Absorption of reaction intermediates

Rg. 12. Time dependent absorption of reaction intermediates A to D observed upon UV-photopolymerization of TS-6 crystals at 270 K. AOD is the change in optical density of the sample (from Ref. )... 

This first step is probably photon induced, but we cannot rule out that the temperature rise, which will take place during irradiation, is also important. A temperature rise can also increase the efficiency of photochemical reactions [297]. It would be very difficult to calculate a temperature rise, because it is closely related to the absorption depth of the laser irradiation and depends on the lifetime and absorption of reaction intermediates. The lifetime is strongly dependent on the complexity of the molecules. The more complex the molecule, the longer the lifetime. In the condensed phase, as in the case of PI, such intermediates can last for time periods of the order of nanoseconds (laser pulse r 20 ns). The importance of this to the UV laser decomposition of PI lies in the UV absorption characteristics of free-radical intermediates. Their strongly delocalized electrons will result in a more intense absorption of the incoming radiation than that of PI itself. However, their contribution to the absorption will be determined by their stationary concentration, i.e., their rate of formation less their rate of disappearance. We do not have these data therefore we cannot calculate the temperature rise. 

The light absorption of reaction intermediates generated at the electrode surface can be monitored in transmission or in reflection mode, and this allows sensitive detection and/or quantitative kinetic studies. The sensitivity of this technique can be substantially improved by introducing cavity-ring-down cells [14] or multiple-reflection path cells. 

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See also in sourсe #XX -- [ ]

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