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 ^-1<~κ<~3.1^-1, extending from the first through the third Brillouin zones in the solid, S(κ,ω) is remarkably similar to the scattering law expected from a polycrystal. For κ<~2.3 ^-1 the observed spectra satisfy the one-phonon sum rule with mean square displacement 〈u²〉=1.0 ².