Department of Chemistry, UR MOLSYS, University of Liège, Belgium
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Upon light absorption, the 5-atom thioformaldehyde S-oxide sulfine (H2CSO) molecule undergoes a rich and unexpected dynamics resulting in the formation of nine photoproducts(1). In this presentation, we will first shed light on the sulfine photochemistry using state of the art molecular modeling and then investigate its photoexcitation by attosecond and few-femtosecond laser pulses using the XFAIMS mixed-quantum classical method that we recently developed.
We shed light on the sulfine photochemistry by reproducing in silico the dynamics following the photoexcitation of the sulfine to its lowest excited state(2). The nonradiative decay to the ground state as well as the following dynamics occurring on the ground state have been modeled with respectively the ab initio multiple spawning(3) (AIMS) method carried out at the MS-CASPT2 level and the Born-Oppenheimer molecular dynamics carried out at the DFT/PBE0 level. Amongst the nine photoproducts observed experimentally, we retrieve them all but surprisingly most of them (8) are formed on the ground state on a sub-picosecond timescale. Therefore the dynamics occurring on the hot ground state cannot be described by statistical methods such as RRKM because of the highly non-statistical character stemming from the excited-state dynamics. This unexpectedly rich chemistry occurring on the hot ground state challenges our view of typical photochemical or photodecomposition reactions in which it is usually the variety of conical intersections that leads to a variety of photoproducts.
We also investigated the photoexcitation the sulfine by attosecond and few-femtosecond laser pulses. With the recent developments of these short pulses (4), new opportunities for the controlling and probing of the electronic motion in atoms and molecules emerge. Due to the short nature of the laser pulse, the bandwidth can reach several eV and a band of several electronic states can be accessed, which can lead to an ultrafast charge migration. Therefore it becomes paramount to model the interaction with laser pulses. In this goal, we developed the eXternal Field Ab-Initio Multiple Spawning (5) (XFAIMS) method to model both the photoexcitation and nonradiative relaxation in medium-sized molecules. It is based on the well-known AIMS method in which we added the interaction with the electric field and adapted the spawning algorithm to fully model experiments from the photoexcitation of the molecule by the laser pulse to the nonradiative relaxation and/or its dissociation.