Interaction of He+ and Ne+ ions with Ni(100)-K and Cu(100)-K surfaces having variable potassium coverage.

Abstract

This paper presents an electron-spectroscopic study employing slow ${\mathrm{He}}^{+}$ and ${\mathrm{Ne}}^{+}$ ions incident on Ni(100) and Cu(100) surfaces with varying amounts of adsorbed potassium. Investigation of the changes in electron-energy spectra as the work function of the surface is varied probes the pattern of resonance tunneling, excitation conversion, and Auger electron-ejection processes. When the surface is clean the work function \ensuremath{\varphi} is high and the mode of electron ejection is the two-electron process of Auger neutralization of the incident ions yielding a folded two-electron spectrum. As the macroscopic \ensuremath{\varphi} is reduced by K adsorption the surface develops more and more regions near K atoms where the local \ensuremath{\varphi} is small, permitting resonance neutralization of the incident ion to form a metastable atom which on Auger deexcitation ejects electrons in a one-electron spectrum.This paper reports an investigation of this transition from the two-electron, kinetic energy spectrum to the one-electron spectrum and of the specific characteristics of each of these spectral types. Results are compared in detail with a published study by other investigators of the interaction of thermal He metastable atoms with a Cu(110)-K surface having variable K coverage. This comparative study has illuminated the differences observed in the spectra produced by incident 12-eV ions which turn to metastables in the high-K-coverage regime and thermal metastables which turn to ions at low K coverage. We have demonstrated that the local work function of ``clean'' sites decreases as K coverage increases in the low-coverage regime by interpreting small observed changes in the kinetic energy spectra of ejected electrons. This, in turn, leads to an estimate of the variation of the mean separation of K atoms with increasing K adsorption. We present new data for ion neutralization at the clean Cu(100) surface. From these we obtain the deconvolution of the kinetic energy spectrum that is the initial-state transition density for comparison with photoemission spectra. These results are also compared in detail, particularly with respect to energy broadening, with the results of an investigation by others employing ions descendent from incident thermal metastables.