Molecular Activation and Deactivation

Strictly speaking, the processes (17.1a) and (17.1b) have to be described by the set of kinetic equations for the population of different energy levels of the reactant molecule AB considered in Section III.8. If we neglect (17.1b), both activation and deactivation are described by the kinetic equations (8.34) for the nonequilibrium distribution function a(E). To solve these equations, it is necessary to introduce explicitly the rate constant k(E, E ) of the transition between the energy levels E and E of the molecule AB. 

Until now, it is virtually impossible to evaluate the function k(E, E ) for polyatomic molecules. For this reason, the theory of collisional activation and deactivation is to a considerable extent based on hypotheses concerning the general properties of the function k(E, E ). The two alternative hypotheses substantially simplifying the microscopic kinetic equations are the strong-collision hypothesis and that of stepladder activation and deactivation [336, 339, 486]. 

According to the strong-collision mechanism each collision AB + M leads to deactivation of the active molecule whereas activation is the result of transitions E E for which the initial state is specified by the equilibrium distribution function. In other words, the mean square of the transfer energy ((AE) ) is assumed to be large compared to (kT). In this case, relaxation is described by a simple kinetic equation of the explicit form [Eq. (17.4)]. 

These two mechanisms refer to opposite limiting cases of activation. Relaxation with arbitrary coUisional energy transfer is to be used for the general case [Eq. (8.34)]. An approach of this kind is, for instance, appelid to the description of the deactivation of molecules produced by chemical activation [392] and of the activation at the low pressure limit [470, 487]. The latter will be considered briefly in connection with the discussion of the activation efficiency of different partners. 

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