Measurement of photoreactions with superimposed thermal reactions

3 Measurement of photoreactions with superimposed thermal reactions 

In the case of photochemical reactions superimposed by fast thermal reactions during the time of measurement the irradiation is cut off and the irradiation time stopped. But the thermal reaction proceeds simultaneously and the overall reaction time will exceed the irradiation time. This principle of two time scales will increase problems with kinetic analysis. For this reason one tries to avoid interruption of the irradiation for such photochemical reactions. 

A set-up is therefore required in which absorbance is measured without interruption of the irradiation. Then the reaction time for irradiation and that for the overall reaction can be taken equally. This fact is a prerequisite in photokinetic analysis. These isochronic conditions are achieved by the setup given in Fig. 4.10. 

The sample is placed between two monochromators. Therefore the stray light of the irradiation source is reduced drastically. For this reason it is not necessary to interrupt the irradiation during the measurement of absorbance. 

Both the monochromators are adjusted to the same wavelength, the reference is taken, and the absorbance of the sample is measured. Afterwards the next wavelength is selected. In this type of measurement the irradiation times differ for the various wavelengths. Therefore the data points have to be synchronised as explained in Fig. 4.6. 

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