A new method is introduced for the measurement of photochemical or photobiological reaction rates. Based on linear response theory, the method uses a randomly modulated light source rather than a pulsed one, and uses cross-correlation analysis to extract the desired kinetic information. In operation, a randomly varying light source illuminates the reaction of interest, and the resulting fluctuations in reactant (or product) concentrations are monitored. Mathematical or instrumental cross correlation of the light source variations with those in component concentration then produces the familiar kinetic decay curve. Because the new method produces only minute concentration fluctuations, it can measure reaction rates near equilibrium; moreover, equilibrium position can be shifted by superimposing on the fluctuating light intensity a dc level. Also, because cross correlation is used, the method enjoys high signal-to-noise ratios. In the present study, the utility and practicability of the new technique are demonstrated through examination of the recombination kinetics of photolytically generated iodine atoms.

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