DEPTH OF ORIGIN OF ESD IONS - ENHANCEMENT OF F- FROM PF3 RU(0001) UPON PASSAGE THROUGH XENON OVERLAYERS/

We have studied the electron stimulated desorption of F- from a monola yer of PF3Ru(0001); the ions pass through ultrathin overlayer films of Xe. Surprisingly, we find the F- yield to be 4 times higher in the pr esence of a Xe overlayer one monolayer thick than in the absence of Xe . This enhancement is accompanied by a dramatic change in the ion angu lar distribution: While F- ions desorb from PF3Ru(0001) in a hexagonal array of off-normal beams, they desorb in a broad normal distribution in the presence of a Xe overlayer. We discuss a model which attribute s the increase in F- to a decrease in the neutralization probability o f F- with the surface, and the change in angle to elastic scattering.