Resonant Charge Transfer in Atom–Metal Surface Reactions

Abstract

In recent years there has been a rising interest in atom–metal surface interactions. Especially resonant charge exchange in atom–metal surface scattering has been the subject of many theoretical and experimental studies. Atoms which easily surface ionize like the alkalis when adsorbed at a metal surface form ionic bonds between adsorbate and substrate resulting in a surface dipole. A surface coverage of about half a monolayer then results in a dipole layer, by virtue of which the work function of the metal is decreased by a factor of three. Especially the pioneering work of Gadzuk resulted in a theoretical description and calculation of this effect from first principles. These theoretical considerations also offer the possibility of a quantitative prediction of surface ionization. Recently the possibility of surface ionization – positive as well as negative ionization – by scattering ions or neutrals from a metal surface has been explored very intensively. This large interest is mainly due to the possibilities of forming intense H− and possibly Li− ion beams by scattering the respective positive ions from low workfunction metal surfaces, like cesium covered tungsten. The intense negative ion beams after neutralization will be needed for heating plasmas in connection with fusion. This paper will be concerned with the theoretical and experimental progress in the field of surface ionization.