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

The dissociative chemisorption of O-2 on Fe(110) has been studied usin g molecular beam techniques. Dissociative chemisorption is facile, wit h an initial sticking coefficient of similar to 0.3 for a translationa l energy of 50 meV, incident at a room temperature sample. The stickin g probability increases with translational energy and above 150 meV di ssociation is a direct, activated process. Measurements of the angular and surface temperature dependence of dissociative chemisorption indi cate that dissociation at lower energies is hindered by translational energy perpendicular to the surface, consistent with precursor trappin g into a physisorption state prior to dissociative chemisorption. As t he surface temperature is decreased the initial sticking probability i ncreases slightly, with a complicated coverage dependence and a greatl y increased dissociation probability at higher O coverages. Coincident ally, the saturation uptake of O, increases from just over a monolayer near room temperature to nearly 4 monolayers O at 160 K. For room tem perature adsorption the sticking probability shows a minimum at 0.5 mo nolayer coverage of O-2 with a slight increase towards Theta=1 monolay er, prior to saturation at high exposures. The sticking coefficient at Theta = 0.5 is sensitive to surface temperature and is associated wit h the formation of a relatively stable oxide layer. An abrupt change i n the S(Theta) dependence is observed both at the completion of the or dered c(2 X 2) phase and at 0.5 monolayer for the activated dissociati on channel.