KINETICS OF OXYGEN DISSOCIATION ON SI(111)7X7 INVESTIGATED WITH OPTICAL 2ND-HARMONIC GENERATION

Optical second-harmonic generation is used as a sensitive, nondestruct ive probe for the thermally activated dissociation of molecular oxygen on Si(111)7 x 7. Isothermal measurements show that the dissociation k inetics strongly depend on the total oxygen coverage. At room temperat ure the 1/e lifetime of chemisorbed O2 is approximately 15 min for sat uration coverage but approximately 50 min for low oxygen coverage. The temperature dependence of the dissociation rates between 80 and 500 K indicates that more than one dissociation channel and/or molecular ox ygen species exist on the surface.