Kinetics of oxygen dissociation on Si(111)7×7 investigated with optical second-harmonic generation

Optical second-harmonic generation is used as a sensitive, nondestructive probe for the thermally activated dissociation of molecular oxygen on Si(111)7×7. Isothermal measurements show that the dissociation kinetics strongly depend on the total oxygen coverage. At room temperature the 1/e lifetime of chemisorbed ${\mathrm{O}}_2$ is ∼15 min for saturation coverage but ∼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 oxygen species exist on the surface.