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

Measurements of the work function $\phi$ of a Si(111) surface held at $T\simeq 20$ K during exposure to ${\mathrm{O}}_2$ 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 $T\lesssim 120$ K, and converts to a final state upon heating the surface. Analysis of the dependence of $\phi$ on temperature and time indicates an activation energy of ∼40 meV for this conversion.