Direct measurements and path integral Monte Carlo calculations of kinetic energies of solid neon

Direct measurements are reported of the kinetic energies 〈E_k〉 of FCC neon at seven temperatures between 4.25 and 20.2 K. The measurements employed the electronvolt spectrometer which is the neutron scattering instrument at the ISIS neutron source, UK. Wavevector transfers Q from 306 to 1158 nm^-1 were used, far larger than previously, in a regime where possible final-state-effect corrections to the observed longitudinal neutron Compton profile J(y) are easily deal with by a simple correctional procedure. The results are of comparable precision with the best previous work, carried out using the HRMECS instrument at IPNS, Argonne. From those previous measurements, Peek et al give a ground-state kinetic energy of 49.1 ± 2.8 K. Path-integral Monte Carlo calculations were performed using both HFD-C2 and Lennard-Jones pair potentials. At 4.13 K, 10.13 K, 15.69 K and 20.31 K the HFD results were 41.6 ± 0.1 K, 42.6 ± 0.1 K, 46.5 ± 0.1 K and 47.8 ± 0.1 K, respectively. These values are broadly in agreement with the current measurements. They are significantly lower than the published values obtained from an HFD-based Wigner-Kirkwood high-temperature expansion including terms up to ℏ⁶.