Single particle dynamics in fluid and solid hydrogen sulphide: An inelastic neutron scattering study

Abstract

Inelastic neutron scattering experiments were performed at intermediate and high momentum transfer, up to 88.2 Å−1, to study the temperature dependence of single hydrogen mean kinetic energy in polycrystalline and liquid hydrogen sulphide (H2S), in the temperature range 16–206 K. Values of the hydrogen mean kinetic energy were extracted, within the impulse approximation, by fitting to the high momentum transfer data a model response function, obtained from a momentum distribution which is the orientational average of a multivariate Gaussian function. The extracted kinetic energies are compared with a harmonic model for the vibrational and roto-translational dynamics. The model makes use of the hydrogen-projected density of states worked out from intermediate momentum transfer data, as well as of optical frequencies determined from Raman and infrared (IR) spectroscopy. A fairly good agreement is obtained in the whole temperature range, while noticeably lower values for the kinetic energy are found if a single atom momentum distribution of isotropic Gaussian shape is assumed in the model response function.