Single-molecule kinetic energy of condensed normal deuterium

Inelastic scattering of 300-meV neutrons allows the study of the liquid (T=20 and 30 K, saturated vapor pressure) and solid (T=4.2 K, saturated vapor pressure) phases of the normal deuterium mixture (2/3 o-${\mathrm{D}}_2$+1/3 p-${\mathrm{D}}_2$) in the region of momentum transfer where a single-molecule response is expected. The spectra are analyzed within the impulse approximation and assuming Gaussian momentum distributions for the translation of the molecules. For the solid, the estimated value of the single-molecule average kinetic energy does not compare unfavorably with those obtained scaling experimental results in parahydrogen solids. In the liquid state, substantial departures seem to exist from the classical liquid behavior, even if up to second-order quantum corrections are taken into account. © 1996 The American Physical Society.