Role of HOPG density of empty electronic states above vacuum on electron emission spectra induced by ions and UV photons

We present results of secondary electron emission (SEE) induced by UV photons and low energy He ions interacting with a pristine and ion damaged surface of highly-oriented pyrolytic graphite (HOPG). A critical examination of the changes in the SEE spectra produced by the radiation damage allows us to identify the origin of the different features of the SEE spectra. In this way, we conclude that the low energy peak at ~ 3 eV is strongly determined by the structure of the (empty) HOPG density of states above vacuum level. In contrast the high-energy structure at ~ 14 eV could not be explained by such an argument but rather by exciton autoionization.