Steric effect in O2 sticking on Al(111): preference for parallel geometry.

We present experimental evidence that the sticking probability of O2 on Al(111) depends strongly on the alignment of an O2 molecule relative to the surface. The steric effect was measured with an aligned O2 beam prepared by a hexapole magnet. It has been found that, at translational energies <0.2 eV, O2 sticking occurs predominantly when the molecular axis is parallel to the surface, and that the dissociation barrier at the perpendicular geometry is ∼ 0.1 eV higher than at the parallel geometry. The present results indicate that the abstraction process, which would occur at the perpendicular geometry, is a minor event at low translational energies.