High energy inelastic neutron scattering from physisorbed hydrogen in cesium intercalated graphite

The inelastic neutron scattering from H₂ in C ₂₄Cs(H₂)_x has been measured at momentum transfers of 8 < Q < 22 Å^-1 as a function of filling, 0.5 < x < 2.0, and temperature from 20 to 150 K. The observed scattering approaches the limiting behavior predicted by the Impulse Approximation (IA) for molecular hydrogen for Q''s greater than 15 Å^-1 and is insensitive to the filling factor and temperature up to kT/M≈0.1, where M is the enthalpy of hydrogen sorption. The results are analyzed using a model for the IA scattering from molecular hydrogen which takes into account the excitation of rotational transitions and the presence of a local orientational potential at the hydrogen adsorption sites. The average translational kinetic energy per molecule, which is determined by the effective local potential for the molecule, can be extracted from the measured scattering. For a powder sample, such as used in this measurement, determination of the kinetic energy requires independent knowledge of the orientational potential of the H ₂ molecules. Using the orientational potential inferred from previous tunneling measurements we extract an average translation kinetic energy of 320 K for the H₂ molecule in the intercalate.