CO adsorption on Pt(1 1 0) investigated by X-ray photoelectron diffraction

The adsorption of CO on a Pt(1 1 0) 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, Tf, resulted in CO layers of increasing coverage. For Tf>350 K, a disordered CO layer was obtained, characterized by a (1×1) LEED pattern, but also by an increasing intensity of inter-molecular scattering peaks in C 1s XPD with increasing CO coverage. This behavior appears to indicate a rising degree of short range order in the adlayer. A well-ordered (2×1)-CO pattern at a coverage of 1.0 was obtained for final temperatures below 300 K. XPD measurements provided evidence of 22° tilted CO molecules, with the projected tilt parallel to the [0 0 1] azimuth of the Pt(1 1 0) surface. The latter is consistent with a p2mg symmetry of the (2×1)-CO structure. A second well-ordered c(8×4)-CO pattern was prepared upon adsorption at T<240 K, which was characterized by an O 1s doublet, assigned to on-top and bridge adsorbed CO, at a total coverage of 1.09. The corresponding XPD C 1s intensity distribution, measured over a large solid angle, indicated tilted and perpendicular CO, and inter-molecular scattering peaks distinctly different from those of the (2×1)p2mg-CO layer. A structure model for the c(8×4)-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 (2×1)p2mg and c(8×4) layers. Good consistency between the experimental and theoretical C 1s angular intensity distributions was found, involving both tilted and perpendicular CO.