Threefold-hollow adsorption

The c(4 x 2)-2CO structure observed20 at Ni(lll) at room temperature has CO occupying both fee and hep threefold hollow adsorption sites with a surface coverage of 0.5 ML. So as to maximise the 0-0 distance, the molecular axis is tilted away from the surface normal towards atop positions. Corrugation of the adlayer is attributed to a CO-induced buckling of the surface nickel atoms, which is manifested by height differences between adjacent CO molecules (Figure 8.6). 

V has been elucidated by x-ray adsorption spectroscopy [5]. Gold has an fee lattice, and the Au(lll) surface forms a triangular lattice with a lattice constant of 2.89 A. Copper atoms are smaller than gold atoms, and they adsorb in the threefold hollow sites (see Fig. 4.12), forming a triangular lattice commensurate with that of the substrate ... 

In a considerable number of cases both sets of modes have been observed in on-specular VEEL spectra, and the deduction has been made that the symmetry of the surface complex is Cs (or less) (145,146,151,152,160,162). The question remains whether this implies a twofold bridged adsorption site or a neighbor-induced asymmetry within an essentially C3 site, as already described. However, there are examples of species on Pt(lll) (150), Ni(ll 1) (117), and Cu(lll) (161) surfaces for which MSSR as applied to VEEL spectra clearly indicates C3v symmetry of the surface complex, without significant differences in the other frequencies as observed off-specular. These favorable cases may arise from particularly regular arrays of adsorbed species, the presence of which could very profitably be confirmed by LEED. We deduce that the CH3 adsorption sites are intrinsically C3v as far as the bare surface is concerned, i.e., on-top or threefold hollow in nature with the threefold axis of the CH3 group perpendicular to the surface. 

Laser-induced desorption via the DIET process is a structure-sensitive phenomenon. Firstly, we describe the recent results for adsorbed NO on Pt(l 1 1), since the adsorption structure of this system has been misunderstood for a long time. Adsorbed species giving rise to the 1490 cm-1 NO stretching vibrational mode had been believed to be adsorbed at bridge sites [34, 35]. Recently it has been shown that this species is adsorbed at the threefold fee hollow site. This problem was pointed at first using LEED analysis by Materer et al. [36, 37]. A similar problem is the occupation of the fee and hep threefold hollow sites in a ratio of 50/50 described by Lindsay et al. [38] on the basis of a photoelectron diffraction investigation of NO on Ni(l 1 1) at a coverage of 0.25 monolayer. 

The (d/z/ + dz, ) orbital relevant to the threefold hollow species is preferentially filled by electrons due to a strong eg character. Then, the threefold fee and hep hollow site adsorption is suppressed by the s-d hybridization in the alloy. For CO adsorption a similar discussion is made using the dyy orbital instead of the (dyy + dz, ) orbital and the bridge site adsorption is also suppressed. It is emphasized that the characteristic properties of this alloy are represented by the global picture. 

In this study four different adsorption sites have been considered, namely the twofold-bridge, the threefold hollow, the diagonal fourfold hollow and the aligned fourfold hollow sites, see Fig. 3. 

To study the adsorption on the twofold-bridge and threefold hollow sites we used a Cu9(5,4) cluster as shown in Fig. 4(a), where the numbers inside brackets indicate the number of metal atoms in the first and second layers respectively. To study the adsorption on the aligned-fourfold-hollow and diagonal-fourfold-hollow sites we used the same cluster but on an inverted position, that is, we used a Cu9(4,5) cluster as shown in Fig. 4(b). 

In light of the open questions related to CO adsorption/dissociation on Rh(l 1 1), Pery et al 314) carried out a systematic SFG/AES study of CO on Rh(l 1 1), at pressures from 10 to 1000 mbar and temperatures from 300 to 800 K. Figures 38a and b show a series of SFG spectra recorded at 300 K and a comparison of spectra at 10 mbar before and after the atmospheric pressure gas exposure. All spectra are dominated by a single vibrational peak at 2053-2075 cm typical of CO terminally bonded to a single Rh atom, with a small peak at about 1900 cm characterizing CO on threefold hollow sites (see, e.g., the 500-mbar spectrum). The intensity difference between the two peaks again points to the lower sensitivity of... 

A somewhat similar case of an ensemble effect was proposed for CO adsorption on a Pt25Ni75(lll) alloy surface [63]. Whereas CO adsorbs on-top of Pt atoms [64], on Ni(lll) it adsorbs in the threefold hollow sites [65]. The HREELS (high-resolution electron energy loss spectroscopy) peak associated with CO adsorbed on threefold Ni sites almost vanishes if the sample prepara-... 

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