Evidence for “inclined” CO on Pd(210)

Abstract

The ESDIAD method (Electron Stimulated Desorption Ion Angular Distributions) has been used to verify an unusual bonding configuration for CO adsorbed on the rather open Pd(210) surface. In a previous IR reflection-absorption study of CO on Pd(210) (where top layer atoms with the nearest neighbor distance of 2.73 Å do not exist) the results indicated that CO was bridge-bonded to two Pd atoms via the carbon atom. Such bridge-bonding can only occcur in several distinct sites between atoms in the first and second atom layers, so that the axes of adsorbed CO molecules are expected to be inclined away from the normal by ∼ 18°. The ESDIAD data are consistent with the infrared observations at low coverages (two-fold symmetric triangular ESDIAD patterns dominated by off-normal ion emission in appropriate symmetry directions). Furthermore, the ESDIAD results indicate that the bending vibrational amplitudes in orthogonal directions for bridge-bonded CO are different, in agreement with recent calculations. At the saturation CO coverage at 90 K, the data indicate that CO is bonded normal to the surfaces. The relative yields of O + and CO + ions are sensitive to the bonding mode, with O + alone seen for bridge-bonded CO at low coverages and both O + and CO + ions observed at higher coverages.