Observation of a metastable precursor for adsorption of oxygen on Si(111) and the activation energy for chemisorption

Measurements of the work function of a Si(111) surface held at T20 K during exposure to O2 provide direct evidence for a precursor state preceding dissociative chemisorption. A striking feature of the precursor is its large surface dipole, resulting in a large positive work-function shift ( 1.6 eV). The precursor is effectively stable for T120 K, and converts to a final state upon heating the surface. Analysis of the dependence of on temperature and time indicates an activation energy of 40 meV for this conversion.