The Microscopic Dynamics of Liquid and Solid Parahydrogen

We have measured the almost-pure incoherent scattering function of liquid and solid parahydrogen, using inelastic neutron scattering. The experiment was carried out on TOSCA, a time-of-flight, inverse-geometry, crystal-analyser spectrometer, operating on the pulsed neutron source ISIS (UK). The measured double differential cross section gives a direct experimental access to the microscopic dynamics of condensed hydrogen. From the high energy region of the spectrum, where the Impulse Approximation for the center of mass motion applies, we have been able to extract the translational mean kinetic energy, which turns out to be density dependent. The density behaviors of the liquid and solid mean kinetic energy are slightly different. In the low energy region we used the Gaussian approximation to compare, in the liquid phase, the results of a novel quantum simulation calculation with the experimental data. Results are encouraging, but suggest further work, possibly beyond the Gaussian approximation.