Relation between Anharmonicity of Free-Energy Profile and Spectroscopy in Solvation Dynamics: Differences in Spectral Broadening and Peak Shift in Transient Hole-Burning Spectroscopy Studied by Equilibrium Molecular Dynamics Simulation

Solvation dynamics is used to monitor the time-dependent fluctuation of solvents, which plays an essential role in chemical reactions in solution. Transient hole-burning spectroscopy, in which a ground-state depletion (hole) formed by a laser pulse is observed, can be used to monitor solvation dynamics. Previous experiments demonstrated that the hole bandwidth relaxes an order of magnitude slower than the hole peak shift in organic solute-solvent systems. However, the detailed mechanisms behind this are still unclear. In this study, we developed a methodology to calculate transient hole spectra using equilibrium molecular dynamics simulation, in which a series of time-dependent system ensembles is accumulated to derive the appropriate dynamic properties. The simulated transient hole spectra adequately reproduced previous spectroscopic results. The different hole bandwidth and peak shift dynamics are ascribed to a non-Gaussian property or anharmonicity of the free energy profile with respect to the solvation coordinate.