Unoccupied band states and ion induced electron emission

JULIO FERRÓN; RICARDO VIDAL; RAÚL BARAGIOLA

San Sebastian

Workshop; Quantum 13, Brandt Ritchie Workshop; 2013

Centro de Física de Materiales; Donostia-San Sebatián

            The experimental probing of unoccupied conduction bands near surfaces provides a strong test of theories, which need to account for the breaking of symmetry and periodicity but by the finite lifetime of electrons and holes. The latter is of particular importance in a material like such as carbon, which can be tested in multiple allotropic forms. In a previous publication we have reported a study of electron emission from highly-oriented pyrolitic graphite (HOPG) induced by 1-5 keV He+ and Li+ ions. The energy spectra of emitted electrons, N(E) obtained over a wide range of polar and azimuthal angles showed a broad peak at E = 10 ? 20 eV for He+, but not for Li+ projectiles. A theoretical analysis explained the observations as due to electron promotion in He-C (but not in Li-C) collisions leading to the formation of a transient electron-hole pair that decayed into vacuum by autoionization. The results did not follow usual correlation with structure in the final density of states f-DOS that helped interpret previous studies of secondary electron emission (SEE) by electron impact and inverse photoemission.      Other recent studies on graphene have returned to the question of whether these peaks in the energy distribution of secondary electrons can be explained by invoking only structure in the density of initially empty states in the conduction band.  We need to expand the discussion about the influence of empty DOS in the energy distribution. For instance works of Hoffmann on electron induced, Marchand empty states in ups and Masada in MIS. This particular point, the influence of empty density of states (e-DOS) has been soundly discussed in the past regarding electron, photon (UPS), and metastable-atom electron spectroscopy. We have now improved and expanded these studies by using an electron energy analyzer that has a narrow angle of collection around the surface normal of an HOPG sample. We used as projectiles 2-5 keV He+, 200-1000 eV electrons and 21.2 eV (He-I) and 40.8 eV (He-II) ultraviolet photons incident on either pristine or ion-damaged surfaces.