Direct observation of adsorbate dynamics from low-frequency vibration on a step defect—CO on Pt(112)

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Abstract
The relative shape of the hindered translation potential energy well for CO chemisorbed on the step sites of a Pt(112) single crystal was determined with digital electron stimulated desorption–ion angular distribution (ESDIAD). The angular displacement of the CO molecule in its hindered translation well predominately determines the half-width at half-maximum (HWHM) of the neutral a3π CO species (CO*) produced by ESD, a species which may be imaged in an ESDIAD apparatus. Variations in the CO* ESDIAD HWHM as a function of temperature are due to population of higher levels of the hindered translational mode. By monitoring the CO* ESDIAD HWHM for CO coverages on the step below 50% step saturation (0.17 monolayer) as a function of temperature, we show that the steepness of the hindered translation potential well is different for CO vibrations up, down, and along the step edge, following the trend: down the steps>up the steps>along the steps. There is no coverage dependence in the CO* ESDIAD HWHM values up or down the steps, but the HWHM values along the steps at 0.17 monolayer CO are significantly broader than those from lower coverages in the temperature range of 150–350 K. We interpret this to indicate the production of transient structures of tilted CO on adjacent step sites formed from preferential diffusion of CO molecules laterally on the step sites. Thus, the experiments probe directly the anisotropy of lateral diffusion of the adsorbate on step sites.