Quantum dynamics and spectroscopy of dihalogens in solid matrices. II. Theoretical aspects and G-MCTDH simulations of time-resolved coherent Raman spectra of Schrödinger cat states of the embedded I2Kr18 cluster

Abstract

This companion paper to [D. Picconi et al., J. Chem. Phys. 150 (2019)] presents quantum dynamical simulations, using the Gaussian-based multiconfigurational time-dependent Hartree (G-MCTDH) method, of time-resolved coherent Raman four-wave-mixing spectroscopic experiments for the iodine molecule embedded in a cryogenic crystal krypton matrix. These experiments monitor the time-evolving vibrational coherence between two wave packets created in a quantum superposition (i. e. a 'Schrödinger cat state') by a pair of pump pulses which induce electronic B 3 Π u (0 + ) ←− X 1 Σ + g transitions. A theoretical description of the spectroscopic measurement is developed, which elucidates the connection between the nonlinear signals and the wave packet coherence. The analysis provides an effective means to simulate the spectra for several different optical conditions with a minimum number of quantum dynamical propagations. The G-MCTDH method is used to calculate and interpret the time-resolved coherent Raman spectra of two selected initial superpositions for a I 2 Kr 18 cluster embedded in a frozen Kr cage. The time-and frequency-dependent signals carry information about the molecular mechanisms of dissipation and decoherence, which involve vibrational energy transfer to the stretching mode of the four 'belt' Kr atoms. The details of these processes and the number of active solvent modes depend in a non-trivial way on the specific initial superposition.

Efficient simulation of time- and frequency-resolved four-wave-mixing signals with a multiconfigurational Ehrenfest approach

“…By integrating the DA into the framework of nonlinear response functions, one can calculate the four-wave-mixing signals of molecular aggregates with both singly and doubly excited electronic states taken into account [38,39]. The G-MCTDH method was recently used to evaluate the time-resolved coherent Raman spectra of the embedded I 2 Kr 18 , offering a valuable tool to map calculated signals to wave packet coherence [40]. Very recently, a single-trajectory semiclassical thawed Gaussian approximation has been applied for the evaluation of third-order response functions [41].…”

On-the-fly ab initio semiclassical evaluation of third-order response functions for two-dimensional electronic spectroscopy

“…67 Recently, we extended our on-the-fly implementation of the single-Gaussian approach to simulate frequency-and time-resolved pump-probe spectra, 68 similar to the earlier work by the groups of Cina 69 and Engel 70 on model potentials. Although ensembles of thawed Gaussians were largely discarded and replaced by frozen Gaussians due to the numerical instabilities that often appear in nonadiabatic dynamics simulations, thawed Gaussians are being reintroduced, e.g., in the semiclassical hybrid dynamics [71][72][73] or Gaussian-based MCTDH, [74][75][76] and especially for spectroscopic applications, [77][78][79][80] due to their ability to describe couplings between different degrees of freedom. 81…”

Semiclassical vibrational spectroscopy with Hessian databases

“…Several computational methods for calculating quantum molecular vibrational spectra have been established. To name some popular ones, we recall the vibrational selfconsistent field and virtual space configuration interaction (VSCF/VCI) method, [1][2][3][4] the multi-configuration time dependent Hartree (MCTDH) approach, [5][6][7][8][9] the use of contracted basis functions with an eigensolver, 10,11 collocation methods, 12,13 and second-order vibrational perturbation theory (VPT2). 14 Despite the advance these methods provide in dealing with the exponential scaling of quantum approaches with the number of vibrational degrees of freedom, the investigation of large molecules or complex supra-molecular systems remains quite challenging.…”