POSITION AND WIDTH OF A NEGATIVE-ION STATE IN FRONT OF A SURFACE - FORMATION OF C-(S-4) IONS BY ELECTRON-CAPTURE

A new method for determining the position and width of negative ion states in front of metal surface is presented. This coupled angular mode (CAM) method consists of studying the electron scattering by the superposition of an e--atom (molecule) interaction and an e--surface interaction. The surface interaction destroys the spherical symmetry of the e--atom (molecule) scattering and mixes the various angular scattering modes. The e--atom (molecule) interaction is treated in the effective range approximation and then the present method is an extension of ERT to anisotropic environments; typically, it describes a negative ion in the field of its image change. An application to the case of C- ion is presented. This system present multichannel and orientation effects. Indeed, the partial widths of C- corresponding to the various orientations of C are found to differ by an order of magnitude. These results are used to discuss the experimental results of van Pinxteren et al. ]Europhys. Lett. 10 (1989) 715[ on C- formation by collision on a cesiated tungsten surface.