Intensity quenching, 394 Kinetic spectroscopy

The spectral-kinetic parameters of the narrow band at 375 nm enable its confident identification as Eu + luminescence, which is confirmed by emission of synthetic BaS04 artificially activated by Eu (Fig. 5.15a). Such emission was also detected and interpreted by steady-state spectroscopy (Tarashchan 1978). It is interesting to note that very often such a band is absent in natural barite and appears only after heating in air at 600-700 °C (Fig. 4.31b). Such a transformation is reversible, at least partly. Under X-ray excitation the intensity of the UV band diminishes, and a new blue-green emission appears (Fig. 5.16). This shows some kind of transformation, which takes place in the barite lattice under these conditions. Several possibihties exist. It is possible that in barite the luminescence is quenched by the components with high-energy phonons. The water and organic matter may represent the latter. They are removed after... 

The lack of excited state involvement for 21 is in contrast to the behavior in systems that have substantial equilibrium concentrations of the syn conformer. To examine the effect that conformation has on ketene growth kinetics, 4-diazo-3-isochromanone (22), ° a cyclic analogue (phenyl version) of 21 that is locked in the syn conformation, was also recently studied by TRIR spectroscopy. In this case, a carbene IR band is observed at 1686 cm that decays with a lifetime of 526 50 ns in Freon-113. The ketene IR band at 2116 cm, however, in dramatic contrast to the data observed with acyclic diazocarbonyl 21, is produced faster than the experimental time resolution. Ketene 24, therefore, is formed entirely from a non-carbene source, presumably the excited state of 22 (Scheme 8). In agreement with this hypothesis, oxygen and methanol quench carbene 23, but they leave the initial intensity of the ketene IR band unaffected. These... 

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