The structure of CO on the Pt(112) stepped surface—a sensitive view of bonding configurations using electron stimulated desorption

Abstract

The structure of chemisorbed CO on the steps of Pt(112) {Pt[3(111)×(001)]} was monitored as a function of coverage by the digital electron stimulated desorption-ion angular distribution method (ESDIAD), digital low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD). The ESDIAD method applied to the desorption of an electronically excited, neutral CO species (the metastable a3 π-CO state) avoids the influence of image potential effects on the trajectory of desorbing species, yielding true desorption angles, equivalent to Pt-CO bond orientations. CO adsorbs exclusively on step sites at low coverage with a 20° ‘‘downstairs’’ tilt from the [112] direction (designated ‘‘0°’’). LEED indicates (2×n) order (i.e., two-fold order along the steps but no order up/down the steps) exists when the steps are half-filled (about 0.19 ML). At 0.24 ML, CO is still adsorbed only on the step sites but one-dimensional CO–CO repulsions between nearest neighbors result in CO tilting along the steps. Terrace CO (+13°) adsorption is observed above 0.24 ML before all the step sites fill. All step CO molecules reorient with new tilt angles up (0°) and down (−38°) the steps when the steps sites saturate (0.53 ML), and (3×1) order is observed by LEED. The surface reorders at saturation (0.79 ML) to (2×1) order. Changes in CO desorption energies, pre-exponential factors and sticking coefficients with coverage in the literature for Pt[n(111)×(001)] surfaces can be directly correlated with structural transformations on the partially filled steps due to CO–CO repulsions.