Density dependence of mean kinetic energy in liquid and solid hydrogen at 19.3 K

We have measured, using the TOSCA spectrometer at ISIS, the inelastic incoherent neutronscattering spectrum of liquid and solid para-hydrogen along the T=19.3 K isotherm. From the high-energyregion of the spectrum, where the Impulse Approximation for the Centre of Mass motion applies, we havebeen able to extract the mean translational kinetic energy, which, as expected, turns out rather differentfrom the classical value and density dependent. We find that the density behaviours in the liquid and thesolid phase are slightly different. This confirms a similar feature already observed in liquid and solid heliumat T=6.1 K [M. Celli, M. Zoppi, J. Mayers, Phys. Rev. B 58, 242 (1998)]. The spectra from the solidphase have been also analysed in the low-energy region and allowed us to derive the Debye-Waller factor ofsolid para-hydrogen as a function of density. The comparison with the available experimental data in theliterature is rather good and confirms the excellent performances of TOSCA in the spectroscopic analysisof the condensed phases of para-hydrogen.