Ab Initio Simulation of Femtosecond Time-Resolved Multipulse Spectroscopies Applied to the Heptazine···H₂O Complex

In multidimensional 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 [Gelin, M. F.; J. Chem. Theory Comput. 2021, 17, 2394] has been generalized to multipulse spectroscopies. Pump-push-probe (PPP) spectroscopy (involving three laser pulses) and pump-stimulated 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···H₂O complex illustrate that pump-stimulated experiments provide rich information on the ultrafast radiationless electronic relaxation dynamics of the excited electronic states of the heptazine···H₂O complex which complements the information obtained by conventional PP or 2D experiments.