Speaker
Description
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.
