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

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Multidimensional time-resolved spectroscopy uses multiple laser pulses to study molecular dynamics.
• Transient absorption pump-probe (PP) spectroscopy is a key technique for this.

Purpose of the Study:

• To generalize the quasi-classical doorway-window (DW) methodology for multipulse spectroscopic techniques.
• To develop efficient simulation protocols for pump-push-probe (PPP) and pump-stimulated 2D (P-2D) spectroscopy.

Main Methods:

• Generalization of the quasi-classical doorway-window (DW) methodology.
• Application to pump-push-probe (PPP) and pump-stimulated 2D (P-2D) spectroscopy.
• Integration with *ab initio* on-the-fly electronic-structure calculations.

Main Results:

• The generalized DW approximation provides computationally efficient and conceptually simple simulation protocols.
• Simulations of PPP and P-2D spectra for the heptazine···H2O complex were performed.
• These methods effectively capture ultrafast radiationless electronic relaxation dynamics.

Conclusions:

• The generalized DW methodology is suitable for *ab initio* on-the-fly calculations in multipulse spectroscopy.
• Pump-stimulated experiments provide complementary information to conventional methods.
• This approach enhances the understanding of excited-state dynamics in molecular systems.