Conformational studies of molecules in the excited triplet state by time-resolved Fourier-transform infrared spectroscopy

为了完成这个研究项目，一个具有异步时间分辨能力的傅里叶变换红外分光光度计与红外显微镜结合使用。使用红外显微镜是必要的，以观察激光诱导的光谱变化，从一个小点在固体样品松散聚焦激光脉冲照射。连续波Nd: YAG激光器在1064 nm处的基谱线和532 nm处的二次谐波在非线性晶体（BBO）中混合，产生355 nm处的三次谐波。在调q操作下，三次谐波的最高功率为15 mW，该操作以2 kHz的重复率提供持续时间为70 ns的脉冲。由此获得的三次谐波用于激发放置在红外显微镜样品台上的KBr盘中的分子。用上述分光光度计在室温下的测量中，观察到一些可分配给最低激发态的吖啶的红外波段。然而，对于测试的其他有机化合物，由于化学反应（可能是氧化），局部加热和样品蒸发的影响可以灵敏地检测到。为了抑制这些副作用，已经尝试用微型低温恒温器将样品在真空中冷却到120 K左右，该低温恒温器可以放置在红外显微镜的样品台上。干氮气或高压氩气可用作冷却剂。利用该装置，观察了各种芳香族化合物如二苯甲酮、4-苯基二苯甲酮和苄基苯胺的激光诱导红外光谱变化。即使在120 K时，也观察到局部加热的影响，而样品的蒸发和化学反应似乎被抑制到令人满意的程度。有必要将局部加热的影响与真正的光物理和光化学过程引起的光谱变化分开。

To perform this research project, a Fourier-transform infrared spectro-photometer with an asynchronous time-resolving capability has been used in combination with an infrared microscope. The use of an infrared microscope is mecessary to observe laser-induced spectral changes from a small spot in the solid sample irradiated by loosely focused laser pulses. The fumdamental line at 1064 nm of a continuous-wave Nd : YAG laser and its second harmonic at 532 nm are mixed in a nonlinear crystal (BBO) to generate the third harmonic at 355 nm. The highest power of the third harmonic is 15 mW under a Q-switched operation which gives pulses with 70 ns duration at a repetition rate of 2 kHz. The third harmonic thus obtained is used to excite molecules in a KBr disk placed on the sample stage of the infrared microscope. In measurements by using the above spectrophotometer at room temperature, some infrared bands assignable to acridine in the lowest excited triplet state have been observed. For other organic compounds tested, however, effects due to chemical reaction (presumably oxidation), local heating, and evaporation of the sample are sensitively detected. To suppress these side-effects, attempts have been made to cool the sample down to about 120 K in vacuum with a miniature cryostat, which can be placed on the sample stage of the infrared microscope. Either dry nitrogen of argon gas of high pressure is used as the coolant. By using this apparatus, laser-induced infrared spectral changes have been observed for various aromatic compounds such as benzophenone, 4-phenylbenzophenone, and benzylideneaniline. Even at 120 K,effects of local heating are observed, whereas evaporation of the sample and chemical reaction seem to be suppressed to a satisfactory degree. It is necessary to separate effects of the local heating from spectral changes arising from genuine phtophysical and photochemical processes.