Temperature dependence of the kinetic energy in condensed argon

Abstract

Using eV neutron scattering, direct measurements have been made of the average single-particle kinetic energies, 〈${\mathit{E}}_{\mathit{k}}$〉, for a series of condensed Ar samples over the range 18--85 K. These temperature-dependent measurements are on a system with A>22 amu in the regime below equipartition. The scattering from liquid-Ar samples, for which it is assumed that 〈${\mathit{E}}_{\mathit{k}}$〉 can be determined with sufficient accuracy from a quantum expansion, has been used to provide information about the total corrections to be made to account for neutron time-of-flight spectrometer resolution and multiple scattering. Data were obtained for wave-vector transfers in the range 192--254 ${\mathrm{nm}}^{\mathrm{\ensuremath{-}}1}$, to check for possible systematic contributions to experimental uncertainty. The directly measured 〈${\mathit{E}}_{\mathit{k}}$(T)〉 is compared to expectations from thermodynamic experiments, from previous neutron-scattering measurements of collective vibrational modes of the solid, and from various theoretical models. The directly measured temperature dependence provides evidence for use in the interpretation of other experiments which use Ar as an ``inert'' host.