Mobility and trapping of molecules during oxygen adsorption on Cu(110)

Adsorption of oxygen on Cu(110) at 4 K has been investigated by scanning tunneling microscopy. We have observed that weakly bound, ''trapped'' molecules coexist with pairs of atoms which are preferentially oriented along [1(1) over bar0$] and [001]. Molecules and atoms are both adsorbed in hollow sites. Clustering of O-2 at step edges perpendicular to [1(1) over bar0$] indicates substantial anisotropic mobility of the molecular precursor. It is concluded that precursor dynamics and multidimensionality of the potential energy surface have a dominant influence on the dissociative chemisorption of O-2 on Cu(110).