Quasi elastic neutron scattering characterization of the relaxation processes in a room temperature ionic liquid

We report the first quasielastic neutron scattering measurements on a room temperature ionic liquid: 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]. Data were collected using a medium resolution spectrometer as a function of temperature in the range 250–320 K. The dataunequivocally indicate the existence of two different relaxation processes: a fast, localized motion occurring in the subpicosecond range and a slower process spanning the subnanosecond regime. These results provide experimental support to recently published molecular dynamics simulations. Evidence for slower, unresolved dynamics (under the present experimental conditions) is also obtained. Both temperature and momentum transfer dependence of the intermediate incoherent dynamic structure factor were investigated, after Fourier transformation into the temporal domain. The fast process shows no appreciable Q- and T-dependence. On the other hand the slow process shows evidence of a complex, non-Debye and non-Arrhenius behavior.