Ab Initio Simulation of Femtosecond Time-Resolved Multi-Pulse Spectroscopies applied to the Heptazine$\cdots$H$_2$O Complex

Abstract

In multi-dimensional time-resolved spectroscopic experiments, multiple (more than two) short laser pulses with variable pulse delay times are employed for the time-resolved exploration of the photoinduced dynamics of molecular chromophores. In the present work, the quasi-classical doorway-window (DW) methodology recently developed for transient absorption pump-probe (PP) spectroscopy [M. F. Gelin et al., J. Chem. Theory Comput. 2021, 17, 2394] has been generalized to multi-pulse spectroscopies. Pump-push-probe (PPP) spectroscopy (involving three laser pulses) and pump-induced two-dimensional (P-2D) spectroscopy (involving five laser pulses) are considered as specific examples. The quasi-classical DW approximation results in conceptually simple and computationally efficient simulation protocols which are suitable for implementation with $ab$ $initio$ on-the-fly electronic-structure calculations. Simulations of PPP and P-2D spectra performed for the hydrogen-bonded heptazine$\cdots$H$_2$O complex illustrate that pump-stimulated experiments provide much richer information on the ultrafast radiationless relaxation dynamics of the excited electronic states of the heptazine$\cdots$H$_2$O complex than conventional PP and 2D experiments.