Transient absorption spectroscopy excitation sources

In the previous sections, it has been shown how powerful the time-resolved fluorescence techniques are in real time probing of photoinduced processes and in allowing the determination of reaction rates from fluorescence lifetimes. The present section is devoted to the method of UV/vis transient absorption spectroscopy, which is a key method in probing non emissive species and is thus crucial to detect photoreaction products or intermediates following optical excitation of molecules in their electronic excited states. When carried out on short time scales, i.e. with femtosecond to subnanosecond excitation sources, fluorescent species can also be detected by their stimulated emission. Combining time-resolved fluorometry and transient absorption spectroscopy is ideal for the study of photochemical and photophysical molecular processes. 

Fig. 8. Transient absorbance change at 700 nm (left panel), 810 nm (center) and 690 nm (right) in a PS-I particle induced by 1.5-ps, 610-nm pulses (after changes due to excited antenna chlorophylls had been subtracted). Figure source Wasielewski, Fenton and Govindjee (1987) The rate of formation of [PTOCT Aq ] in photosystem I particles from spinach as measured by picosecond transient absorption spectroscopy. Photosynthesis Res 12 184-186.

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