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.