Interband transitions and core excitation in highly oriented pyrolytic graphite studied by inelastic synchrotron x-ray scattering: Band-structure information

The dynamic structure factor S(q,ω) of electrons in highly oriented pyrolytic graphite for q∥c and q⊥c with 0.37<q<2.06 a.u. was measured at 0.8 eV resolution by inelastic scattering of synchrotron x radiation. The energy-transfer regions both of interband transitions and of core excitations were investigated. By means of Kramers-Kronig transformation Re[ε(q,ω)] and Im[ε(q,ω)] were obtained, where peaks of the latter were attributed directly to maxima of projections off the joint density of states achieved by both dipole selection rules and q-dependent matrix elements. Making use of both x-ray emission and photoemission data, limits for the energy positions of some conduction bands could be established by means of the Im[ε(q,ω)] data. As far as weakly-$k_z$-dispersing bands are concerned, their energy positions were found to be in good agreement with recent band-structure calculations. On the other hand, the measured energy ranges of the bands that could be identified as belonging to the recently proposed interlayer states disagree with most of the relevant band-structure calculations, though they are not far from the results of other spectroscopies. The strict bulk origin of the inelastic-scattering-based band-structure information obtained is stressed. The experimental data are also discussed in view of the interpretation of previous low-resolution inelastic-scattering data in terms of correlation-induced fine structure for q⊥c.