Hybrid equilibrium-nonequilibrium molecular dynamics approach for two-dimensional solute-pump/solvent-probe spectroscopy.

There has been a tremendous effort in accessing liquid-phase solvation dynamics using ultrafast spectroscopies for decades. It is recently demonstrated that one can go beyond tracking the relaxation of the solute-solvent interaction energy as reported in traditional time-dependent fluorescence spectroscopy. Combining a resonant solute pump with a subsequent nonresonant 4-wave-mixing light-scattering solvent probe gives rise to two-dimensional solute-pump/solvent probe (SPSP) spectroscopy, which displays the nonequilibrium relaxation of the intermolecular vibrations after the solute gets electronically excited. However, the response function of the SPSP spectra has been challenging to calculate, even on the classical-mechanical level, due to the difficulty of evaluating the surviving Poisson bracket. In this work, a hybrid equilibrium-nonequilibrium molecular dynamics simulation approach is proposed where the Poisson bracket can be estimated using nonequilibrium molecular dynamics. Applying the resulting numerically exact formalism and the previously proposed hybrid instantaneous-normal-mode/molecular-dynamics approach to a preferential solvation model system reveals that the SPSP spectroscopy provides an alternative measure for solvation dynamics, which is more sensitive to the local solvent structures than the traditional energetic dynamics measured in the time-dependent fluorescence spectroscopy.