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.

Figures (5)

• Figure 1: Scheme of pulse sequences and their respective time delays for three-beam pump-push-probe (a) and five-beam pump-2D (b) configurations. The pump pulse is shown in blue and the push and probe pulses are shown in red.
• Figure 2: (a) SE contribution to the integral PPP signal of Hz$\cdots$H2O as a function of the push-probe delay time T2 and the probe carrier frequency $\hbar\omega_{pr}$. (b) SE contribution to the integral PP signal of Hz$\cdots$H2O as a function of the pump-probe delay time T and the probe carrier frequency $\hbar\omega_{pr}$. The pulse durations of pump, push and probe pulses are 5 fs, 1 fs and 5 fs, respectively. The pump pulse is in resonance with the maximum of the UV absorption spectrum ($\hbar\omega_{pu}$ = 4.29 eV). The push pulse is in resonance with the maximum of the excited-state absorption spectrum ($\hbar\omega_{push}$ = 2.8 eV). The signal intensities are given relative to the maximum of the SE component of the TA PP signal. The data for (b) are taken from Ref. pios_hz_h2o_pumpprobe.
• Figure 3: (a) ESA contribution to the integral PPP signal of Hz$\cdots$H2O as a function of the push-probe delay time T2 and the probe carrier frequency $\hbar\omega_{pr}$. (b) ESA contribution to the integral PP signal of Hz$\cdots$H2O as a function of the pump-probe delay time T and the probe carrier frequency $\hbar\omega_{pr}$. The pulse durations of pump, push and probe pulses are 5 fs, 1 fs and 5 fs, respectively. The pump pulse is in resonance with the maximum of the UV absorption spectrum ($\hbar\omega_{pu}$ = 4.29 eV). The push pulse is in resonance with the maximum of the excited-state absorption spectrum ($\hbar\omega_{push}$ = 2.8 eV). The (negative) signal intensities are given relative to the maximum of the ESA component of the TA PP signal. The data for (b) are taken from Ref. pios_hz_h2o_pumpprobe.
• Figure 4: (a-d) SE contribution to the rephasing P-2D signal of Hz$\cdots$H2O as a function of the excitation and detection frequencies at waiting times T2 = 20, 40, 60, 80 fs. (e-h) SE contribution to the rephasing 2D signal of Hz$\cdots$H2O as a function of excitation and detection frequencies at waiting times T = 20, 40, 60, 80 fs. The pulse durations of excitation and detection pulses are 0.1 fs. The signals for each waiting time have been rescaled to the same maximum intensity for better visibility.
• Figure 5: (a-d) ESA contribution to the rephasing P-2D signal of Hz$\cdots$H2O as a function of the excitation and detection frequencies at waiting times T2 = 20, 40, 60, 80 fs. (e-h) ESA contribution to the rephasing 2D signal of Hz$\cdots$H2O as a function of excitation and detection frequencies at waiting times T = 20, 40, 60, 80 fs. The pulse durations of excitation and detection pulses are 0.1 fs. The signals for each waiting time have been rescaled to the same maximum intensity for better visibility.