The adsorption of O at (110) surfaces at 4K was investigated by means of scanning tunnelling microscopy. It was observed that weakly-bound, so-called trapped molecules, coexisted with pairs of atoms which were preferentially oriented along [1¯10] and [001]. Both molecules and atoms were adsorbed at hollow sites. The clustering of O2 at step edges, perpendicular to [1¯10], indicated a substantial anisotropic mobility of the molecular precursor. It was concluded that precursor dynamics and multi-dimensionality of the potential energy surface exerted a predominant effect upon the dissociative chemisorption of O2 on (110).

B.G.Briner, M.Doering, H.P.Rust, A.M.Bradshaw: Physical Review Letters, 1997, 78[8], 1516-9