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 [11̄0] and $[001]$. Molecules and atoms are both adsorbed in hollow sites. Clustering of O₂ at step edges perpendicular to [11̄0] 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 O₂ on Cu(110).