Ultrafast Internal Conversion Dynamics through the on-the-Fly Simulation of Transient Absorption Pump–Probe Spectra with Different Electronic Structure Methods

Abstract

An on-the-fly surface-hopping simulation protocol is developed for the evaluation of transient absorption (TA) pump−probe (PP) signals of molecular systems exhibiting internal conversion to the electronic ground state. We study the nonadiabatic dynamics of azomethane and the associating TA PP spectra at three levels of the electronic-structure theory, OM2/MRCI, SA-CASSCF, and XMS-CASPT2. The impact of these methods on the population dynamics and time-resolved TA PP signals is substantially different. This difference is attributed to the strong non-Condon effects that must be taken into account for the proper understanding and interpretation of time-resolved TA PP signals of nonadiabatic polyatomic systems. This shows that the combination of the dynamical and spectral simulations definitely provides more accurate and detailed information on the microscopic mechanisms of photophysical and photochemical processes. Hence the simulation of time-resolved spectroscopic signals provides another important dimension to examine the accuracy of quantum chemistry methods.

Automatized protocol and interface to simulate QM/MM time‐resolved transient absorption at TD‐DFT level with COBRAMM

“…The time-resolved TA spectrum can be computed from an ensemble of trajectories independently propagated that sample the evolution of the wavepacket along the potential energy surfaces. 9,12,13,31 Considering a single trajectory evolving on the PES of an active state a, at each time step the TA spectrum can be obtained by calculating vertical excitations from state a. The SE is computed considering the oscillator strength (f) with respect to the ground state, while the ESAs are evaluated considering f of transitions between state a and the higher-laying excited states calculated.…”

“…8 Many approaches have been reported to simulate such signals with varying compromises between accuracy and cost, documenting an immense interest of the computational photochemistry community to these kinds of simulations and the importance of developing tools in this field. [9][10][11][12][13][14][15][16][17][18][19] The most rigorous and accurate way to compute TA signal is based on quantum dynamics (QD) simulations, which allow to explicitly consider the electric field of the pulses in the simulations, to vary their strength, temporal and spectral profiles, and other parameters. 18,20 QD simulations are usually performed on a potential energy grid and the cost associated with pre-computing these grids, exponential increasing with the number of degrees of freedom (DOF), limits their applicability to two or three DOFs.…”

Equation-of-Motion Methods for the Calculation of Femtosecond Time-Resolved 4-Wave-Mixing and N-Wave-Mixing Signals

“…As input for the calculation of spectroscopic signals, the ab initio DW methodology uses the electronic energies and TDMs calculated along classical trajectories which evolve in the electronic ground state {0} (for GSB) and in the manifold {I} of low-lying excited electronic states (for SE and ESA). See also ref 478 Nonperturbative Equation-of-Motion Formalism. Traditionally, time-resolved nonlinear spectroscopy is performed with relatively weak laser pulses.…”

“…As input for the calculation of spectroscopic signals, the ab initio DW methodology uses the electronic energies and TDMs calculated along classical trajectories which evolve in the electronic ground state {0} (for GSB) and in the manifold {I} of low-lying excited electronic states (for SE and ESA). See also ref 478 Traditionally, time-resolved nonlinear spectroscopy is performed with relatively weak laser pulses. The goal is the acquisition of information on the dynamics of the unperturbed (field-free) material system, in contrast to laser control of chemical dynamics, where the intention is the active steering of the molecular system toward a desired target, e.g.…”

Enhanced Electron Transfer and Spin Flip through Spin–Orbital Couplings in Organic/Inorganic Heterojunctions: A Nonadiabatic Surface Hopping Simulation

“…In addition, empirical fitted models employed in novel systems may lead to poor or misleading descriptions of the same process, e.g., the exciton dissociation in nonfullerene organic solar cells. − Therefore, the knowledge gap in the field requires new tools, preferable nonempirical methods, capable of yielding comprehensive descriptions of ultrafast processes at hybrid interfaces. Recently, first-principles calculations, namely, ab initio nonadiabatic molecular dynamics (NA-MD) methods are being used extensively to provide detailed insights into ultrafast dynamics mechanisms because NA-MD has the advantage to achieve atomic-level insights into the studied system. − …”