Coadsorption of Cs and CO on Ru(001): Formation of Cs + CO Islands and Isotope Exchange of the CO Located Inside the Islands

H. Kondoh, H. Orita, and H. Nozoye
[J. Phys. Chem., Vol.99, pp.8790-8798, 1995]

The CO adsorption on Cs-covered Ru(001) surfaces has been studied by means of temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). When CO molecules adsorb on the Cs-preadsorbed surfaces at 85 K, two-dimensional Cs + CO islands are formed for Cs coverages lower than 0.25, which exhibit p(2 x 2) LEED patterns irrespective of both Cs and CO coverages. The p(2 x 2) Cs + CO island has an almost constant [CO]:[Cs] stoichiometry of 2:1 at Cs coverages below 0.2. This stoichiometry changes to 4:3 when the islands cover the whole surface (q_Cs = 0.25). The local density of Cs in the island is 0.25 independent of Cs coverage. The C-O stretching frequency of the CO species located outside the island shifts from 2075 to 2000 cm^-1 in proportion to Cs coverage. The C-O stretching mode of the inside CO species appears at much lower frequencies (1800-1580 cm^-1), which depend on the [CO]:[Cs] ratio. The former and the latter frequency shifts are associated with a weak long-range and strong short-range effect of Cs, respectively. The isotope exchange between the isotope-labeled CO species occurs exclusively inside the islands. The temperature onset of the isotope-exchange reaction is estimated to be 450-500K as shown in Fig. 1. This is comparative to the temperature onset of the desorption from the 2:1 state, while it is lower by 100 K than that from the 4:3 state, which results in a distinct difference in the exchange fraction for these states. The formation mechanism of the p(2 x 2) Cs + CO island is discussed based on the recently proposed models of the alkali + CO coadsoption process.

Fig. 1. Change of the full width of the half-maximum (fwhm) for the C-O stretching mode of pure ¹²C¹⁶O (solid circles) and of a 1:1 mixture of ¹²C¹⁶O and ¹³C¹⁸O (open circles) adsorbed on Cs/Ru(001-(theta_Cs = 0.24) as a function of the flashing temperature. At the top of the figure, the ratio between the two fwhm's (fwhm(mixture)/fwhm(pure)) is also plotted (solid triangles). The ratio starts to decrease at 450 K.