Recombination in the absence of atomic desorption

In this section, the temperature is never allowed to rise so high that direct desorption of adatoms plays a significant role. Because their recombination behaviours are so different, we consider separately the cases of (a) moderately strong molecular chemisorption, (b) weak or endothermic molecular adsorption and (c) exceptionally strong chemisorption. 

The energetics of this case are illustrated in Fig. 2(b) when molecular adsorption is still exothermic and in Fig. 2(c) when the adsorption is endothermic. 

In the case of very strong chemisorption [see Fig. 2(d)], the desorption of molecules is even more activated than the desorption of adatoms. At temperatures not high enough for appreciable desorption of adatoms, the very large value of — AU2 t ensures values of 6 close to unity. The virtual absence of molecular desorption rules out the Langmuir— Hinshelwood process, so that recombination is restricted to the Rideal— Eley mechanism, which is now activated, causing recombination to be 

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