Kinetic examinations of fast reactions

It was seen earlier (Subsection 5.4.3) that at least the spirit of the fluctuation-dissipation theorem can be utilised in chemical kinetics, since rate constants can be evaluated from equilibrium fluctuation measurements. 

According to the classical approach of experimental reaction kinetics, the reagents have to be mixed, then the temporal changes of the components have to be followed. The classical methods are not appropriate for the study of fast reactions, since the reaction is much more rapid than the mixing. 

From the theoretical point of view measurements based on fluctuation phenomena are better, since the equilibrium fluctuation may be interpreted as spontaneous perturbation and relaxation. Consequently, kinetic information is available without perturbing the system externally. In practice, chemical fluctuation measurements refer to small systems, therefore the problem is discussed in this section. Fluctuation measurements can be classified as follows (Romine 1976) indirect measurements, and direct measurements, which may be based on the time representation of fluctuations, or the frequency representations of fluctuations. 

The indirect measurements are derived from static not kinetic measurements, where the effect of chemical fluctuations appears as a disturbance. It is well-known (e.g. Gordon, 1969) that the connection between the function describing the shape of the spectrum line and the associated physical quantity 

The direct measurement of fluctuations seems to be feasible, given current and anticipated measurement techniques. The measurement of optically active systems is particularly favourable, since laser-based absorbance measurements, fluorescence spectroscopy, circular dichroism, rotary dispersion and Raman spectroscopy are all avariable. 

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