Energy accommodation during heterogeneous recombination

The extent to which the molecules formed by recombination are in thermal equilibrium with the catalyst is of fundamental interest for the light it sheds on the nature of the interaction with the surface at the instant of reaction. It is also of practical interest, particularly in the use of thermal probes for the determination of atom concentrations, where the need to take account of factors influencing energy transfer processes has not always been recognised. Fresh interest in the phenomenon has been stimulated by the demands of space technology for information on surface heating due to recombination during re-entry into the earth s atmosphere. 

The parameter used to characterise the energy transfer to the surface is the energy accommodation coefficient (]3) which is defined as the fraction of the total energy of recombination adsorbed by the catalyst. 

One of the objectives of the following numerical exemplification of this simple model is to show that a prerequisite of any discussion of energy accommodation in the recombination process is a knowledge of its mechanism. 

Substituting — 2AI7i, = D(X2) — AU2, (see Fig. 2), we obtain 0 = 1. When molecular adsorption is activated, the limiting value of /3 is given by 

When the adatom participating in the recombination derives from the adsorption of a molecule, then reaction (VIb) has brought about the 

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