Evidence for anisotropic vibration of diatomic adsorbates—NO and CO chemisorbed on stepped Pt(112)

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Abstract
Both NO and CO preferentially chemisorb on step sites of the Pt(112) crystal orienting the intermolecular bond in the downstairs direction. Using the digital electron stimulated desorption-ion angular distribution method (ESDIAD), an elliptical angular distribution of the desorbing O+ ions was detected from the NO/Pt(112) system, with the longer axis of the ellipse normal to the step-edge direction. On the other hand, the O+ ESDIAD pattern from the CO/Pt(112) system shows an approximately cylindrical symmetric shape. Heating of the crystal leads to broadening of the ion desorption patterns in both cases without change in the patterns’ elliptical or circular cross-sectional geometry. These results are interpreted as being due to ion desorption from NO molecules bonded to two Pt atoms on the step edge and vibrating with a longer amplitude in the direction perpendicular to the step. In the case of CO, bonded to a single Pt atom, the amplitude of vibration is approximately the same in directions parallel and perpendicular to the step edge. Thus, in certain cases ESDIAD patterns may be used to determine the hybridization state of adsorbates.