Lifetime and screening of the C 1s photoemission in graphite

High-resolution soft-x-ray photoemission spectra of the C 1s line of graphite have been analyzed to determine the various contributions to the linewidth and line shape. The natural lifetime width Γ is 210 meV, much larger than those of small molecules (for which Γ≃110 meV), or that calculated for the isolated carbon atom (for which Γ=56 meV). The phonon width is no larger than 50 meV. The general trend toward an enhanced Auger decay rate from the isolated atom to the small molecule and solid is indicative of significant contributions from interatomic Auger processes. However, the large increase in width from molecules to solids must be due to another phenomenon, perhaps to the formation of the excitonic final state in the solids. Our line-shape analysis confirms that the line has an asymmetry like that associated with metallic screening, which had been earlier related to the excitonic final state in graphite, but the singularity index is significantly smaller than that obtained in the previous analysis.