High-energy neutron inelastic scattering from adsorbed hydrogen molecules

Abstract

High-energy neutron scattering to the ν=1 state of molecular hydrogen is reported for hydrogen trapped in stage II caesium graphite. Despite the electron rich environment of the intercalate the vibrational frequency is 510±10 meV, close to the free molecule value and so there is no evidence of H 2 − formation. The kinematics of the scattering of 828, 1226 and 1546 meV neutrons from both the ν=0 →ν=0 and ν=0 →ν=1 transitions has been studied. Both transitions show recoil effects which obey the scaling relationships of the impulse approximation. No “recoil shifts”, compared to other low Q studies, of the individual vibrational and phonon density of states singularities are observed when 98 meV incident neutrons are used. The relationship between the observed phonon spectrum and the “recoil shifted” excitations for higher E 0 is discussed.