Reactive and inelastic scattering dynamics of hyperthermal oxygen atoms on a saturated hydrocarbon surface

Abstract

The dynamics of the initial interactions of hyperthermal O atoms with a saturated hydrocarbon surface have been investigated by directing an O-atom beam at a continuously refreshed liquid squalane surface and monitoring time-of-flight and angular distributions of inelastically scattered O atoms and reactively scattered OH and H2O. These products are formed through thermal and nonthermal processes. The inelastic scattering processes may be described in terms of the limiting cases of direct inelastic scattering (nonthermal) and trapping desorption (thermal). The initial step leading to production of volatile OH and H2O is believed to be direct H-atom abstraction to form OH. Once formed, the OH may scatter directly into the gas phase before thermal equilibrium with the surface is reached, or it may undergo further collisions and reactions with the surface. These secondary interactions include trapping and desorption of OH and abstraction of a second hydrogen atom to form H2O. Interactions that occur before thermal equilibrium with the surface can be reached lead to products that exit the surface at hyperthermal velocities, while those that occur in thermal equilibrium with the surface yield products that leave the surface at thermal velocities given by the surface temperature. Direct, single-collision scattering events that produce O and OH are described with a kinematic picture that allows the determination of the effective surface mass encountered by an incident O atom, the atom–surface collision energy in the center-of-mass frame, and the fraction of the center-of-mass collision energy that goes into translation of the scattered gaseous product and the recoiling surface fragment. Center-of-mass velocity-flux maps for OH indicate either single-collision events through a largely collinear O–H–C transition state or multiple-collision events in which initially formed OH scatters inelastically from the surface.