The role of an oxygen adsorbate on the secondary emission properties of low energy ion-bombarded magnesium

The absolute secondary electron and anion emission probabilities for low energy (<500 eV) O−, N2+, Ar+, Ne+ and He+ impacting an oxygen-adsorbed Mg(0 0 0 1) single crystal surface have been measured as a function of oxygen exposure and of incident ion impact energy. Secondary anion emission was observed throughout the range of adsorbate coverage studied, from that of a “clean” metal (defined by a demonstrated adsorbate-induced work function shift) to that rendering an insulative metal oxide, and was determined to comprise mostly O−. Both electron and anion emission were observed to be largely unaltered during the metal–metal oxide transition. Secondary anion and electron kinetic energy spectra were also measured; no significant dependence on oxygen exposure was observed. The relevance of secondary emission mechanisms that are driven by the ionization potential of the incident ion ranges from limited to none in our experiment in the case of incident O−; we suggest that decay of a collisionally-excited MgO surface state may account, in part, for secondary emission related to the presence of adsorbed oxygen.