Electronic response of metal surfaces to electron exchange with keV-ions

Electron spectra arising from 'simple' doubly charged ion-metal surface systems under well defined experimental conditions, have been investigated. Spectra were taken for grazinng incidence He2+-Cu(110) collisions. Auger capture and resonant capture followed by autoionization were found to be the main contributing neutralization processes. A model was developed to calculate the electron spectra taking these processes into account. From a comparison of the measurements to the model calculations, a quantitative determination of the contributions from the various processes and, information on several properties of the metal surface was obtained. A sensitivity of the electron spectra to the time dependence of the electronic response of the metal surface has been observed: the energy of the emitted electrons depends on the time dependent electronic state, in which the ion-metal-surface system is initially formed to the ionization event. Comparison of the measurements to the calculations shows the necessity of assuming a finite response time of the metal surface of at least the order of 10-16 seconds, i.e. the ion-metal-surface is left in an excited state. A similar finite response time had to be assumed for Ne2+ and Ar2+ ions impacting on the same metal surface. The influence of oxygen adsorption on the electron spectra was studied by taking measurements for the three ions incident on oxygen covered Cu(110). The contribution to the spectra from autoionization considerably decreased. Measurements were also performed for Ni(110) with the same incident ions. The spectra mainly consist of contributions from Auger-capture processes. Resonant capture, followed by autoionization plays a minor role as compared to corresponding spectra for Cu(110). Upon oxygen adsorption, the contribution from autoionization significantly increased. For all three ions again the necessity to allow for a finite response time of the metal surface followed. (author). 84 refs.; 42 figs.; 4 tabs