NEGATIVE PARTICLE-EMISSION FROM A CS/RU(0001) SURFACE DURING EXPOSURE TO NO AND NO2

Exposure of a monolayer of Cs adsorbed on a Ru(0001 ) surface to NO leads at first to dissociative chemisorption and in a later stage to molecular adsorption. During this latter stage emission of exoelectrons signals the possibility for non-adiabatic processes, while with a clean surface efficient quenching of electronically excited states via resonance ionisation dominates. The behavior of NO closely resembles that of 02; however, in the latter case 0- ions are also ejected. Interaction with NO2 leads to qualitatively different results: Emission of exoelectrons is observed during the initial stage of dissociative chemisorption while at higher exposures NO2- ions are ejected as well. The experimental findings are rationalized in terms of a model which takes the different electron affinities of the incident particles into account. The observation of a pronounced dependence of the yield of exoelectrons on the substrate temperature suggests the participation of a metastable intermediate phase.