π*andσ*Excitons in C1sAbsorption of Graphite

We have studied the ${\pi }^{*}$ and ${\sigma }^{*}$ threshold structures in the C $1s$ x-ray absorption spectrum of graphite using core level autoionization spectroscopy. We confirm that the ${\sigma }^{*}$ threshold feature is an exciton. We find that the prominent ${\pi }^{*}$ feature is also excitonic, though it lies approximately 1 eV above the ${\pi }^{*}$ threshold. This excited state is localized on a time scale of fs, even though it is degenerate with propagating states of the crystal.