Description
We report on theoretical calculations aiming at interpreting a recent experiment at the Linac Coherent Light Source (LCLS) free-electron laser (FEL), in which time-resolved x-ray photoelectron and absorption spectroscopy (TRXPS and TRXAS) were employed to monitor the photoinduced isomerization of gas-phase acetonitrile molecules following ionization by an intense infrared (IR) pulse. TRXPS/TRXAS provide information on the structural reorganization of the molecule by accessing the energies associated with the ionization/excitation of a core electron. X-ray pulses resonant to different core states can then allow tracking these dynamics with additional spatial selectivity. Using ab-initio molecular dynamics (AIMD) simulations, we first identify the most important isomers emerging during the molecular motion and rearrangement following IR ionization on a femtosecond timescale. For these representative geometries, we perform CASSCF/CASPT2 electronic-structure calculations to predict the associated core-hole ionization and transition energies which are expected to emerge in the TRXPS and TRXAS signals. Our calculations allow us to determine the spectral signatures associated with the different geometries involved in acetonitrile's isomerization dynamics.
