Accounting for Electronic Coherences Induced by Broadband Pulses by Using Pulse-Independent Trajectories

Recent advances in the generation of ultrashort, few-femtosecond laser pulses in the ultraviolet-visible domain are now enabling the coherent excitation of several electronic states in neutral molecules, with new opportunities for the manipulation of molecular dynamics on ultrafast time scales. Current time-resolved pump-probe experiments can monitor the ensuing coupled electron-nuclear dynamics with ultrashort resolution. Computational modeling of the observables measured in such experiments can be very challenging for medium-sized and large molecules because of (i) the nontrivial treatment of pump-generated coherences with mixed quantum-classical methods and (ii) the high computational cost of probe-step calculations, which cannot be afforded when many different pump pulses have to be considered, as e.g., in control schemes. In this work, we present two trajectory-surface-hopping approaches that include, a posteriori, the effect of the pump-generated coherences on the ensuing coupled electron-nuclear dynamics, thus avoiding performing a different coupled electron-nuclear dynamics calculation for every individual pump pulse. The effectiveness of both approaches is exemplified in glycine molecules excited by short ultraviolet pump pulses. We compare the results of both approaches with those obtained by including pump-generated coherences from the very beginning, showing an excellent agreement and confirming the important role of such initial coherences in the early nonadiabatic dynamics. Our results pave the way for both accurate and flexible simulations of pump-probe experiments or control studies in molecules excited by broadband laser sources.