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

TL;DR

This paper extends a quasi-classical methodology for simulating multi-pulse time-resolved spectroscopy, enabling efficient ab initio simulations of complex experiments like PPP and P-2D, demonstrated on a heptazine-water complex.

Contribution

It generalizes the doorway-window approach to multi-pulse spectroscopies, allowing efficient ab initio simulations of complex ultrafast spectroscopic experiments.

Findings

Simulations reveal richer information from pump-stimulated experiments.

The methodology is computationally efficient and suitable for on-the-fly electronic-structure calculations.

Application to heptazine-H2O complex demonstrates enhanced insight into ultrafast dynamics.

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.