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 four-particle two-time correlation function is used to analyze the applicability of collisional and hydrodynamical models of vibrational energy relaxation (VER). The solute-solvent binary dynamics is separated from collective equilibrium correlations by means of Green's functions. The collective contributions manifest themselves mainly via the solute-solvent radial distribution function (RDF), which reflects peculiarities of the particular solvent thermodynamical (e.g., supercritical) state. The binary dynamics is also closely related to many-body equilibrium correlations, as initial conditions sample microscopic system states in the vicinity of the solute which are the 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 approximation for force-time correlation functions, while Monte Carlo simulations are used for calculating RDFs. The results are compared with molecular dynamics simulations at low, intermediate and high densities. It is shown that at near-critical conditions as well as far from the critical point the key contribution to VER comes from the short and intermediate time behavior of the force-time correlation function. In configuration space only short range binary solute-solvent correlations are important. Analytical estimations, Monte Carlo and molecular dynamics simulations clearly show that the dynamics of VER can only be understood on the basis of a detailed description of local solute-solvent interactions and correlations.