Adsorption and Thermodesorption Spectra

The stage of adsorption is the simplest elementary process among the other surface processes. It can be both a main process in adsorption and one of the stages of complex interface process. At least one of the adsorption or desorption stage is always presented in any surface process. In the theory of desorption process, the AC was introduced independently for the mono-and bimolecular desorption processes by different authors [107,108] in 1974. In both papers the quasi-chemical approximation has been used. Flowever, actual computations [107] have been performed at e — 0 (the collision model). They have shown that TDS slitting is caused even by a slight repulsion e 0.05 des. The expressions obtained for the desorption rates have been applied to TDS computations for H2/W(100), CO/W(210), and N2/W(100) [109,110]. 

Some important data concerning the role played by the effects of the correlation and AC interaction with the neighbors on the TDS appearance have been obtained in other work [114]. TDS splitting is due to the correlation effects of the interacting adspecies, therefore the splitting is absent if the mean field approximation (MFA) is used. For the TDS to split, the correlation effects should be supplemented with a sufficient difference between e and e since no splitting occurs at small differences (as in Ref. [107]). The magnitude of the parameter e is as important as that of e. The TDS for CO/W(210), CO/Mo(100), Cs/W and CO on a number of metals have been described qualitatively [124,125], 

The importance of considering the correlation effects is a principal for the adsorption and desorption rates in the case of dissociating molecules [126]. If the correlation effects are absent this violates the self-consistent describing of the system s kinetic and equilibrium characteristics. 

The kinetic theory of interface processes, including the adsorption one, on heterogeneous surfaces was constructed in Refs. [80,85,89]. An example of an application of the theory taking into account both lateral interactions and surface heterogeneity is the TDS for H2 on Ir(110)-2 x 1 described in Refs. [127,128]. The elementary lattice consists of three sites (Fig. 8.8) one strong (site of the first type) and two weak (sites of the second and third types). The sites of the third type are separated only formally for an easier presentation of the distribution function - the molecular characteristics of these sites are the same as those of the sites of second type. 

This suggests the existence of strong repulsions between adsorbed atoms localized on weak sites and a strong attraction between atoms on strong sites. Because the strong sites are isolated, the attraction between adsorbed atoms localized on these sites involves only the third coordination shell. It should be noted that the authors reporting the experimental TDS [129] have commented upon these curiosities of peak evolution and have drawn some conclusions about the lateral interactions. However, they have not arrived at any quantitative estimates. 

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