The (2CO+O)(2×2)/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(2×2) unit cell using very high (∼10⁵ 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 (fee) sites is proposed. From the line position of the ν_C-O mode of linearly bonded ¹³C¹⁶O molecules coverage-dependent chemical shifts of this mode are determined. Also, relative changes of the vibrational polarizability α_ν of the linear ¹²C¹⁶O ν_C-O mode and of the dielectric constant ε = 1+α_cθU₀ of the adlayer, as an extra CO molecule is squeezed into the O(2 × 2) unit cell, can be determined; by the presence of the CO molecules in hollow sites the linear ¹²C¹⁶O ν_C-O mode suffers a pronounced chemical blue-shift of 25.4cm^-1. α_ν is reduced by 22%, and ε is increased by 12%. The ν_M-CO mode is shifted by 15.5 cm^-1 to lower energies.