Recombination in the presence of atomic desorption

At sufficiently high temperature, the desorption of adatoms becomes appreciable. This is not only because the high temperature reduces the contrast in the exponential factors of D and D2, but because the sparse coverage increases the relative significance of Dx by virtue of its dependence on 0 (as opposed to 02 ). Any factor (e.g. low pressure) which favours very low values of 0 will contribute to the relative importance of Dx. 

Reference to eqn. (66) shows that the growth in the d NgZ 1 term with continued increase in temperature will reduce 7 and this effect is illustrated in Fig. 5 for the hydrogen—tungsten system (T 1500 K). Eventually, 7 will become negative and when this happens, the parameter is no longer a convenient way of accounting for the processes occurring. 

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