BIMODAL ANGULAR AND VELOCITY DISTRIBUTIONS OF CO2 DESORBING AFTER OXIDATION OF CO ON Pt(111)

Abstract

Time-of-flight (TOF) distributions of CO 2 molecules desorbing after the oxidation of CO on Pt (111) were investigated at various desorption angles ϑ for surface temperatures Ts 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 nozzle 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 Ts appeared in a cosine-shaped angular distribution and molecules with appreciably higher kinetic energies (mean kinetic energy normal to the surface <E kin >⊥≈9kTs) were observed in a narrow angular distribution proportional to cos 8 ϑ. The partition of the total desorbing CO 2 flux to these two channels depended on Ts and ϑ and the total oxygen dose d0 accumulated during an experiment. With increasing d0 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.