Deep inelastic neutron scattering from fluid hydrogen and deuterium: From vibrational excitations to the impulse approximation

Experimental deep inelastic neutron-scattering data from fluid deuterium at a temperature of 20.7 K, collected over a wide range of momentum transfer q (28^-1<q<70^-1), are presented. These experimental results, together with data previously collected from fluid hydrogen, are compared with a quantum-mechanical calculation, based on a Morse anharmonic intramolecular interaction. For both fluids it is shown that the present calculation is able to reproduce the experimental response function $F(y,q)$ over a wide momentum-transfer range, which spans from the molecular rotovibrational structure up to the molecular dissociation regime. Thus this calculation represents a clear improvement on previous models, based on harmonic or impulse approximations, which provide a good description of these fluids in the two opposite limits only, i.e., at the lowest and highest q values, respectively.