Simulation of the femtosecond optical response of a solute in water

We present microscopic semiclassical simulations of the femtosecond nonlinear-optical polarization (${\mathit{P}}^{\mathrm{}\left(3\right)\mathrm{}}$) of a solute in water. ${\mathit{P}}^{\mathrm{}\left(3\right)\mathrm{}}$ is dominated at short times by intermolecular vibrations that cannot be accounted for by the Bloch equations. The applicability of the Bloch, stochastic, and Brownian oscillator models to the interpretation of photon-echo and pump-probe measurements is discussed. We find that only microscopic semiclassical simulation and the Brownian oscillator model can account for the essential features of ${\mathit{P}}^{\mathrm{}\left(3\right)\mathrm{}}$.