Speaker
Description
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)
