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 desorptionndash;ion angular distribution (ESDIAD). The angular displacement of the CO molecule in its hindered translation well predominately determines the halfhyphen;width at halfhyphen;maximum (HWHM) of the neutrala3pgr; CO species (COast;) produced by ESD, a species which may be imaged in an ESDIAD apparatus. Variations in the COast; ESDIAD HWHM as a function of temperature are due to population of higher levels of the hindered translational mode. By monitoring the COast; ESDIAD HWHM for CO coverages on the step below 50percnt; step saturation (0.17 monolayer) as a function of temperature, we show that thesteepnessof the hindered translation potential well is different for CO vibrations up, down, and along the step edge, following the trend: down the stepsup the stepsalong the steps. There is no coverage dependence in the COast; 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 150ndash;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.
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