Free radicals, absorption spectra lifetime

The absorption band around 520 nm is very similar to that of polystyrene excimer (2,3,5). The decay follows first order kinetics with a lifetime of 20 ns. The decay rate agrees with that of the excimer fluorescence and excimer absorption. The longer life absorptions, attributed to the triplet states and free radicals (2,5), were observed at wave lengths <400 nm, although the anionic species of polystyrene with the absorption maximum at 410 nm as seen in solid films (cf. Figure 5) was not observed. Figure 9 shows the absorption spectrum observed in the pulse radiolysis of CMS solution in cyclohexane. 

The absorptions at both 500 nm and 320 nm follow first order kinetics with a lifetime of 420 ns. This absorption species is neither the excimer of polystyrene nor free cationic species of polystyrene. Although the excimer of polystyrene has an absorption band around 500 nm, the lifetime is only 20 ns. Further the free cationic species of polystyrene should live for a longer time in this solution, and the absorption band should exist in a longer wavelength region (6). These considerations of lifetime and absorption spectrum lead us to conclude that the absorption spectrum shown in Figure 12 is due to the charge transfer-radical complex between polystyrene and Cl radical (2,4,17). A very similar... 

The absorption spectrum of 22 is nearly identical to the sum of the spectra of unlinked model compounds. The long wavelength band of the free base porphyrin is observed at 650nm in chloroform, whereas that of the zinc porphyrin is at 590nm. Excitation of a chloroform solution of 22 with a 15 ns pulse of 650 nm laser light leads to the formation of a carotenoid radical cation which can be detected by transient absorption spectroscopy (Figure 14). This ion arises from the charge separated state C -Pzh-P-Qa-Qb, and is formed with a quantum yield of 0.83. The lifetime of the species is 55/iS. 

Intermediates in the radiation chemistry of high polymers include ions and trapped electrons, radicals and excited states. Free radicals trapped after irradiation have been studied mainly by electron spin resonance (ESR) and in some cases by chemical methods and by ultraviolet or infrared spectroscopy. The detection of free radicals during radiolysis has been performed by pulse radiolysis and also by ESR. Trapped ions and radical-ions were characterized by absorption spectroscopy and thermoluminescence while pulse radiolysis allows their detection during irradiation. Excited states, owing to their very short lifetime, could be observed only by pulse radiolysis or by the measurement of the luminescence spectrum and decay time during steady irradiation. 

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