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

我们研制了微电脑控制的纳秒和皮秒漫反射激光光解系统，可以测量激发态的紫外吸收光谱以及不透明和散射材料中的瞬时化学物种，并分析它们的动态行为。纳秒系统在这个国家是独一无二的，而皮秒研究在本项目中是第一次进行。利用这些系统，我们演示了各种散射物质的光物理和光化学过程，揭示了影响吸收光谱形状的因素，并在此基础上研究了瞬变物质的衰减曲线。为了获得准确可靠的数据，我们提出了一个经验公式Ao=AAexp(BA)，这里Ao是校正后的吸光度，A是在漫反射激光光解测量中用作测量的吸光度。这种校正方法似乎是卓有成效的，例如，校正后的二苯甲酮微晶的T-T吸收光谱与其对应的单晶的T-T吸收光谱非常相似。为了解释这个方程的物理意义，我们用凯贝尔卡理论和威尔金森方法对漫反射现象进行了模拟。以散射系数、基态和暂态的摩尔消光系数和激发强度为参数。我们得出结论：%吸收与瞬时吸收之间存在一定的关系，其中基态摩尔消光系数和激发强度可以调节。这意味着漫反射激光光解在固体有机光化学中是很有前途的，因为透射式激光光解对于溶液体系是卓有成效的。

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.