Collective excitations in liquid para-H2:: A neutron polarization-analysis study

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