  Two-particle spectroscopy with correlated electron pairs is used to establish
the causal link between the secondary electron spectrum, the
$(\pi+\sigma)-$plasmon peak and the unoccupied band structure of highly
oriented pyrolitic graphite. The plasmon spectrum is resolved with respect to
the involved interband transitions and clearly exhibits final state effects, in
particular due to the energy gap between the interlayer resonances along the
$\Gamma$A-direction. The corresponding final state effects can also be
identified in the secondary electron spectrum. Interpretation of the results is
performed on the basis of density functional theory and tight binding
calculations. Excitation of the plasmon perturbs the symmetry of the system and
leads to hybridisation of the interlayer resonances with atom-like $\sigma^*$
bands along the $\Gamma A$-direction. These hybrid states have a high density
of states as well as sufficient mobility along the graphite $c$-axis leading to
the sharp $\sim$3\ eV resonance in the spectrum of emitted secondary electrons
reported throughout the literature.
