Rate constant for very fast reactions

Wiesner s expression used different symbols, but this is not important.) This expression strictly holds only for a first-order reaction and Vetter [559] provides a more general expression. However, the above expression is sufficient for most simulation purposes. The equation for fi holds in practice only for rather large values of the rate constant for small values below unity, fi becomes greater than the diffusion layer thickness, which will then dominate the concentration profile. At the other end of the scale of rate constants, for very fast reactions, // can become very small. The largest rate constant possible is about 1C)1 ° s 1 (the diffusion limit) and this leads to a fx value only about 10 5 the thickness of the diffusion layer, so there must be some sample points very close to the electrode. This problem has been overcome only in recent years, first by using unequal intervals, then by the use of dynamic grids, both of which are discussed in Chap. 7. 

The fastest reactions in solution are limited in their reaction rates by mass transfer. For most bimolecular reactions this limitation arises because the two reactants must diffuse together in order to form a reactant pair. Therefore it is interesting to estimate the magnitude of the rate constant for a diffusion-con-trolled reaction and to compare it with experimentally determined rate constants for very fast reactions. The treatment presented here applies to bimolecular reactions involving either molecules or ions. 

Another three orders of magnitude decrease in the laser pulse width brings us to femtosecond pulses with which rate constants for very fast reactions, such as direct dissociations on repulsive surfaces, can be measured (Zewail, 1991). However, the resolution now degrades to 1000 cm . The complex experimental set-up required for this work has been described by Zewail and co-workers (Felker and Zewail, 1988 Khundkar and Zewail, 1990). 

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See also in sourсe #XX -- [ ]

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