Orientational processes in liquid nitromethane studied by depolarized light scattering and transient optical Kerr effect

Abstract

The dynamics of nitromethane in the liquid phase is investigated by steady-state (Raman and Rayleigh) and time-resolved (optical Kerr effect) spectroscopic experiments performed at variable temperature. Both experiments show that the entire relaxation process is completed in a few picoseconds and that the dynamics can be described by assuming a biexponential function for the molecular response. The prominent dynamical feature at longer times appears to be that of a diffusionally reorienting symmetric top. The time dependence for orientational correlation at the shorter times observed in the coherent optical processes (Rayleigh and OKE) suggests that perturbative phenomena in the subpicosecond time scale (collisional and cage effects) are operative prior to the onset of the diffusional regime. Reorientational times from steady-state coherent and incoherent light scattering experiments enable us to exclude that pair-particle orientational correlation is effective. The same orientational activation energy was estimated from all the experiments. The reorientation times closely follow the η/T (η shear viscosity) linear dependence, conforming to the predictions of slip rather than stick Stokes–Einstein–Debye hydrodynamic theory.