Direct observation of molecularly-aligned molecules in the second physisorbed layer-CO/Ag(1Â 1Â 0)

We report the direct observation of oriented second-layer physisorbed molecules on a single crystal surface by electron stimulated desorption. Experiments and simulations show that the orientation of the second-layer physisorbed CO molecules on Ag(1 1 0) is the result of both electrostatic and dispersion forces from the underlying chemisorbed CO and Ag atoms. At 25 K, the physisorbed C-O bond is tilted and azimuthally oriented with the C-O bond axis inclined in an azimuthal plane at 45° to the principal Ag(1 1 0) azimuthal crystallographic directions. The O atom in CO is directed outward, giving an O+ beam at 43° to the normal.