On-the-fly simulations of transient absorption pump–probe spectra: Combining mapping dynamics with doorway-window protocol

We have constructed an ab initio protocol for the simulation of transient-absorption (TA) pump-probe (PP) signals of realistic polyatomic systems. The protocol is based on interfacing the doorway-window representation of spectroscopic signals with the on-the-fly mapping Hamiltonian dynamics approach at the symmetrical quasi-classical/Meyer-Miller level. The methodology is applied to the simulation of TA PP signals of two molecular systems, azobenzene and cis-hepta-3,5,7-trieniminium cation. For both molecules, the TA PP spectra were demonstrated to give a direct fingerprint of the excited state wavepacket dynamics and internal conversion, which permits the monitoring of the isomerization pathways en route to the final photoproducts.