Redhead approximation

Redhead [89] gives the approximate equation EjRTm - ln(A7) //3)- 3.64. Check the usefulness of this equation by comparing with the answers to Problems 5 and 6. 

Expression (2-16) is approximately correct for first-order desorption and for values of vt[ between 108 and 1013 K l. It is very often applied to determine from a single TDS spectrum. The critical point however is that one must choose a value for v, the general choice being 1013 s, independent of coverage. As we explain below, this choice is only valid when there is little difference between the entropy of the molecule in the ground state and that in its transition state 125, 27], The Redhead formula should only be used if a reliable value for the prefactor is available ... 

Finally, a caution about the Redhead equation. The requirement n=l is not explicit in the equation. For this reason nothing prevents that the equation is applied in situations where the order of the desorption is unknown or, even worse, when the order is known to be different from 1. To describe E obtained in this fashion as an approximation is an understatement, completely wrong is a better description. 

For first-order desorption kinetics (i.e., n = 1), Redhead [331] gave an approximate relationship between the activation energy Ea and the temperature Tp at which the desorption rate is a maximum... 

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