Collective and single-particle excitations in liquid neon

The neutron inelastic scattering from orthobaric liquid neon at 26.9 ^∘{}K has been measured with a triple-axis crystal spectrometer operating in its constant-Q mode. The scattering was measured as a function of frequency ν for 34 values of Q in the range 0.8-12.5 ^-1. Corrections for residual multiple scattering and instrumental resolution were applied to the theoretical cross sections before comparing with experiment. In the region of collective behavior, Q4 ^-1, the quasielastic peak that is observed is in qualitative agreement with the Kerr - Singwi theory and with the memory-function theory, which is applied to neon in the present work. The observed scattering at constant Q(<1.6^-1) shows a pronounced inelastic wing at values of ν where the memory-function theory predicts a small inelastic peak. No such wing is contained in the Kerr-Singwi theory. The latter theory evidently overestimates the damping of the collective motion while the former underestimates it. The observed scattering in the region $Q>4$ ^-1 is characteristic of single-particle scattering. The peak position follows, but lies somewhat lower than, the recoil frequency Q²4πm, where m is the mass of a neon atom, and the width of the scattering function increases approximately linearly with Q. Superimposed on this general behavior are well-defined oscillations characteristic of the coherent nature of the scattering. The single-particle scattering in liquid neon is very similar to that observed in liquid helium. The data in the single-particle region are in reasonable agreement with the Kerr-Singwi model and in even better agreement with a truncated Gram-Charlier expansion. The earlier data of Chen et al. are corrected for the nonlinearity inherent in a time-of-flight experiment and are found to agree with the present results in showing that there are no well-defined propagating excitations in liquid neon for $Q>~0.8$ ^-1.