4He adsorbed in cylindrical silica nanopores: Effect of size on the single-atom mean kinetic energy

This paper reports a study of the short-time dynamics of helium confined in silica nanopores (xerogel powder), with average pore diameters of 24 and 160 Å. The longitudinal momentum distribution of helium adsorbed in xerogels has been determined via deep inelastic neutron scattering (DINS) measurements performed on the VESUVIO spectrometer at the ISIS spallation source. DINS measurements, in the attosecond time scale, (i.e., 10−16–10−15 s), were performed at a temperature of T=2.5 K and saturated vapor pressure conditions, with 95% pore volume filling. The average wave-vector transfer q was about 130 Å−1. For confined helium, significant changes in the values of the single-particle mean kinetic energies ⟨EK⟩ are found in the bulk phase. These are 32.6±8.7 K for the 24 Å and 24.4±5.3 K for the 160 Å pore diameters, remarkably higher than ⟨EK⟩=16.2±0.4 K, the value of normal liquid 4He at T=2.5 K and saturated vapor pressure conditions. The results are interpreted in terms of a model where 4He atoms are arranged in concentric annuli along the cylindrical pore axis, with ⟨EK⟩ mainly dependent on the ratio between the atomic “effective” diameter and the pore diameter. The number of solid layers close to pore surface is found to be strongly pore-size dependent with one single solid layer for 24 Å diameter pore and three solid layers for 160 Å diameter pore.