Spectral frequency distribution for liquid deuterium determined by neutron scattering

The generalized spectral distribution, which is the frequency-domain representation of the velocity autocorrelation function, is determined for liquid molecular deuterium for a broad range of frequencies from neutron-scattering experiments. Two well-defined features centered at frequencies of 3.8 and 7.6 meV are seen superimposed on a broad, slowly decaying tail. The latter seems reminiscent of what has been observed for hydrogen in the dense-gas phase whereas the former is shown to arise from finite-frequency collective excitations and quantized molecular rotations.