Quasiequilibrium theory QET

The most widely accepted theory of unimolecular reactions of polyatomic ions remains the quasiequilibrium theory (QET) [591, 720, 883], which is a treatment in the spirit and tradition of absolute reaction rate theory. Thus it is assumed that the rate of reaction of an ion is slow relative to the rate of energy flow among its vibrational modes and that each reaction may be described as a motion along a reaction coordinate which is separable from all other internal coordinates and which passes through a critical configuration (the transition state ). It is further assumed that ions formed in excited electronic states rapidly redistribute such electronic energy over vibrational levels of the ground electronic state. One further assumption is necessary, and that is that the time involved in the ionization process is short compared with subsequent reaction times. The QET model is taken as the theoretical basis of this review. QET leads to 

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Statistical theories treat the decomposition of the reaction complex of ion-molecule interactions in an analogous manner to that employed for unimolecular decomposition reactions.466 One approach is that taken by the quasiequilibrium theory (QET).467 Its basic assumptions are (1) the rate of dissociation of the ion is slow relative to the rate of redistribution of energy among the internal degrees of freedom, both electronic and vibrational, of the ion and (2) each dissociation process may be described as a motion along a reaction coordinate separable from all other internal... 

Figure 43. Schematic diagram of CH3+ fragment ion angular distribution from He (2 S) + CH4 compared with quasiequilibrium theory (QET).

Perhaps the point to emphasise in discussing theories of translational energy release is that the quasiequilibrium theory (QET) neither predicts nor seeks to describe energy release [576, 720], Neither does the Rice— Ramspergei Kassel—Marcus (RRKM) theory, which for the purposes of this discussion is equivalent to QET. Additional assumptions are necessary before QET can provide a basis for prediction of energy release (see Sect. 8.1.1) and the nature of these assumptions is as fundamental as the assumption of energy randomisation (ergodic hypothesis) or that of separability of the transition state reaction coordinate (Sect. 2.1). The only exception arises, in a sense by definition, with the case of the loose transition state [Sect. 8.1.1(a)]. 

In the ion-molecule community RRKM theory is widely known as quasiequilibrium theory (QET) see Ref. [6],... 

The subsequent step, unimolecular dissociation, has been explained fully in terms of RRKM and quasiequilibrium theory (QET) theories (see Section 6.7). 

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