Recombinant desorption angular distributions

There have been a large number of measurements of angular distributions in desorption which show sharply peaked distributions and these have recently been reviewed by Kislyuk [44]. In some cases the products of reaction on fcc(l 1 0) surfaces are found to be peaked at an angle to the surface normal along the [001] azimuth (Fig. 9) notably for N2 produced by NO [77, 78] or N20 [79] decomposition, C02 formed by CO oxidation [80, 81] and CO formed by C + O recombination [82]. Sharply peaked distributions indicate a repulsive energy release which lies at an angle to the surface normal [83]. This occurs either because reaction takes place on (111) facets on the reconstructed (1x2) missing row surface (e.g., CO formation on Pt(l 1 0)-(l x 2) surface [82]) or, as in the case of N20 decomposition, because the symmetry of the transition state creates a repulsion which is directed away from the surface normal [84, 85]. 

Madey and Netzer have examined the adsorption of HjO on Ni(lll) at 80 K, as well as the effect of preadsorbed oxygen, by ESDIAD (electron stimulated desorption ion angular distribution), TPD and LEED. At room temperature, water does not adsorb on Ni(lll). Water adsorbed on Ni(lll) at 80 K desorbs at 170 K (from the first monolayer) and 150 K (from an ice multilayer) without evidence for any decomposition products. For oxygen predosed Ni(lll) surfaces there is an extra peak at 275-300K in HjO TPD spectra, which is due to the interaction between water and adsorbed oxygen. The authors suggested the formation of OH above 120 K which recombined above 200 K to desorb as water and oxygen. Similar results have also been reported on Ni(110)5, Ni(100)5 and stepped Nidll). ... 

It is also the case that measurements of gas-phase molecular distributions are used to analyze the products of recombinative desorption. In this case, as indicated in Fig. 16, the angular distribution is observed to peak along the normal to the surface, being proportional to cos"6 with n>2 and sometimes up to 12. Highly excited vibrational states are often found (Kubiak et al. 1984 Brown and Bernasek 1985 Mantell et al. 1986 Karikorpi et al. 1987 Harris et al. 1988b). We will have much more to say about such distributions in Section VI. The review by Comsa and David (1985) deals in detail with this subject. 

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