DISSOCIATION AND ELECTRONIC-TRANSITIONS IN LOW-ENERGY GRAZING COLLISIONS OF CO2+ IONS WITH W(110) SURFACES

Electron energy spectra from slow (50 eV) CO2+ molecular ions collidin g under grazing incidence (5 degrees) with W(110) surfaces are reporte d. The surface work function was varied by the exposure to alkali atom s. The electron emission is caused by Auger processes, such as Auger c apture for clean W(110) and at moderate alkali coverages, and Auger de excitation of core excited CO2 states (formed by the resonant capture of a surface electron by the CO2+ projectile) at larger alkali covera ges (> 0.3 monolayers). A comparison is made with the spectra of the e lectrons ejected in CO+ collisions (also reported here). The CO+ and C O2+ induced spectra are very similar for coverages beyond 0.5 monolaye rs. This can be explained by assuming that in this coverage range effi cient dissociation of the core excited CO2 states is in competition wi th the Auger process. The dissociation apparently leads to the formati on of excited CO fragments whose Auger deexcitation is observed. 0(-) ions are seen in the spectra of the ions scattered into 90 deg with r espect to the beam direction. These could originate from oxygen atoms from the dissociation process which have captured a surface electron t hus forming 0(-) ions.