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 ${\mathrm{\AA }}^{-1\mathrm{}}<~\kappa <~3.1{\mathrm{\AA }}^{-1\mathrm{}}$, 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 $\kappa <~2.3$ ${\mathrm{\AA }}^{-1\mathrm{}}$ the observed spectra satisfy the one-phonon sum rule with mean square displacement $〈u^2〉=1.0\mathrm{}$ ${\mathrm{\AA }}^2$.