An XPS investigation of the chloridation of Ag(110)

XPS has been used to investigate the adsorption of chlorine on Ag(110) and to follow its subsequent chloridation. The first monolayer is adsorbed very rapidly and is followed by a slower chloridation process. The mode and rate of the latter is very temperature sensitive. Previous work has shown that, at room temperature and above, any absorbed chlorine beyond the first monolayer can dissolve in the bulk of the crystal, whereas at lower temperatures some kind of layer-by-layer chloride growth takes place at the surf ace. The X-ray induced Auger spectra of silver show new peaks in the MNN region shifted by 2.2eV to lower kinetic energy and this shifting is entirely due to relaxation energy differences in the final states between the metallic and ionic species. However, after 7 monolayers equivalent of chloride have absorbed, there appears to be more substrate signal remaining than would be expected from a simple layer model. This effect is explicable in terms of a Stranski-Krastanov form of crystalline growth, with nucleated patches of crystallite on the surface, separated by regions of single monolayer adsorption. Upon heating such a layer the spectroscopies show that these islands are removed by chlorine dissolution in the crystal above 250 K, leaving only the adsorbed monolayer at the surface but that, just prior to AgCl desorption, the chlorine reversibly segregates back to the surface (at ~ 650 K). Finally, the chlorine signal is lost between 700–850 K due to AgCl desorption into the gas phase.