The geometry of CO on Ru(001): Evidence for bending vibrations in adsorbed molecules

As a test of the utility of the ESDIAD method (Electron Stimulated Desorption Ion Angular Distributions) in studies of the geometry of adsorbed molecules, the chemisorption of CO on Ru(001) has been examined. Data previously reported using UPS (ultraviolet photoemission spectroscopy) and EELS (electron energy loss spectroscopy) have indicated that CO is terminally bonded to the Ru surface through the C atom, with the CO axis perpendicular to the surface. The ESDIAD results for CO confirm this orientation; for all CO coverages in the temperature range 90 K to ~ 350 K, the angular distributions of O⁺ and CO⁺ ESD ions are centered about the surface normal. The widths of the ion beams are temperature dependent; for both O⁺ and CO⁺, the half widths at half maximum, α, of the ion cones are ~16° at 300 K, and ~12° at 90 K. This temperature dependence, coupled with a simple model calculation, indicates that the dominant factors contributing to the width of the ESD ion beams are the CO surface bending vibrations, i.e., initial state effects. Thus, the data suggest that both the directions and widths of ESDIAD beams are determined largely by the structure and dynamics of the initial adsorbed state.