AN ANGULAR RESOLVED ELECTRON-ENERGY-LOSS INVESTIGATION OF HIGHLY ORIENTED PYROLITIC GRAPHITE ELECTRONIC-STRUCTURE

Angular resolved electron energy loss measurements on highly oriented pyrolitic graphite have been performed in reflection kinematics and at grazing incidence angles. Because of the selected kinematics it has been possible to change, almost independently, either the momentum transfer parallel to the surface (q(parallel-to)) or the angle between the momentum transfer and the graphite c axis (q^c). The present measurements have proved to be very sensitive to the surface structure of the sample and have allowed the assignment of the prominent loss at about 6 eV to a pi --> pi* excitation mainly localized perpendicular to the c axis. Possible contributions from surface localized states have also been detected. This transition shows negligible dispersion in q(parallel-to) and its energy centroid appears to be shifted with respect to previous surface and bulk measurements. Furthermore, the measured cross-sections are brought to the same relative scale for each grazing angle, and the loss and scattering functions are derived from data taken in reflection kinematics. The relevance of the kinematic prefactor in deriving transition energies from loss spectra is discussed with an emphasis on the role of q(parallel-to) zeroes.