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 inci dent, scattered, and ejected electrons of 20, 18.8, and 1.2 keV, respe ctively. Thin (congruent-to 150 angstrom) films of highly oriented pyr olitic graphite were obtained by cleaving followed by plasma etching i n an Ar-O2 mixture. The (e, 2e) spectra were sensitive to oxygen conta mination of the film due to the plasma etching. Clean, oxygen-free gra phite films were obtained by annealing in vacuum. Spectra of these fil ms show distinct peaks in their momentum and energy distributions, but in between the peaks in the momentum distributions there is an additi onal contribution of unknown origin. The sharp peaks can be associated with the sigma 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 d ensity were difficult because of the lack of a full understanding of t he additional contributions, but qualitatively the calculated and meas ured momentum densities agree. A comparison is made with similar measu rements of amorphous carbon films. An assessment of the potential of t he (e, 2e) technique as applied to solids is made.