THE KINETICS OF O-2 DISSOCIATIVE CHEMISORPTION ON FE(110)

The dissociative chemisorption of O-2 on Fe(110) has been studied using molecular beam techniques. Dissociative chemisorption is facile, with an initial sticking coefficient of similar to 0.3 for a translational energy of 50 meV, incident at a room temperature sample. The sticking probability increases with translational energy and above 150 meV dissociation is a direct, activated process. Measurements of the angular and surface temperature dependence of dissociative chemisorption indicate that dissociation at lower energies is hindered by translational energy perpendicular to the surface, consistent with precursor trapping into a physisorption state prior to dissociative chemisorption. As the surface temperature is decreased the initial sticking probability increases slightly, with a complicated coverage dependence and a greatly increased dissociation probability at higher O coverages. Coincidentally, the saturation uptake of O, increases from just over a monolayer near room temperature to nearly 4 monolayers O at 160 K. For room temperature adsorption the sticking probability shows a minimum at 0.5 monolayer coverage of O-2 with a slight increase towards Theta=1 monolayer, prior to saturation at high exposures. The sticking coefficient at Theta = 0.5 is sensitive to surface temperature and is associated with the formation of a relatively stable oxide layer. An abrupt change in the S(Theta) dependence is observed both at the completion of the ordered c(2 X 2) phase and at 0.5 monolayer for the activated dissociation channel.