Development of time-resolved diffuse reflectance laser photolysis method and its application to solid photochemistry

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We have developed microcomputer-controlled nanosecond and picosecond diffuse reflectance laser photolysis systems, which make it possible to measure UV absoprption spectra of excited states as well as transient chemical species in opaque and scattering materials and to analyze their dynamic behavior. The nanosecond system is the unique one in this country, while the picosecond study was performed for the first time in the present project. Using these systems, we demonstrated that photophysical and phtochemical processes of various kinds of scattering material and revealed the factors affecting absorption spectral shape and rese as well as decay curves of transient species.On the basis of these experimental results. we proposed an empirical equation, Ao = aA exp(bA), in order to get accurate and reliable data.Here Ao is a corrected absorption and A is % absorption used as a measure in diffuse reflectance laser photolysis measurement. This correction method seemed to be fruitful, for example, the corrected T-T absorption spectrum of benzophenone microcrystals was quite similar to the corresponding one of its single crystal. In order to interpret the physical meaning of this equation, we performed a simulation of diffuse reflectance phenomena due to Kebelka's theory and Wilkinson's method. Scattering coefficient, molar extinction coefficients of the ground and transient states, and excitation internsity were examined as paprmeters. We have come to the conclusion that there is a definite relation between % absorption and transient absorption where molar extinction coefficient of the ground state and excitation intensity can be abjusted. This means that diffuse reflectance laser photolysis is promising for solid organic photochemistry as transmittance laser photolysis is fruitful for solution systems.