CarbonK-edge fine structure in graphite foils and in thin-film contaminants on metal surfaces

We report absorption and extended-fine-structure studies on the carbon $K$ edge in microcrystalline, polycrystalline, and oriented crystalline (monochromator) graphite. On the basis of a density-of-states analysis, strong excitonic effects about the core hole are inferred. The fine structure at the absorption edge is found to differ for the three materials. This is explained by the orientational dependence of the polarization selection rules for electric dipole transitions from an $s$ core to $\sigma$ and $\pi$ conduction-band states. The extended x-ray fine structure (EXAFS) above the edge exhibits similar oscillations for the bulk crystalline samples (polycrystalline and monochromator graphite), but considerably weaker structure is observed for the microcrystalline sample. A Fourier analysis of the oscillations yields the nearest-neighbor shell separations. Finally, it is shown that the intensity modulations caused by carbon contamination on optical Au and Pt mirror surfaces that are exposed to high-intensity synchrotron radiation closely resemble those above the C $K$ edge in bulk crystalline graphite. This indicates the formation of graphitic overlayers on such mirrors even under ultrahigh-vacuum (<1 × ${10}^{-9\mathrm{}}$ Torr) conditions. These findings furthermore demonstrate the feasibility of EXAFS studies of adsorbates on surfaces by monitoring the (totally) reflected radiation off the substrate.