Ultrafast solvent response upon a change of the solute size in non-polar supercritical fluids

Non-polar solvation dynamics has been investigated using steady-state absorption and emission spectroscopy of the NO A 2Σ+(3sσ) Rydberg state in fluid Ar over a wide range of densities spanning the supercritical regime. Equilibrium molecular dynamics simulations were implemented to derive a new isotropic NO A(3sσ)-Ar pair potential which was further used to investigate the role of local density enhancements on the solvation process by non-equilibrium molecular dynamics simulations. These density inhomogeneities were found to have no influence on the solvation dynamics. Furthermore, the latter was shown to take place in a strongly non-linear regime, especially at low temperatures. This process results from the dramatic change of solute-solvent short range interaction associated with the large solute size change upon excitation to the Rydberg state.