Structure of CO on the Pt(112) Stepped Surface: A Sensitive View of Bonding Configurations Using Electron Stimulated Desorption.

摘要

The structure of chemisorbed CO on the steps of Pt(112) Pt(3(111)x(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 a 3 pi - 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 x 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 x 1) order is observed by LEED. (JES)