Bimodal Angular And Velocity Distributions Of Co2 Desorbing After Oxidation Of Co On Pt(111)

Time-of-flight (TOF) distributions of CO2 molecules desorbing after the oxidation of CO on Pt(111) were investigated at various desorption angles theta for surface temperatures T-s in the range 550-800 K. The Pt(111) surface was exposed to a continuous flow of O-2 from a doser and to a chopped CO nestle beam. Surface residence-time effects proved to be unimportant or were elucidated. Bimodal TOF distributions composed of two parts were obtained: molecules with a Maxwellian velocity distribution corresponding to T-s appeared in a cosine-shaped angular distribution and molecules with appreciably higher kinetic energies (mean kinetic energy normal to the surface (E(kin))perpendicular to approximate to 9kT(s)) were observed in a narrow angular distribution proportional to cos(8) theta. The partition of the total desorbing CO2 flux to these two channels depended on T-s and theta and the total oxygen dose d(o) accumulated during an experiment. With increasing d(o) the Maxwellian component markedly increased at the expense of the fast component. The observed trends suggest almost only fast desorption from clean Pt(111). We therefore assume Maxwellian desorption to occur mainly from surface sites gradually produced under the influence of oxygen.