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 t hat 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 mo del comprising linearly bonded CO molecules on top sites and ''bridge' '-bonded (in terms of internal stretch frequency) CO molecules in thre efold (fcc) sites is proposed. From the line position of the nu(C-O) m ode of linearly bonded (CO)-C-13-O-16 molecules coverage-dependent che mical shifts of this mode are determined. Also, relative changes of th e 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 prono unced 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.