A model for the adsorbed state

In order to discuss the factors which relate rates of desorption of atoms and molecules to concentrations of adsorbed species, we require a fairly detailed description of the adsorbed state. We do not possess the knowledge necessary for a truly quantitative account, but will adopt a simple model of the overlayer which will permit us to develop the discussion in a semi-quantitative way. In this manner, we will be able to discover the important ingredients of the problem and even at times to obtain results which are in fair agreement with experiment. However, we ought to recognise at the outset that the simplicity of our model will severely limit its value and, indeed, there will be occasions when we will find it difficult even to maintain self-consistency. 

We assume that the chemisorption of hydrogen, oxygen and nitrogen entails dissociation into atoms according to the equation 

We make the further assumption that the surface is crystallographically perfect and isotropic, so that no matter in which direction the adatom moves parallel to the surface, it experiences the same variation in potential (see Fig. 1). 

When the temperature is sufficiently low, the probability of an adatom s surmounting the potential energy barrier between adjacent sites will be negligible and then each adatom occupies an adsorption-site, of which there are Ns per unit area. When RT is large relative to the barrier height, adatoms are free to translate over the surface without sensing its atomic 

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