CO ADSORPTION ON CLEAN AND C-COVERED, O-COVERED AND H-COVERED MO(110) SURFACES - AN IRAS STUDY

The adsorption of CO on clean and O-, C- and H-precovered Mo(110) surfaces has been studied using infrared reflection absorption spectroscopy (IRAS). For a low CO exposure (less-than-or-equal-to 2 L) at a sample temperature of 90 K, CO on a Mo(110) surface shows no IR features in the 1800-2200 cm-1 frequency region, and is thus likely bound to the substrate with the C-O axis tilted with respect to the surface normal. By an exposure of approximately 4 L, CO features at 2004 and 1888 cm-1 are evident, and interpreted to correspond to linear and bridging CO, respectively. Saturation CO on Mo(110) at 90 K shows a single peak at 2033 cm-1 which splits into two peaks at 2031 and 2014 cm-1 upon heating to 160 K due to the formation of ordered CO islands. Adsorption of O2 induces two CO IR peaks at 2062 and 1983 cm-1 which are believed to arise from CO molecules that have an indirect (via the Mo substrate) and a direct electronic interaction with O, respectively. CO on a C-covered Mo(110) surface displays an IR peak at 2085 cm-1. The invariant frequency of this peak at various C coverages indicates that the C forms two-dimensional islands which grow via a mechanism of expansion at the island edges. Coadsorption of H with CO on a Mo(110) surface results in a splitting of the linear CO band. The peak induced at 2008 cm-1 is attributed to CO adsorbed in the vicinity of H.