Positive ion neutralisation on chlorine covered silver surfaces

We report a study of the effect of chlorine adsorption on the interaction of positive ions with the silver surfaces. Neutralisation rates of He+ were measured and an ion scattering spectroscopy (ISS) study of Cl covered Ag(111) was performed. The Cl ISS peak remains vanishingly small during the Cl chemisorption submonolayer phase and then rapidly increases above 1.5 L. The neutralisation rate of He+ is found to increase during the first phase and then decrease above 1.5 L, an effect that we attribute to A.-Cl island formation. This points out a chemical environment dependance of the Cl detection with ISS due to changes in particular quasi-resonant neutralisation rates involving Cl(3s) core levels. In the case of H+, Ne+ and Ar+ neutralisation on Ag(111) (110) and (100) a striking oscillation of the neutralisation rate is found for the three surfaces at low coverages. A first minimum is reached around 0.5 to 2/3 of the full coverage and a maximum is then attained at full coverage. Angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and metastable deexcitation spectroscopy (MDS) measurements allow one to describe these features in terms of the modifications in Auger neutralisation (AN) rates due to changes of the density of states as adsorption changes from smaller to higher packing of adsorbed Cl. Beyond the saturation limit the neutralisation rate is found to decrease again on Ag(111) but remains constant on Ag(110) and Ag(100). This is ascribed to the fact that, in the exposure range reported, AgCl formation occurs only on the (111) face. On the two other faces an ordered chemisorption phase remains stable until higher coverages.