The complex behaviour of the CO/Pd(110) adsorption system was monitored via high-resolution X-ray photo-electron spectroscopy of both the C 1s and the surface core-level shift of the Pd 3d features, from initial exposures to a monolayer coverage. For low exposures (<0.45ML) the surface core-level shifted from −0.5 to +0.43eV. The C 1s emission first appeared as a low-energy band at around 284.8eV; characteristic of CO in high-coordination sites, and indicating that reconstruction began at the initial exposure. Upon increasing the exposure up to saturation at room temperature (CO coverage of 0.75ML) with its (4 x 2) CO low-energy electron diffraction structure, a second higher binding-energy band attributable to two-fold bridged molecules appeared and became dominant. Concomitant with this was a further change in the surface core-level shift to +0.63eV. The features observed for a CO coverage of 0.75ML coverage were fully consistent with the model of CO adsorption on a (1 x 2) missing-row reconstructed surface where the C 1s peaks at 286.3 and 286.1eV binding energy were assigned to two-fold bridge sites on the ridges and in the grooves, respectively. In the process of reconstruction, the latter appear before the former. At low temperatures the (2 x 1)p2mg “herring-bone” CO overlayer on the unreconstructed (110) surface was characterised by a large surface core-level shift of +0.94eV and a single C 1s feature at 286.3eV, proving that this structure involves CO adsorbed in two-fold bridge sites.

High-Resolution Core-Level Photoemission of the CO-Induced Pd(110) Surface Reconstruction. M.G.Ramsey, F.P.Leisenberger, F.P.Netzer, A.J.Roberts, R.Raval: Surface Science, 1997, 385[1], 207-15