Microscopic description of vibrational energy relaxation in supercritical fluids: On the dominance of binary solute-solvent contributions

The representation of the frequency dependent friction coefficient via a fo ur-particle two-time correlation function is used to analyze the applicabil ity of collisional and hydrodynamical models of vibrational energy relaxati on (VER). The solute-solvent binary dynamics is separated from collective e quilibrium correlations by means of Green's functions. The collective contr ibutions manifest themselves mainly via the solute-solvent radial distribut ion function (RDF), which reflects peculiarities of the particular solvent thermodynamical (e.g., supercritical) state. The binary dynamics is also cl osely related to many-body equilibrium correlations, as initial conditions sample microscopic system states in the vicinity of the solute which are th e most important for VER. VER rates along a close to critical isotherm are calculated on the basis of the breathing sphere model and the Douglas appro ximation for force-time correlation functions, while Monte Carlo simulation s are used for calculating RDFs. The results are compared with molecular dy namics simulations at low, intermediate and high densities. It is shown tha t at near-critical conditions as well as far from the critical point the ke y contribution to VER comes from the short and intermediate time behavior o f the force-time correlation function. In configuration space only short ra nge binary solute-solvent correlations are important. Analytical estimation s, Monte Carlo and molecular dynamics simulations clearly show that the dyn amics of VER can only be understood on the basis of a detailed description of local solute-solvent interactions and correlations.