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Description
Differential very low energy electron diffraction spectra have been measured on a Highly Oriented Pyrolitic Graphite (HOPG) surface in the range of landing energies from 0-1600eV. The reflectivity in the band gap, in between the interlayer resonances, differs from unity, implying that the vacuum wave function can penetrate the surface, but it is strongly damped via excitation of $\pi$- and $\pi +\sigma$-plasmon excitation. This is also true, albeit to a much lesser extent for wave functions for allowed states in the one electron band structure. Measurements of time correlated electron pairs (electron coincidence spectroscopy) show that Plasmon decay leads to emission of secondary electrons via the interlayer resonances. The results exemplify the momentum exciton picture of plasmon excitation and decay.
