Collective excitations in liquid hydrogen observed by coherent neutron scattering

Coherent scattering of neutrons by liquid parahydrogen shows the existence of well-defined collective excitations in this liquid. Qualitative similarity with the scattering from liquid helium is found. Furthermore, in the range of observed wave vectors, 0.7 ${\AA }^{-1}<\kappa <3.1{\AA }^{-1}$, extending from the first through the third Brillouin zones in the solid, $S(\kappa ,\omega )$ is remarkably similar to the scattering law expected from a polycrystal. For $\kappa <2.3$ Å⁻¹ the observed spectra satisfy the one-phonon sum rule with mean square displacement 〈u²〉=1.0 Ų.