The effect of precursor states

In Sect. 3.2.2, the effects of precursor states on adsorption kinetics have been discussed. Since even the earliest adsorption experiments show evidence for the influence of weakly bound intermediate states, following the principle of microscopic reversibility it might be expected that desorption kinetics would show the influence of such species. However, desorption experiments are usually carried out at considerably higher temperatures and average lifetimes in such states will be much lower. 

Shanabarger [294, 295] was the first to use a model based on such effects, studying the systems H2/Fe (films) and H2/Ni (films). He proposed that hydrogen is dissociated on these surfaces and that the formation of a molecular precursor bottle-necks the desorption process. This results in isothermal desorption techniques measuring only the desorption energy of this precursor. This is obviously a misconception as the rate-limiting step in the process has to be desorption from the chemisorbed state and thus kinetics are measured principally from this state, although values may be affected by the precursor to a small extent. 

The first work to examine precursor effects in a quantitative manner was performed only recently by King [298]. It is based on the reversal of the kinetic model described in Sect. 3.2.2. The rate of transfer from the chemisorbed state to the precursor is given by 

Because of the reasons discussed earlier, it is necessary to write that 

F being a fraction since not all the molecules reaching the precursor desorb. By describing the possible events for a molecule in terms of probabilities, F was found to be given by 

---