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Thermal Population Fluctuations in Two-Dimensional Infrared Spectroscopy Captured with Semiclassical Mechanics.

Research paper by Prashanth P Ramesh, Roger F RF Loring

Indexed on: 01 Feb '18Published on: 01 Feb '18Published in: Journal of Physical Chemistry B



Abstract

Time-resolved two-dimensional (2D) infrared spectra of the asymmetric stretch mode of solvated CO2 show distinct features corresponding to ground and excited state thermal populations of the bend modes. The time-dependence of these peaks arises in part from solvent-driven thermal fluctuations in populations of the lower frequency bend modes through their coupling to the higher frequency asymmetric stretch. This observation illustrates the capacity of multidimensional vibrational spectroscopy to reveal details of the interactions among vibrational modes in condensed phases. The optimized mean-trajectory (OMT) method is a trajectory-based semiclassical approach to computing the vibrational response functions of multidimensional spectroscopy from a classical Hamiltonian. We perform an OMT calculation of the 2D vibrational spectrum for two coupled anharmonic modes, with the lower frequency mode undergoing stochastic transitions in energy to mimic solvent-induced fluctuations in quantum populations. The semiclassical calculation reproduces the influence of thermal fluctuations in the low-frequency mode on the 2D spectrum of the high-frequency mode, as in measured spectra of solvated CO2.