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

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.