Collective excitations in liquid para-H-2: A neutron polarization-analysisstudy

Neutron time-of-flight spectroscopy combined with polarization analysis has enabled the experimental separation of the coherent and spin incoherent co ntributions to inelastic scattering from Liquid para-hydrogen. The results from this experiment are analyzed in terms of self and distinct molecular c ontributions and extend the range of the observed distinct response beyond the wave numbers corresponding to the first peak in the static structure fa ctor of the Liquid. The collective response is adequately described in term s of two damped harmonic oscillators. While the observed wave number depend ences resemble acoustic longitudinal and optical branches in long-range-ord ered systems, the possibility of a multiexcitation origin for the higher-en ergy branch cannot be ruled out on the basis of the present experimental ev idence. [S0163-1829(99)13101-1].