Anisotropy in the atomic momentum distribution of pyrolytic graphite

Abstract

Deep inelastic neutron scattering using neutron resonance absorption techniques on the pulsed source ISIS at the Rutherford Appleton Laboratory has been used to determine the mean atomic kinetic energy in polycrystalline and pyrolytic graphite. An analysis of the recoil distribution widths shows a significant anisotropy in the atomic momentum distribution measured parallel and perpendicular to the hexagonal layers. Comparison between the observed mean kinetic energies and those derived from a phonon density-of-states model are in agreement perpendicular to the hexagonal layers but at variance parallel to the layers. A small asymmetry in the recoil lineshape has also been observed and is attributed to the influence of final-state corrections to the impulse approximation.