HOT-ATOM MECHANISM IN PHOTODESORPTION OF MOLECULAR-OXYGEN FROM A STEPPED PLATINUM (113) SURFACE

The photodesorption of oxygen admolecules was studied on a stepped Pt( 113) = (s)2(111) x (001) surface with 193 nm irradiation at 110 K. Mul tidirectional desorptions were found to collimate at +/- 12 - 20 degre es and +/- 45 - 49 degrees off the surface normal and also along the s urface normal in a plane along the trough. The first component is alwa ys dominant, and the weak second component only appears at higher oxyg en coverages. The normally directed desorption is not significant. The translational energy of desorbing O-2 peaks around 15-20 degrees and 50 degrees, confirming the inclined desorptions. It is proposed that t hese inclined components are due to the desorption induced by the impa ct of oxygen admolecules with hot oxygen atoms from the photodissociat ion of adsorbed molecular oxygen, emitted along the trough. A simple c osine distribution was found to fit the thermal desorption from oxygen admolecules and also the recombinative desorption of oxygen adatoms. The 193 nm irradiation also produces additional, less tightly bound ox ygen adatoms, which yield a desorption component collimated at 15 degr ees from the surface normal in the step-down direction. (C) 1998 Ameri can Institute of Physics.