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 $\left[1\overline{1}0\right]$ and $\left[001\right]$. Molecules and atoms are both adsorbed in hollow sites. Clustering of ${\mathrm{O}}_2$ at step edges perpendicular to $\left[1\overline{1}0\right]$ 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 ${\mathrm{O}}_2$ on Cu(110).

• Received 18 September 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.1516