Revealing electronic state-switching at conical intersections in alkyl iodides by ultrafast XUV transient absorption spectroscopy

Abstract

Conical intersections between electronic states often dictate the chemistry of photoexcited molecules. Recently developed sources of ultrashort extreme ultraviolet (XUV) pulses tuned to element-specific transitions in molecules allow for the unambiguous detection of electronic state-switching at a conical intersection. Here, the fragmentation of photoexcited iso-propyl iodide and tert-butyl iodide molecules (i-C${}_3$H${}_7$I and t-C${}_4$H${}_9$I) through a conical intersection between ${}^3$Q${}_0$/${}^1$Q${}_1$ spin-orbit states is revealed by ultrafast XUV transient absorption measuring iodine 4d core-to-valence transitions. The electronic state-sensitivity of the technique allows for a complete mapping of molecular dissociation from photoexcitation to photoproducts. In both molecules, the sub-100 fs transfer of a photoexcited wave packet from the ${}^3$Q${}_0$ state into the ${}^1$Q${}_1$ state at the conical intersection is captured. The results show how differences in the electronic state-switching of the wave packet in i-C${}_3$H${}_7$I and t-C${}_4$H${}_9$I directly lead to differences in the photoproduct branching ratio of the two systems.