Experimental determination of the translational kinetic energy of liquid and solid hydrogen

TOSCA is a novel inelastic spectrometer operating on the pulsed neutron source ISIS (UK). It covers a wide momentum and energy range, even though its kinematic region is close to a line in the (k, E) plane. Its use is mainly intended for vibrational spectroscopy. However, taking advantage of its good resolving power, we have carried out a test experiment aimed to use this instrument to measure the centre of mass kinetic energy of molecular hydrogen. The experiment was successful and we have obtained the translational kinetic energy of liquid and solid para-hydrogen improving the overall accuracy by almost an order of magnitude with respect to previous determinations. The data are compared with the results of a Path Integral Monte-Carlo simulations with almost perfect agreement. We have demonstrated that TOSCA can be used for measuring the translational kinetic energy of small molecular systems, taking advantage of the intrinsic incoherence that is introduced in the scattering process by the intra-molecular transitions.