different slopes of inner shell potentials with bond extension and Fermi resonance coupling in the X-ray. Precision measurements of bond length changes can be as small as 0.0001 Angstrom. Real time observations of Jahn-Teller distortion and passage through conical intersections are achieved. Open shell radicals have characteristic features of singly occupied orbitals and energetic shifts upon bond cleavage, and spin splittings, which can be viewed from the localized atomic perspective. Corresponding theory work by collaborators provides a powerful assessment of the dynamics through X-ray spectroscopic investigations. Progress in revealing the full potential of time-resolved X-ray spectroscopy for the investigation of numerous novel features in molecular photochemistry is discussed.

J. H. Ou, D. Hait, P. Rupprecht, J. E. Beetar, T. J. Martínez, S. R. Leone, “Attosecond probing of coherent vibrational dynamics in CBr4,” J. Phys. Chem. A 128, 9208–9217 (2024).

A. D. Ross, D. Hait, V. Scutelnic, D. M. Neumark, M. Head-Gordon, S. R. Leone, “Measurement of coherent vibrational dynamics with X-ray transient absorption spectroscopy simultaneously at the carbon K- and chlorine L2,3- edges,” Commun. Phys. 7, 304 (2024).

E. Haugen, D. Hait, V. Scutelnic, T. Xue, M. Head-Gordon, S. R. Leone, “Ultrafast X-ray spectroscopy of intersystem crossing in hexafluoroacetylacetone: chromophore photophysics and spectral changes in the face of electron-withdrawing groups” J. Phys. Chem. A 127, 634-644 (2023).

M. Epshtein, B. N. C. Tenorio, M. L. Vidal, V. Scutelnic, Z. Yang, T. Xue, A. I. Krylov, S. Coriani, S. R. Leone, “Signatures of the bromine atom and open-shell spin-coupling in the X-ray spectrum of bromobenzene cation,” J. Am. Chem. Soc. 145, 3554-3560 (2023).

E. Ridente, D. Hait, E. A. Haugen, A. D. Ross, D. M. Neumark, M. Head-Gordon, S. R. Leone, “Femtosecond symmetry breaking and coherent relaxation of methane cations at the carbon K-edge,” Science 380, 713-717 (2023).

Speaker: Stephen Leone (University of California, Berkeley)

9:40 AM Exploring the photocycle of the [Fe(BPAbipyH)]2+ CO2 reduction catalyst using ultrafast X-ray techniques

A self-induced photosensitive bis(pyridyl)amine-bipyridine-iron(II) framework [Fe(BPAbipyH)]2+ can convert CO2 to CO without the addition of an external photosensitizer. Direct irradiation of FeDPABipyH by visible light leads to CO2 reduction to CO with >95% selectivity and >800 TON in 24 h in a mixed-solvent solution (acetonitrile: ethanol = 1:1)1. This is two times higher than what is achieved in pure acetonitrile. The ns optical transient absorption (OTA) results show that the excited state lifetime is 297ns in the mixed-solvent solution, while a much shorter 16ns lifetime is seen in pure acetonitrile solution, which is consistent with the simple concept that the longer the excited state lifetime, the more CO2 reduction can take place.
To investigate the details of this long-lived process, ultrafast X-ray techniques have been applied to probe the excited state dynamics and identify the reaction intermediates. Time resolved XAS at a time delay of 2ns was collected at 1W2B of the Beijing Synchrotron Radiation facility (BSRF) where 343 nm was used to photoactivate the catalyst. The fitting of the difference spectrum at 2ns is consistent with a spin crossover (SCO) excitation from low-spin (LS) to high-spin (HS), analogous to other Fe(II) complexes2-4. Spin crossover excited states are normally populated after relaxation through charge transfer and ligand field electronic excited states, which can occur on ultrafast timescales (< 1ps)5,6. To probe the ultrafast charge and spin dynamics we also measured XES in pure acetonitrile and RXES in mixed solvent (acetonitrile:ethonal = 1:1) using 400nm excitation at the FXE instrument of the EuXFEL7.8. The XES data shows a strong Kβ transient signal at 100 fs time delay, which is consistent with 5T2 high-spin character. The RXES shows changes at 1s → 3d and 1s →ligand transitions in the pre-edge. The XAS data, measured at the SACLA XFEL, tracks the excitation process and relaxation of the sample in both pure acetonitrile and mix (acetonitrile:ethanol = 1:1) solvent. From the preliminary data analysis, after ultrafast excitation within 100fs, the excited state has two or three relaxation processes like the a, b, c in the figure. The process before and after a (~300fs) seems like different. Between a(~300fs) and c(~750fs), the excited process become slower. The WAXS measured in ESRF shows different transient state with different solvents at low Q which may reveal the effect of solvent on the relaxation of excited states after 100 ps. These results from the ultrafast X-ray measurements and the insight they provide into the photochemistry provide important insight into the self-sensitized catalystic cycle of FeDPABipyH. This talk will present the experiments’ results and our current interpretation.

Speaker: Dr Hao Wang (EuXFEL)

10:00 AM Coupled nuclear and electronic dynamics during proton transfer observed with combined experimental and computational resonant inelastic x-ray scattering

Proton transfer processes lie at the heart of many (bio)chemical reactions. Excited state intramolecular proton transfer (ESIPT) systems are of particular interest due to possible application as e.g. photoluminescence sensors or white-light emitting materials. In ESIPT, the proton transfer process is initiated through excitation into a 𝛑𝛑* state with UV or visible light and leads to fluorescence from the product excited state. The Stokes-shifted fluorescence makes ESIPT systems a promising candidate for optoelectronic applications [1]. To investigate the coupled nuclear and electronic motion during and following ESIPT, we here consider an ESIPT model system, 10-hydroxybenzo[h]quinoline (HBQ). In the ground state, HBQ in solution mainly exists in its enol form. Upon excitation from the enol ground state, ESIPT to a keto excited state takes place within 13 fs [2]. We probed the coupled nuclear and electronic structure of HBQ using resonant inelastic X-ray scattering (RIXS) spectroscopy at the N and O K-edges. The atom-specificity of the X-ray probe allows for directly probing the local electronic structure around the proton donor (O) and acceptor (N) atoms. Here, we present a combined experimental and computational approach to elucidate the coupled electronic and nuclear motion during and following ESIPT.

We simulated RIXS spectra along the excited state trajectory of the proton transfer using excited state ab initio molecular dynamics simulations combined with time-dependent density functional theory calculations. The results highlight the potential of RIXS to observe the coupling between different electronic states via off-diagonal peaks containing complementary information to XANES and UV/vis spectra [3]. The excited state spectra specifically show how the time-dependent changes in the intramolecular hydrogen bond are encoded in the dynamics of the RIXS spectral features during and following ESIPT at the proton donor and acceptor sites. Further, the calculations show that some of the RIXS features can be used as a ruler probing the proton transfer distance during the ultrafast chemical process.

The computational results complement our experimental study using femtosecond X-ray pulses at the newly commissioned chemRIXS endstation at LCLS-II. We recorded RIXS spectra at the O and N K-edges of the enol ground state and of the keto excited state following 390 nm excitation. The experimental data highlight the differences in electronic structure at the proton donor and acceptor sites and their changes following proton transfer. The experimental results are in good agreement with the computational data providing detailed insights into a prototypical proton transfer process.

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[1] Kwon, J. E., & Park, S. Y. Advanced Materials, 23(32), 3615–3642, 2011.
[2] Kim, C.H, and Joo, T. Phys. Chem. Chem. Phys., 2009, 11, 10266–10269.
[3] Nimmrich, A., Govind, N. and Khalil, M., J.Phys. Chem. Lett., 2024, 15 (51), 12652-12662.

Speaker: Amke Nimmrich (University of Washington)

10:20 AM Resolving Mechanistic Pathways in Bioinorganic Catalysis via Ultrafast X-ray Spectroscopy

The most efficient and sustainable means of storing energy from raw sources involves harnessing chemical bonds. Achieving this requires the development of catalysts that are not only cost-effective but also exhibit high efficiency and selectivity. Many of these catalysts are inspired by bioinorganic systems where transition metal centers mediate complex redox transformations. Advanced X-ray spectroscopic techniques have become invaluable tools for probing relevant intermediates involved in such transformations. In particular, the use of ultrafast X-ray spectroscopy provides valuable insight not only into photophysical processes but also into ground state reactivity opening new avenues for exploration in bioinorganic chemistry.
Heterometallic systems, such as the FeMn cofactor in the ribonucleotide reductase-like enzyme R2lox, have garnered interest due to their unique chemical reactivity compared to their homo analogues. R2lox exhibits an unusually efficient light-induced decarboxylation—a rare photoactivity for natural enzymes outside of photosynthesis. This raises fundamental questions about the role of each metal site in the enzyme’s reactivity, specifically the identity of the metal involved in the metal-ligand charge transfer (MLCT) that initiates the photochemical process. These questions have been addressed through element-specific femtosecond X-ray absorption spectroscopy (fs-XAS) targeting both Fe and Mn centers in R2lox enzyme and relevant synthetic model complexes. We will present our fs XAS results on Fe-Mn and Fe-Fe enzymes and discuss the photoinduced process in the light of their comparison.

Speaker: Rebeca Gomez Castillo (Max Planck Institute CEC (Germany))

10:40 AM → 11:10 AM Coffee Break

11:10 AM → 12:40 PM Session 12 - Structural Dynamics II: Structural Dynamics II

11:10 AM Towards femtochemistry X-ray studies of catalysis on surfaces under operando

Catalytic processes on surfaces are ubiquitous and a profound understanding has become even more important in order to support the energy transition: virtually every process that can convert carbondioxide into fuels and even the production of alternative energy carriers, like hydrogen or ammonia, involves processes on surfaces. In order to rapidly tailor optimal catalysts that respect the new process conditions that are not based anymore on natural gas or crude oil, requires a complete understanding of the underlying catalyst configurations, reaction steps and energy landscapes.

X-ray studies at free-electron lasers have shown to allow to dissect reaction sequences and big steps have been made to study catalysts under operando conditions - often with surprising results. Now the time is ripe to combine both methodologies and an appropriate instrument is being developed. Furthermore, non-linear X-ray methods are being developed and hold great promise to allow for X-ray studies with even higher sensitivity to active surfaces. When this gets combined with the study of operating catalysts, we have a whole new toolbox at hand to understand catalytic reactions while and where bonds are being formed and broken.

Speaker: Martin Beye

11:40 AM Phonon Transport and Polaron Formation with Mode, Momentum and Time Resolution using Ultrafast Electron Diffuse Scattering (UEDS)

The nature of the couplings within and between lattice and charge degrees of freedom is central our understanding of material properties. These interactions are essential to phenomena as diverse as thermoelectricity, superconductivity, charge density waves, and carrier and phonon transport. Despite their fundamental role in a broad range of processes, detailed momentum-dependent information on the strength of electron-phonon coupling (EPC) and phonon-phonon coupling (PPC) across the entire Brillouin zone has proved to be very difficult to obtain.

This talk will describe an emerging pump-probe technique, ultrafast electron diffuse scattering (UEDS), that directly provides such information from the perspective of the phonon system by measuring the time-dependence of phonon-diffuse scattering from single-crystal samples [1,2]. Recent examples and proposals for the application of UEDS to a range of phonon-related phenomena in layered and monolayer materials will be discussed. Specifically, the direct connection that can be made between UEDS measurements and ab-initio computations for inelastic carrier scattering [3], phonon transport [4] and polaron formation [5] processes will be emphasized. We will show that UEDS can reveal signatures of chiral electron-phonon coupling [3] and phonon hydrodynamic transport, including second sound oscillations [4]. Such signatures are provided by the time, momentum, and branch resolved information on the nonequilibrium state-of-excitation of the phonon system that UEDS provides. We will also show that phonon diffuse scattering signatures of polarons in materials are directly emblematic of the underlying polaron wavefunction [5].
The combination of new time and momentum resolved experimental probes of nonequilibrium phonons with novel computational methods promises to complement the qualitative results obtained via model Hamiltonians with a first principles, material-specific quantitative understanding of polarons and their properties.

[1] M. J. Stern et al, Phys. Rev. B 97 (2018) 165416.; Waldecker, L. et al. Phys. Rev. Lett. 119, 036803 (2017). Chase, T. et al Appl. Phys. Lett. 108, 041909 (2016).
[2] T. Britt et al, NanoLett , 22 (2022) 4718.
[3] T. L Britt and B. J. Siwick, Phys. Rev. B 107 (2023) 214306.
[4] L. Kremeyer, J. Haibeh, B. J. Siwick and S. C. Huberman, Structural Dynamics, 11 (2024) 024101.
[5] T. L Britt, F. Caruso and B. J. Siwick, Computational Materials, 10 (2024) 178

Speaker: Prof. Bradley Siwick (Department of Physics and Department of Chemistry, Center for the Physics of Materials (CPM), McGill University, Montreal, Canada)

12:00 PM Applications of optical crystallography to ultrafast X-ray crystallography for structural dynamics.

Ultrafast X-ray crystallography has emerged as a powerful technique to resolve electron density dynamics on femtosecond time scale in molecular crystals, using X-ray Free Electron Lasers. However, a multitude of ultrafast phenomena, in particular optical non-linear processes, remain to be fully established for this application. There are multiple avenues to combine the techniques of optical crystallography, non-linear optical spectroscopy and ultrafast X-ray crystallography. These give rise to non-linear optical crystallography [1-5], coherent control methodology [6], and also the combination of coherence theory developed for Raman spectroscopy together with crystallographic data [7]. I will also discuss technical developments and predictions for future capabilities once we will have the availability of high repetition rate XFEL instruments.
1) van Thor JJ (2019) Coherent two-dimensional electronic and infrared crystallography, The Journal of Chemical Physics, Vol: 150, 124113
2) Bressan G, van Thor JJ, (2021) Theory of two-dimensional spectroscopy with intense laser fields, The Journal of Chemical Physics, Vol: 154, Pages: 1-10
3) Hutchison CDM, Fadini A, van Thor JJ (2022) Linear and Non-Linear Population Retrieval with Femtosecond Optical Pumping of Molecular Crystals for the Generalised Uniaxial and Biaxial Systems, APPLIED SCIENCES-BASEL, Vol: 12
4) van Thor JJ (2019) Advances and opportunities in ultrafast X-ray crystallography and ultrafast structural optical crystallography of nuclear and electronic protein dynamics Struct. Dyn. 6, 050901
5) Marius Kaucikas, Karim Maghlaoui, Jim Barber, Thomas Renger and Jasper J van Thor (2016) Ultrafast infrared observation of exciton equilibration from oriented single crystals of photosystem II. Nature Communications. 7, 13977
6) Christopher D.M. Hutchison, et al. Optical control of ultrafast structural dynamics in a fluorescent protein. (2023) Nature Chemistry. 15, 1607–1615
7) Samuel Perrett et al. Application of Density Matrix Wigner Transforms for Ultrafast Macromolecular and Chemical X-ray Crystallography. (2024) J. Chem. Phys. 160(10):100901

Speaker: Jasper van Thor (Imperial College London)

12:20 PM Probing Ultrafast Photoinduced Structural Dynamics in Molecular Solutions using Angular X-ray Cross-Correlation Analysis

Understanding photoinduced chemical reactions at the electronic, atomic and molecular scales and on relevant timescales is critical for controlling reaction pathways, rates and efficiency. This fundamental knowledge is essential for applications such as catalysis, photoswitching, and light-harvesting. X-ray free-electron lasers (XFELs) produce intense and ultrashort x-ray pulses, allowing to probe chemical processes experimentally with high spatial and temporal resolution. Here we apply femtosecond XFEL pulses to track ultrafast optically excited dynamics of a model photocatalyst Ir$_2$(dimen)$_4$]$^{2+}$ (dimen=1,8-diisocyano-p-menthane) in solution [1].

Time-resolved x-ray solution scattering (TRXSS) with an XFEL directly probes the time-dependent structure of a solution through the atomic pair distribution function (PDF). While this established approach simplifies the interpretation of experimental scattering data, the one-dimensional shape of the PDF in solutions limits the structural information that can be extracted. Here we employ an alternative approach to analyze TRXSS measurements based on the application of angular cross-correlation functions (CCFs) [1,2], which was originally proposed to facilitate biological structure determination from solution x-ray scattering [3-5].

We perform a model-assisted analysis of correlations in scattered x-rays, which allows us to elucidate various aspects of photoinduced changes in photoexcited molecular ensembles [1]. We unambiguously identify that in our experiment the photoinduced transition dipole moments in [Ir$_2$(dimen)$_4$]$^{2+}$ molecules are oriented perpendicular to the Ir–Ir bond. The analysis also shows that the ground state conformer of [Ir$_2$(dimen)$_4$]$^{2+}$ with a larger Ir–Ir distance is mostly responsible for the formation of the excited state. We also reveal that the ensemble of solute molecules can be characterized with a substantial structural heterogeneity due to solvent influence.

[1] R. P. Kurta et al., Phys. Chem. Chem. Phys. 25, 23417 (2023)
[2] P. Vester et al., Struct. Dynamics 6, 024301 (2019)
[3] Z. Kam, Macromolecules 10, 927 (1977)
[4] R. P. Kurta, M. Altarelli, I. A. Vartanyants, Adv. Chem. Phys. 161, Ch.1 (2016)
[5] R. P. Kurta, L. Wiegart, A. Fluerasu, A. Madsen, IUCrJ 16, 635 (2019)

Speaker: Ruslan Kurta (European XFEL)

12:40 PM → 2:10 PM Lunch Break

2:10 PM → 3:40 PM Session 13 - Materials I: Materials II

2:10 PM Ultrafast dynamics of colloidal plexcitonic nanohybrids studied by 2D electronic spectroscopy

The burgeoning field of polaritonic chemistry explores the interaction between molecules and confined electromagnetic field modes, enabling new chemical reactivities. Colloidal plexcitonic materials are particularly promising due to their easy and cost-effective preparation. Plexcitons are hybrid states resulting from the combination of plasmon resonances of metal nanostructures with molecular excitons. They enable the confinement of electromagnetic fields at the nanoscale and the establishment of strong couplings between light and matter, potentially leading to controllable and adjustable dynamic phenomena. However, the ultrafast coherent and incoherent dynamics of colloidal plexciton nanohybrids are not well understood. In this study, 2D electronic spectroscopy was used to investigate the ultrafast dynamics of these systems, focusing on identifying possible quantum coherent interactions after photoexcitation. By comparing the response of different nanohybrids and uncoupled components, the most relevant nonlinear photophysical processes underlying the femtosecond coherent and incoherent dynamics were identified, advancing our understanding and potential applications of these nanomaterials. Particularly noteworthy is the detection of clear signatures of 'vibronic plexcitons' dynamics, representing the first experimental observation of such excitations in colloidal systems, to the best of our knowledge.

Speaker: elisabetta collini (Università di Padova)

2:40 PM Probing Surface and Interface Carrier Dynamics via Ultrafast Scanning Electron Microscopy

Abstract
Conventional photon-pump/photon-probe time-resolved techniques face inherent limitations in resolving the intricate surface and interface carrier dynamics due to constraints in spatial resolution and penetration depth. To address these challenges, we utilize scanning ultrafast electron microscopy (SUEM), a state-of-the-art technique that integrates femtosecond temporal resolution with nanoscale spatial precision, enabling direct visualization of carrier transport and recombination at surfaces and interfaces in optoelectronic materials. In SUEM, an ultrashort laser pulse initiates excitation, while a time-delayed electron pulse probes transient modifications in electron density and spatial charge distribution with unparalleled surface sensitivity. This capability is particularly crucial for investigating surface-specific carrier dynamics, which often diverge from bulk behavior in thin films, nanostructures, and two-dimensional materials. By leveraging SUEM’s unique spatiotemporal resolution, this study advances our fundamental understanding of surface and interface charge transport mechanisms, shedding light on fundamental processes that govern the performance of electronic and photonic devices.

Speaker: Lijie Wang (KAUST)

3:00 PM Dynamic control of electron correlations in photodoped charge-transfer insulators

The electronic properties of correlated insulators are governed by the strength of Coulomb interactions, enabling the control of electronic conductivity with external stimuli. In this talk, I will highlight that the strength of electronic correlations in nickel oxide (NiO) can be coherently reduced by tuning the intensity of an optical pulse excitation above the charge-transfer gap. Remarkably, this weakening of correlations persists for hundreds of picoseconds and exhibits a recovery time independent of the photodoping density across two orders of magnitude. A broadening of the charge-transfer gap is also observed, consistent with dynamical screening. The high degree of control achieved over both the energy and temporal dynamics of electronic correlations offers a promising avenue to a full optical control of correlated systems and the Mott transition.

Speaker: Dr Thomas C. Rossi (Helmholtz Zentrum Berlin für Materialien und Energie)

3:20 PM Excitation Energy Transfer and Diffusion in Synthetic Light-Harvesting Nanoparticles

We investigate electronic excitation energy transport within 40-nm organic nanoparticles (ONPs) loaded with cationic rhodamine dyes (R18). Bulky counterions act as spacers which prevent dyes aggregation and quenching, up to dye concentrations as large a 300 mM - i.e. ~1 nm inter-dye distance - and similar to the effective dye concentration in natural, photosynthetic light-harvesting antennas. In this disordered, rigid solution of dyes, the inter-dye interaction remains relatively weak suggesting an incoherent energy hoping mechanism within the dyes.

We implement femtosecond fluorescence up-conversion spectroscopy to monitor the ultrafast fluorescence anisotropy decay of the ONPs dispersed in water solution. Hoping times as fast as 90 fs are obtained. Alternatively, we use a streak camera to monitor the nanoparticles fluorescence decay kinetics due to exciton-exciton annihilation (1) or to energy transfer to a few energy acceptors (traps) embedded within the ONPs. All three types of experiments provide indirect measurements of the exciton diffusion constant and consistently reveal a diffusion length of 80 to 120 nm - i.e. longer than the particles’ diameter. These results rationalize the remarkable “antenna effect” already reported for the nanoparticles, which are applied to bio-sensing and single molecule detection.(2)

(1) Gharbi, A. M.; Biswas, D. S.; Crégut, O.; Malý, P.; Didier, P.; Klymchenko, A.; Léonard, J. Exciton Annihilation and Diffusion Length in Disordered Multichromophoric Nanoparticles. Nanoscale 2024, 16 (24), 11550–11563. https://doi.org/10.1039/D4NR00325J.
(2) Trofymchuk, K.; Reisch, A.; Didier, P.; Fras, F.; Gilliot, P.; Mely, Y.; Klymchenko, A. S. Giant Light-Harvesting Nanoantenna for Single-Molecule Detection in Ambient Light. Nature Photonics 2017, 11 (10), 657–663. https://doi.org/10.1038/s41566-017-0001-7.

Speaker: Jérémie Léonard (Université de Strasbourg & CNRS, IPCMS, UMR 7504, F-67200 Strasbourg, France.)

3:40 PM → 4:10 PM Coffee Break

4:10 PM → 6:00 PM Session 14 - Structural Dynamics III: Structural Dynamics III

4:10 PM The combined electronic and nuclear structure molecular movie for a conical intersection

Many molecular light-energy conversion processes in nature occur on an ultrafast (sub-picosecond) timescale, such as retinal light harvesting, optical switching of green and yellow fluorescent proteins, and nucleobase photoprotection. The conversion process often occurs at conical intersections, which are regions in molecular phase space characterized by degenerate potential energy surfaces and a linear lifting of energy degeneracy.

To fully monitor and understand the ultrafast dynamics of a molecule, a combined knowledge of two realms—nuclear geometry and electronic structure—is required. Specifically, this involves understanding the molecular geometry changes that drive the molecule towards regions of strong coupling among electronic states, as well as the resulting changes in these states.

In this talk, the electronic and nuclear geometry perspectives will be combined in time-resolved studies on the thionated nucleobase 2-thiouracil (2-tUra). The molecule is planar in the ground state. After UV excitation to a ππ state, the molecule is driven via de-planarization towards a conical intersection with a state of dominant nπ electronic character.

We use time-resolved soft x-ray photoemission spectroscopy to gain insight into changes in the molecular electronic structure during internal conversion. The element- and site-selective nature of this method allows us to gather information about valence charge dynamics with angstrom precision in space and on a femtosecond timescale. We observe a charge shift in the UV excitation from the sulfur atom of 2-tUra towards the ring. In addition, coherences in the sulfur XPS exhibit the transfer of population among different electronic states, driven by a coherent nuclear mode around the conical intersection [1].

To investigate the light-induced changes in molecular geometry, we employ Coulomb-Explosion Imaging, a method that utilizes an intense and short x-ray pulse to highly ionize the molecules and subsequently resolve the momenta of the exploding molecular fragments. We use hydrogen atoms as messengers and discover the de-planarization of the molecular geometry on its path towards the conical intersection. The combination of both methods provides unprecedented experimental insight into the light-energy conversion dynamics in molecules [2].

(1) Mayer, D.; Lever, F.; Picconi, D.; Metje, J.; Alisauskas, S.; Calegari, F.; Düsterer, S.; Ehlert, C.; Feifel, R.; Niebuhr, M.; Manschwetus, B.; Kuhlmann, M.; Mazza, T.; Robinson, M. S.; Squibb, R. J.; Trabattoni, A.; Wallner, M.; Saalfrank, P.; Wolf, T. J. A.; Gühr, M. Following Excited-State Chemical Shifts in Molecular Ultrafast x-Ray Photoelectron Spectroscopy. Nat Commun 2022, 13 (1), 198. https://doi.org/10.1038/s41467-021-27908-y.

(2) Jahnke, T.; Mai, S.; Bhattacharyya, S.; Chen, K.; Boll, R.; Castellani, M. E.; Dold, S.; Frühling, U.; Green, A. E.; Ilchen, M.; Ingle, R.; Kastirke, G.; Lam, H. V. S.; Lever, F.; Mayer, D.; Mazza, T.; Mullins, T.; Ovcharenko, Y.; Senfftleben, B.; Trinter, F.; Atia-Tul-Noor; Usenko, S.; Venkatachalam, A. S.; Rudenko, A.; Rolles, D.; Meyer, M.; Ibrahim, H.; Gühr, M. Direct Observation of Ultrafast Symmetry Reduction during Internal Conversion of 2-Thiouracil Using Coulomb Explosion Imaging. Nat Commun 2025, 16 (1), 2074. https://doi.org/10.1038/s41467-025-57083-3.

Speaker: Markus Guhr (DESY)

4:40 PM Chemical dynamics of microsolvated (bio)molecules

Observing molecules in action through the recording of “molecular movies”, i.e., their spatiotemporal evolution during chemical dynamics, with atomic spatial and temporal resolution promises to revolutionize our understanding of the molecular sciences and to provide a time-dependent basis of chemistry. However, most real-world chemistry occurs at or near room temperature, yet the ultrafast dynamics of corresponding elementary chemical processes at this energy scale are largely unexplored. We aim to change this [1].

Experimentally, we build upon our approaches to prepare highly controlled samples that enable advanced imaging methods of individual molecular species and directly in the molecular frame. We prepare highly-controlled molecular samples for advanced ultrafast imaging experiments. This includes the preparation of ensembles of individual molecular species, e.g. single microsolvation environments, single conformers, or even single quantum states. Furthermore, the generated very cold samples are ideally suited to fix the molecules in space in laser-alignment or mixed-field orientation approaches.

I will discuss how we can utilize these highly controlled, ultracold samples to investigate "room-temperature" chemical dynamics. I will present first experimental results and discuss both the chemical information obtained as well as the challenges ahead for disentangling ultrafast elementary steps of general-chemistry in general.

[1] M. S. Robinson and J. Küpper, Unraveling the ultrafast dynamics of thermal-energy chemical reactions, Phys. Chem. Chem. Phys. 26, 1587 (2024).

Speaker: Prof. Jochen Küpper (Deutsches Elektronen-Synchrotron DESY)

5:00 PM Unveiling the wavelength dependent ultrafast relaxation of solvated thymidine with extreme ultraviolet time-resolved photoelectron spectroscopy and simulations

The nucleobases photo-protection mechanism is at the heart of our genetic code stability: the electronic states produced by UV light absorption are rapidly converted to heat by internal conversion and safely dissipated to the environment before reactive pathways can occur. The precise understanding of photo-deactivation in these molecules and the involvement of potentially harmful trapping states thereof is still highly debated. To tackle this challenging problem, we performed extreme ultraviolet time-resolved photo-electron spectroscopy (XUV-TRPES) experiments and simulations of water solvated thymidine, unraveling its ultrafast relaxation. The large ionization spectral coverage of the employed TRPES apparatus[1] (with a probe pulse central frequency of about 30 eV) coupled to its high time-resolution (of about 20 fs) enabled to record the decay of excited states thymidine as well as the appearance of ground-state recovery signals in a single experiment. The involvement of the elusive $^1n\pi^*$ state, which acts as a bridging state between the initially excited $^1\pi\pi^*$ state and the ground-state is proven, while a significant role of such state as a population trap is excluded. This result is shown to be dependent upon the excitation wavelength (266 nm in the present study), which allows us to construct a comprehensive framework that reconciles seemingly contradictory spectroscopic measurements in the literature.[2-5] Support of theory (surface hopping QM/MM dynamics and on-the-fly XUV-TRPES spectroscopy simulation at the ab-initio CASSCF/CASPT2 level of theory) is key to decode and interpret the recorded spectra, and to shed new light on the nucleobases photo-relaxation mechanism.

[1] Phys. Rev. Lett., 128, 133001 (2022)
[2] Proc. Natl. Acad. Sci. USA, 104, 2, 435–440 (2007)
[3] Nat. Commun, 12, 1, 7285 (2021)
[4] J. Am. Chem. Soc., 137, 2931–2938 (2015)
[5] J. Am. Chem. Soc., 145, 3369–3381 (2023)

Speaker: Dr Mario Taddei (University of Bologna)

5:20 PM Solvation Shells and Simulation Cells: Advances in modeling X-ray Solution Scattering for Time-Resolved Studies

The role of the solvent in ultrafast processes has become a focus of interest for many researchers within femtochemistry and beyond. To interpret emerging time-resolved x-ray scattering experiments, we therefore need forward models that accurately predict scattering across the full range of momentum transfer, q. This talk will present new methods to correct finite simulation cell errors in scattering signals calculated from molecular dynamics simulations[1], particularly at low values of q, improving the accuracy of forward models for the solvation structure of solvated systems. Our renormalization scheme, based on excluded volume corrections to radial distribution functions, recovers the correct q = 0 limit. The work also provides practical guidance for integrating MD simulations into experimental structural studies.
To accurately predict the coupling between solute and solvent, we present an analysis of the nature of the interactions governing solvent structure, to guide future strategies for accurate simulations[2]. Lastly, for molecules with significant conformational flexibility, we show how to employ robust conformational sampling techniques to assist the structural modeling of picosecond-timescale excited-state conformational dynamics of an all-organic covalent dimer, which was very recently recorded at the European XFEL.

[1]:Dohn, A. O.; Markmann, V.; Nimmrich, A.; Haldrup, K.; Møller, K. B.; Nielsen, M. M. Eliminating Finite-Size Effects on the Calculation of x-Ray Scattering from Molecular Dynamics Simulations. The Journal of Chemical Physics, 2023, 159. https://doi.org/10.1063/5.0164365.

[2] Zulfikri, H.; Pápai, M.; Dohn, A. O.; Simulating the solvation structure of low- and high-spin [Fe(bpy)3]2+: long-range dispersion and many-body effects. Physical Chemistry, Chemical Physics, 2022, 14.
https://doi.org/10.1039/d2cp00892k

Speaker: Asmus Ougaard Dohn (DTU Physics)

5:40 PM Electron transfer-induced misfolding of prion proteins studied by ultrafast X-ray absorption

The cellular prion protein (PrPC) is a membrane-bound glycoprotein found in the central nervous system of all mammal species and has emerged as an important copper binding protein. It is well established that the redox behaviour of copper bound to PrPC, plays an important role in both the physiological function of the protein, and in the pathogenesis of neurodegenerative diseases known as transmissible spongiform encephalopathies or prion diseases. They are accompanied by an accumulation of a misfolded, not soluble isoform (PrPSC) of the endogenous prion protein (PrPC) [1]. NMR studies on recombinant human PrPC demonstrate that the C-terminal region, which adopts a globular fold that is largely helical but with a small two-strand β-sheet [2], and the N-terminal region, which is unstructured and flexible in solution [3].
A hallmark of this region is the so-called octarepeat (OR) domain. Human PrPC has four OR repeats, which an interact with divalent ions such as Zn(II), Mn(II), Fe(II), although the affinity for Cu(II) is highest [4]. There is evidence that the OR domain is able to reduce Cu(II) via an electron transfer (ET) from a tryptophan (Trp) residue [4]. Density functional theory and molecular dynamics simulations demonstrated that the resulting Cu(I) [5] and reactive oxygen species [6] can lead to formation of the precursor of the pathogenic PrPC structures [7]. Despite the high relevance of the redox behaviour of Cu bound to PrPC, the exact mechanism of misfolding remains unclear.
Up-to-date, no direct measurement of a Trp* to Cu(II) ET and the structural changes accompanying it has been reported. Our approach here is to mimick the biological activity by inducing an ET via a photon within the OR-Cu2+ complex in aqueous solution.
We phototrigger the Cu2+ to Cu1+ reduction by electronically exciting Trp residue (280 nm), present in the N-terminal region of the protein. Preliminary results obtained using femtosecond-resolved X-ray absorption spectroscopy (fs-XAS) technique at the X-ray free electron lasers (XFELs) show unequivocally the ET taking place instead of energy transfer. We have used this technique, in combination with X-ray scattering, to gain deeper understanding of ET processes involving the Cu2+ active site and the subsequent structural changes of the nearby PrPC environment upon photoreduction.

Speaker: Wojciech Gawelda (Universidad Autonoma de Madrid / IMDEA Nanoscience)

6:00 PM → 7:30 PM Poster Session 3

Fri, June 27

8:00 AM → 9:00 AM Registration

9:00 AM → 10:30 AM Session 15 - Materials II: Materials II

9:00 AM Singlet fission contributes to solar energy harvesting in photosynthesis

Photosynthesis, the foundation of most life, begins when sunlight is captured by (bacterio)chlorophyll (BChl) and carotenoid (Crt) pigments. These molecules are arranged so that captured energy migrates rapidly to reaction centres (RC), where it is stored as a charge separation. The complementary absorption of Crt and BChl pigments, and rapid energy transfer between them, underpins solar harvesting. Here we report a Crt-to-BChl energy transfer mechanism mediated by singlet fission (SF), in which a high-energy singlet exciton (with spin quantum number S=0) is converted into two low-energy triplet (S=1) excitons. In purple photosynthetic bacteria, the Crt S2 singlet exciton splits into Crt and BChl triplet excitons on adjacent sites. Once formed, the triplets transfer cooperatively to BChl, and onward to RCs. Energy is transferred from a singlet Crt state, via the spin-protected long-lived triplet pair, to a singlet BChl state. Thus, this novel SF-mediated mechanism augments solar energy harvesting for photosynthesis.

Speaker: Jenny Clark (University of Sheffield)

9:30 AM Deciphering the ultrafast photobehavior of benzothiadiazole-based HOFs and its molecular units: experimental and theoretical insights into their spectroscopic properties in solution and in the solid state

Benzothiadiazole (BTD) derivatives have emerged as versatile building blocks for the fabrication of smart porous materials. Herein, we report on experimental and theoretical studies of two BTD-based H-bonded organic frameworks (HOFs) and their molecular units (MUs) in solutions and in solid state (BTIA and BTTA).1-3 The MUs in solutions exhibit a photoinduced intramolecular charge-transfer (ICT) reaction in the excited species followed by a twisting motion of phenyl moieties. Femtosecond (fs) experiments reveal that the ICT event takes place in ∼300 fs while the phenyl torsion occurs in ∼6 ps and ~10 ps for BTIA and BTTA, respectively. Ester-derivatives show a single emission lifetime of ~7 ns, while the acidvderivatives display lifetimes of ~ 400 ps and ~1 and ~7 ns assigned to different emitting species. Theoretical calculations on the ester compounds agree with the experimental observations. In solid state, the ester derivatives show an abnormally slow ICT event (80 ps) leading to ICT aggregates with lifetimes of ~1 and ~3 ns, whereas single crystal of BTD-HOFs under the microscope exhibit a fast ICT and intermolecular proton-transfer (PT) reactions (<15 ps), producing ICT and ionic species with lifetimes of ~500 ps and ~1 ns, respectively. These results provide new findings for a better understanding of the photobehavior of BTD related HOF materials and will help in the development of new HOFs for photonic applications.
Acknowledgement: This research was supported by PID2020-116519RB-I00 funded by MICIU/AEI/10.13039/501100011033 and by the “European Union, EU”; SBPLY/23/180225/000196 funded by JCCM and the EU through “Fondo Europeo de Desarollo Regional” (FEDER), and 2022-GRIN-34325 funded by UCLM (FEDER). MH thanks MCIN for the FPI fellowship PRE2021-099064 financed by MCIN/AEI/10.13059/501100011033.
References:
[1] Z. Yang, A. Moriyama, R. Oketani, T. Nakamura, I. Hisaki. Chem. Lett. 2021, 50, 1909-1912.
[2] Z. Yang, A. Saeki, A. Inoue, R. Oketani, K. Kamiya, S. Nakanishi, T. Nakamura, I. Hisaki. Cryst. Growth Des. 2022, 22, 4472−4479.
[3] M. de la Hoz Tomás, J. A. Organero, M. R. di Nunzio, T. Hashimoto, I. Hisaki, A. Douhal. J. Mater. Chem. C, 2024, 12, 9112-9129.

Speaker: Abderrazzak Douhal (Universidad de Castilla la Mancha)

9:50 AM RF-compressed, THz-streaked ultrafast electron diffraction at high repetition rates with direct detection

Ultrafast electron diffraction (UED) [1-3] is a powerful tool for tracking the nuclear dynamics of photo-induced gas-phase reactions in real-time with picometre spatial and 150-240-fs temporal resolution [4,5]. However, time-resolving rapidly evolving photo-induced processes, such as photoisomerization [6] in vision (<350-fs predicted total timescale), remains challenging due to space-charge-induced pulse broadening in high-charge electron pulses required for existing low-repetition-rate (≤1-kHz) gas-phase UED setups.

We present a <100-keV UED instrument operating at high repetition rates (30-100+ kHz) that utilizes direct electron detection. Initial results demonstrate the capability to measure time-resolved electron scattering signals with single-electron, temporally uncompressed pulses at 30 kHz [7]. This was possible by using significantly lower bunch charges but at high repetition rate, while also measuring the primary unscattered electron beam which is used to normalise all scattering data, leading to a 10-100 factor improvement in signal-to-noise ratio of detected signals [7]. In a temporally uncompressed mode, we were able to investigate electron-phonon dynamics in aluminium using 10^2 electrons/pulse with sub-400-fs resolution [7].

Recent upgrades to our instrumentation and laser system (CARBIDE, 80-W, 2-mJ) have enabled THz electron streaking at 40-100 kHz, allowing precise temporal characterization of electron pulse compression using a radiofrequency (RF) microwave cavity [8]. We demonstrate 30-fold compression of an electron pulse containing >10^4 electrons, reducing a ~3,000 fs pulse to ~100 fs, making our setup one of the brightest in Europe. Moreover, combining this compression capability with high repetition rate operation (up to 2-MHz) and direct detection makes our set-up one of the highest electron flux UED setups in the world. This combination uniquely positions it for high-sensitivity gas-phase UED measurements while retaining the flexibility to study solid-state thin films.

[1] K. Amini and J. Biegert, Adv. At. Mol. Opt. Phys., Chapter 3., 2020
[2] M. Centurion et al., Annu. Rev. Phys. Chem. 73, 21 (2022).
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Speaker: Kasra Amini (Max-Born-Institut)

10:10 AM Linear and Ultrafast Optical Diffusion-Ordered SpectroscopY sheds new light on nanoparticles, amyloids, and mixed solutions

Linear and ultrafast optical techniques (including Raman, Infrared and Uv/Vis spectroscopy) are excellent tools to investigate molecular structure and dynamics in solution. However, these spectroscopic methods are generally not sensitive to the size of molecules. Inspired by concepts from NMR, we have developed a spectroscopic method that adds a size dimension to optical spectra: optical Diffusion-Ordered Spectroscopy (DOSY). In an optical DOSY spectrum, the(2D)Infrared (1) or Raman (2) or Uv/Vis (3) spectrum is spread out along an additional axis showing the diffusion coefficient (or equivalently, size). The combined sensitivity to spectroscopic signature and diffusion coefficient is achieved by generating a concentration gradient inside a specially designed sample cell and monitoring its equilibration in a spectrally and time-resolved manner. The resulting multi-dimensional DOSY spectrum has Raman or IR frequency or Uv/Vis wavelength on the first axis/axes, and the diffusion coefficient (or size) on the other axis. We are currently exploring a broad range of applications, ranging from investigating ultrasmall nanoparticles and supramolecular complexes to studying amyloids and contaminants in protein solutions.

We believe that optical DOSY will become a valuable tool for investigating structure and dynamics of molecules, aggregates and particles in solution, by providing the size (or size distribution) and unraveling the (multidimensional) optical spectra of mixed solutions.

Speaker: Giulia Giubertoni (University of Amsterdam)

10:30 AM → 11:00 AM Coffee Break

11:00 AM → 12:20 PM Session 16 - Materials III: Materials III

11:00 AM Field-driven virtual charge dynamics in dielectrics

The interaction of intense, few-femtosecond infrared (IR) pulses with solids can induce light-field-driven phenomena, reversibly modifying their electro-optical properties on attosecond time scales. This opens new avenues for ultrafast optoelectronics and petahertz device applications. However, harnessing these coherent light–matter states requires a deeper understanding of the fundamental processes governing strong-field interactions in solids. In particular, the interplay between real and virtual carrier dynamics remains largely unexplored, despite its potential to constrain material response times, especially in wide-bandgap dielectrics.

We employ attosecond transient reflection spectroscopy to probe ultrafast virtual electron dynamics in monocrystalline diamond across a previously unexplored photon energy range (20-50 eV). Absolute pump-probe delay calibration enables a direct, parameter-free comparison with time-dependent density functional theory (TDDFT) simulations, revealing that virtual inter-band transitions (VITs) — often neglected in favour of intra-band motion — play a crucial role in shaping both the timing and adiabaticity of the ultrafast nonlinear response. Since VITs emerge instantaneously as the light field dresses the material, they influence the available bandwidth and response time well before real vertical transitions occur. Further analysis using an independent-particle approximation and a simplified three-band model elucidates the origin and impact of VITs, paving the way for their exploitation in next-generation optoelectronic devices and enabling precise control of material properties on sub-femtosecond time scales.

Speaker: Matteo Lucchini (Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci, 20133 Milano, Italy)

11:30 AM Ultrafast Quantum Optics for Femtochemistry and Biological Applications

Advancements in quantum optics and squeezed light generation have transformed various domains of quantum science and technology. However, real-time quantum dynamics remain an underexplored frontier. Here, we extend quantum optics into the ultrafast regime, providing direct experimental evidence that quantum uncertainty is not a static constraint but evolves dynamically with the system’s state and interactions. Using ultrafast squeezed light generated via a four-wave mixing nonlinear process, we observe the temporal dynamics of amplitude uncertainty, demonstrating that quantum uncertainty is a controllable and tunable physical quantity. This offers new insights into fundamental quantum mechanics in real-time. Additionally, we demonstrate control over the quantum state of light by switching between amplitude and phase squeezing. Our ability to generate and manipulate ultrafast squeezed light waveforms with attosecond resolution unlocks exciting possibilities for quantum technologies, including petahertz-scale secure quantum communication, quantum computing, and ultrafast spectroscopy. We also introduce an ultrafast quantum encryption protocol leveraging squeezed light for secure digital communication at unprecedented speeds. This work paves the way for exploring quantum uncertainty dynamics and establishes the foundation for the emerging field of ultrafast quantum science.

Speaker: Mohammed Hassan (University of Arizona)

12:00 PM Resonantly enhanced X-ray impulsive vibrational spectroscopy in trigonal tellurium

Laser control strategies enable perturbing selected degrees of freedom and driving matter in metastable states often inaccessible under equilibrium conditions. In the optical regime this is achieved through either nonlinear phononics or impulsive vibrational spectroscopy (IVS), in which coherent phonons are launched upon ultrashort laser excitation. The impulsive vibration of the lattice causes macroscopic changes in the system’s refraction index that can be detected using a second probe laser in a pump-probe fashion. Extending this scheme to the X-ray domain promises achieving element-selectivity in the generation process by tuning the pump pulses on-resonance with core electronic excitations.
In this contribution, we present our recent implementation of X-ray impulsive vibrational spectroscopy (XIVS) in trigonal tellurium performed at the FERMI free electron laser (FEL) in Trieste. By tuning the FEL pump photon energy either off- or on-resonant with the tellurium N4,5-edge, we demonstrate a resonant enhancement of the coherent phonon amplitude by a factor of eight that is rationalized in terms displacive excitation of coherent phonons (DECP) mechanism. Ab initio simulations including ultrafast electron and coherent phonon dynamics in presence of electron-phonon interactions show an excellent agreement with our experiment, underscoring the key contribution of band-specific electron phonon coupling (EPC) terms.
These results demonstrate the possibility of harnessing EPC to enhance the light-induced activation of specific phonons, paving the way for element-selective coherent control strategies in functional materials.

Speaker: Dr Oliviero Cannelli (Center for Free-Electron Laser Science, DESY, Notkestraße 85, 22607 Hamburg, Germany)

12:20 PM → 12:30 PM Closing Remarks