Energy-resolved electron-momentum densities of graphite films

We report the results of (e,2e) studies of crystalline graphite films. A noncoplanar asymmetric geometry was used with energies of the incident, scattered, and ejected electrons of 20, 18.8, and 1.2 keV, respectively. Thin (≃150 Å) films of highly oriented pyrolitic graphite were obtained by cleaving followed by plasma etching in an Ar-${\mathrm{O}}_2$ mixture. The (e,2e) spectra were sensitive to oxygen contamination of the film due to the plasma etching. Clean, oxygen-free graphite films were obtained by annealing in vacuum. Spectra of these films show distinct peaks in their momentum and energy distributions, but in between the peaks in the momentum distributions there is an additional contribution of unknown origin. The sharp peaks can be associated with the σ band electrons and modeled as an average of the basal plane momentum density of crystalline graphite. Good agreement in the electron dispersion was obtained. Quantitative tests of the momentum density were difficult because of the lack of a full understanding of the additional contributions, but qualitatively the calculated and measured momentum densities agree. A comparison is made with similar measurements of amorphous carbon films. An asssessment of the potential of the (e,2e) technique as applied to solids is made.