CO adsorption on Pt(110) investigated by X-ray photoelectron diffraction

The adsorption of CO on a Pt(110) surface was studied by X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), and low energy electron diffraction (LEED). CO exposure during cooling the sample from 600 K to several final temperatures, T-f, resulted in CO layers of increasing coverage. For T-f > 350 K, a disordered CO layer was obtained, characterized by a (1 x 1) LEED pattern, but also by an increasing intensity of inter-molecular scattering peaks in C Is XPD with increasing CO coverage. This behavior appears to indicate a rising degree of short range order in the adlayer. A well-ordered (2 x l)-CO pattern at a coverage of 1.0 was obtained for final temperatures below 300 K. XPD measurements provided evidence of 22 degrees tilted CO molecules, with the projected tilt parallel to the [001] azimuth of the Pt(110) surface. The latter is consistent with a p2mg symmetry of the (2 x l)-CO structure. A second well-ordered c(8 x 4)-CO pattern was prepared upon adsorption at T < 240 K, which was characterized by an O Is doublet, assigned to on-top and bridge adsorbed CO, at a total coverage of 1.09. The corresponding XPD C Is intensity distribution, measured over a large solid angle, indicated tilted and perpendicular CO, and inter-molecular scattering peaks distinctly different from those of the (2 x l)p2mg-CO layer. A structure model for the c(8 x 4)-CO layer was developed. Single scattering cluster calculations were performed for real space models of CO layers at low coverage, and for the high coverage (2 x l)p2mg and c(8 x 4) layers. Good consistency between the experimental and theoretical C Is angular intensity distributions was found, involving both tilted and perpendicular CO. (C) 2001 Elsevier Science B.V. All rights reserved.