Four Wave Mixing Studies and Molecular Dynamics Simulations

Abstract

A quantitative microscopic understanding of molecular motion in dense media at times 10 −13 –10−11s is not only the goal of experimental and theoretical studies of molecular dynamics in liquids but also a prerequisite for a comprehensive treatment of ultrafast chemical reactions, in which solute-solvent interactions, local density fluctuations and energy relaxation can all play a deterministic role. In chemical reactions driven by strong laser fields, induced dipole moments and transient orientational ordering could well influence the height and pathway over the reaction barrier. We report here new results from a nonlinear optical study of four wave mixing (4WM) via \(Xi\mathop{{{\text{ }}j}}\limits^{{(3)}} kl\) in several binary organic systems, in which the solute-solvent interactions significantly modified the net nonlinear optical polarisability of the solute molecule. We then describe computer simulations of the optical field-induced anisotropy, from which we can deduce the time evolution of the subsequent relaxation and the orientational pair correlation functions (g(Ω)) describing the molecular interactions, given the experimentally determined orientational component of χ(3) in the 4WM data. These are novel attempts to simulate the nonlinear optical response of a dense medium to an intense but transient optical field, and, through continual refinement of the computational aspects, they will provide an important new tool for theintexpretation and testing of dynamical models in the femtosecond and picosecond time domain.