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 incoh erent neutron scattering spectrum of liquid and solid para-hydrogen along t he T = 19.3 K isotherm. From the high-energy region of the spectrum, where the Impulse Approximation for the Centre of Mass motion applies, we have be en able to extract the mean translational kinetic energy, which, as expecte d, turns out rather different from the classical value and density dependen t. We find that the density behaviours in the liquid and the solid phase ar e slightly different. This confirms a similar feature already observed ill liquid and solid helium at T = 6.1 K [M. Celli, Al. Zoppi, J. Mayers, Phys. Rev. B 58, 242 (1998)]. The spectra from the solid phase have been also an alysed in the low-energy region and allowed us to derive the Debye-Waller f actor of solid para-hydrogen as a function of density. The comparison with the available experimental data in the literature is rather good and confir ms the excellent performances of TOSCA in the spectroscopic analysis of the condensed phases of para-hydrogen.