Unoccupied electronic states in graphite

Soft x-ray appearance potential (SXAP) and ultraviolet bremsstrahlung measurements have been employed to explore the density of unoccupied electronic states of graphite. Deconvolutions of SXAP spectra are in quite good agreement with theoretical predictions. Considerable differences between earlier isochromat results obtained at $\hslash {\omega }_0=1\mathrm{}$ keV and the present measurements at $\hslash {\omega }_0=9.7\mathrm{}$ eV are explained in terms of a symmetry-dependent energetic variation of the radiative transition-matrix element. As a consequence, neither x-ray nor uv isochromat reflect the density of empty states over the full 20-eV region considered here satisfactorily. A properly weighted average of the respective data, however, agrees quite well with theory and SXAP deconvolution.