Anisotropy in the Optical Transitions from the π and σ Valence Bands of Graphite

The dielectric tensor {article}{empty}{document}ε ᵈᵒᶜᵘᵐᵉⁿᵗ (ω) of pyrolytic graphite, cleaved with its surface perpendicular to the c ‐axis, has been determined using polarized synchrotron radiation of photon energies 3 eV ≦ h ω ≦ 40 eV. For this purpose the reflectivities were measured for nine angles of incidence between 15° and 75°, the electric field vector E lying parallel and perpendicular to the plane of incidence. The complex dielectric functions {article}{empty}{document}ε ᵈᵒᶜᵘᵐᵉⁿᵗ ⊥and {article}{empty}{document}ε ᵈᵒᶜᵘᵐᵉⁿᵗ ‖ for E perpendicular and parallel to the c ‐axis were obtained by a least squares fitting of the measured reflectivities to the calculated ones. In addition {article}{empty}{document}ε ᵈᵒᶜᵘᵐᵉⁿᵗ ⊥ was determined independently by a dispersion analysis of the reflectivity at 15° angle of incidence. The results showing strong anisotropy over the whole energy range are compared with previous optical data for {article}{empty}{document}ε ᵈᵒᶜᵘᵐᵉⁿᵗ and electron energy losses. On the basis of recent two and three dimensional band structure calculations the spectra are attributed to direct interband transitions from the π‐ and σ‐type valence bands to π(σ)‐and σ(π)‐conduction bands for E ⊥ c ( E ‖ c ), respectively.