Ultraviofet photoekction spectra from Rh~(C0)16 and from CO adsorbed on a Pd(l11) single crystal surfzce evhtblt cIose slmllarities which become further evident from a comparison of vibrational and thermal properties. These results demonstrate the localized character of the chemisorption bond and justify cluster approximations for its theoretlcal description.
The metal particles used m heterogeneous catalysis are of rather small size and therefore chemisorption studies with systems consistmg of only a few metal atoms with defined geometry would be of some practical interest. On the other hand various attempts are currently made to describe the chemisorption bond in terms of a "surface molecule", i.e. by coupling the adsorbate wavefunctions to only 3 very hmited number of neighbouring sslrface atoms [ I] or by modeling it with (fictitious) small clusters which then for example may be treated by the SCF Xa technique [2]. Such a procedure is justified in part by the fact that calculations of the electronic energy levels of clusters consisting of only a few metal atoms reveal already close similarities with the corresponding bulk band. structures of infinite crystals [3]. On the other hand Muetterties [4] su=ested recently that cluster compounds might be attractive models of metal surfaces for chemlsorption and catalysis.
Carbon monoxide appears to be a suitable candidate for studying the question of the minimum cluster size for the following reasons: it is undoubtedly the most ubiquitous hgand in organometallic chemistry and polynuclear metal carbonyls have been extensively explored in recent years IS]. Moreover the chemisorption of this molecule has already been studied with a large variety of clean single crystal surfaces and it was found that (at least with the fee metals) the bond strength and electronic properties are only slightly