A FT–IRAS study of the vibrational properties of CO adsorbed on Cu/Ru(001). I. The structural and electronic properties of Cu

Abstract

We have used high resolution vibrational spectroscopy (FTIR) and carbon monoxide as a molecular probe to investigate the structural and electronic properties of well annealed copper layers adsorbed on a Ru(001) substrate. Adsorption of CO on annealed Cu–Ru (001) surfaces (θCu<1) at 85 K results in occupation of both Ru and Cu sites which exhibit characteristic C–O stretching frequencies as a function of CO coverage. The latter also indicates formation of Cu islands for 0.25<θCu<1 and chemical modification of the copper film by the underlying Ru, which leads to increased Cu–CO backdonation. From IR line shapes of the C–O stretch from CO-√3×√3-R30° on Ru sites, domain sizes are estimated for the bare Ru patches. Weak chemical modification is still observed for a 3 ML thick Cu film, whereas a 8 ML thick film exhibits all properties of bulk Cu(111), i.e., a coverage dependent red shift (2077–2075 cm−1), a (1×1) LEED pattern and characteristic CO-TDMS features. In contrast to the modification of Cu by Ru, there is little indication of a copper-induced change in the Ru–CO backdonation. The observed decrease in desorption temperature of CO–Ru TDMS features rather is related to a suppression of formation of an ordered √3-CO structure. The mobility of CO from Cu to Ru sites is found to be high and diffusion of CO occurs between 120 and 175 K. We finally observe vibrational coupling between neighboring CO–Cu and CO–Ru which leads to significant intensity transfer and frequency shifts for high θCu, i.e., small size of open Ru patches.