The (2CO+O)(2x2)/Ru(001) layer: preparation, characterization, and analysis of interaction effects in vibrational spectra

The coadsorption system CO+O/Ru(001) has been investigated using TDS, IRAS, LEED and XPS. Our work goes beyond preceding investigations in that we show that it is possible to accommodate two CO molecules in an O(2 x 2) unit cell using very high (similar to 10(5) L) CO exposures. The CO sticking coefficient shows an abrupt decrease by a factor of 60 at the conventional CO saturation coverage of 0.25 ML. A structure model comprising linearly bonded CO molecules on top sites and ''bridge''-bonded (in terms of internal stretch frequency) CO molecules in threefold (fcc) sites is proposed. From the line position of the nu(C-O) mode of linearly bonded (CO)-C-13-O-16 molecules coverage-dependent chemical shifts of this mode are determined. Also, relative changes of the vibrational polarizability alpha(v) of the linear (CO)-C-12-O-16 nu(C-O) mode and of the dielectric constant epsilon = 1 + alpha(e) theta U-o of the adlayer, as an extra CO molecule is squeezed into the O(2 x 2) Unit cell, can be determined: by the presence of the CO molecules in hollow sites the linear (CO)-C-12-O-16 nu(C-O) mode suffers a pronounced chemical blue-shift of 25.4 cm(-1). alpha(v) is reduced by 23%, and epsilon is increased by 12%. The nu(M-CO) mode is shifted by 15.5 cm(-1) to lower energies. (C) 1997 Elsevier Science B.V.