Articles you may be interested in Electron spin echo detection of the microwave-induced recombination of transient radical pairs produced in photochemical reactionsElectron spin resonance emission was observed from some photochemically produced semiquinone radicals in liquid isopropanol. It is proposed that the dynamic electron polarization is due to the optically spin polarized triplets of the parent quinones and their subsequent hydrogen abstraction reaction with retention of polarization in the resultant semiquinone radicals. Theoretical calculations have been carried out using typical zero field parameters for aromatic triplet molecules and the established assumption that the intersystem crossing rates to the three sublevels are unequal. Calculations reveal significant spin polarization of the triplets randomly oriented in an arbitrary magnetic field. Further treatment shows that when the triplet depolarization via relaxation is comparable to the chemical hydrogen abstraction rate, the resultant radical retains the polarization and is thus in the emissive mode.

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The photolysis of 1,4-benzoquinone, duroquinone, and 1,4-naphthoquinone in isopropanol and in dioxane leads to the observation of e.s.r. spectra of the corresponding semiquinone radicals. The rates of the selfdisproportionation of these radicals were measured by the rotating-sector technique. In the case of 1,4-naphthoquinone, unexpected e.s.r. emission was initially observed by photochemically induced dynamic electron polarization of the semiquinone radicals. Photolysis of anthraquinone in isopropanol gives e.s.r. spectra of the anthrasemiquinone radical anions. Their rate of decay was measured and a mechanism for the decay was suggested. The neutral anthrasemiquinone radical, however, was observed when photolyzed in dioxane.La photolyse de la benzoquinone-l,4,duroquinone et naphtoquinone-1,4 dans I'isopropanol et dans le dioxane, permet d'observer les spectres r.p.e. des radicaux semiquinone correspondants. Les vitesses d'autodismutation de ces radicaux ont kt6 mesurees par la technique du secteur tournant. Dans le cas de la naphtoquinone-1,4, on a observe initialement un spectre d'emission r.p.e. inattendu explique par une polarisation electronique dynamique, induite par voie photochimique, des radicaux semiquinone. La photolyse de l'anthraquinone dans l'isopropanol donne des spectres r.p.e. des radicaux anions anthrasemiquinone. Leur duree de vie a ete mesuree et un mecanisme de leur extinction suggere. Cependant, le radical neutre anthrasemiquinone a ete observe par photolyse dans le dioxane.[Traduit par le journa(] Canadian Journal of Chemistry, 50,3052 (1972)

. 62, 403 (1984).Singlet and triplet benzyl radical pairs have been generated on silica gel by photolysis of a benzyl phenylacetate, a dibenzyl ketone, and a dibenzyl sulfone. The extent of geminate radical recombination has been measured and requires that translational motion of radicals occur on the silica gel surface. This motion was affected by the radical pair multiplicity and the photolysis temperature, but was relatively insensitive to the state of hydration of the silica gel and the presence of coadsorbates. The presence of certain rearranged starting materials, which are not formed in solution, amongst the products from photolysis of both dibenzyl ketone and dibenzyl sulfone on silica gel indicates the restrictions on radical movement on the surface on the shorter tirnescale of the benzyl-phenylacetyl and benzyl -benzyl sulfonyl radical pairs.

The diffusion coefficient of hydrogen in crystalline silicon, obtained from recent profiling experiments such as nuclear resonance retention and secondary-ion mass spectroscopy, is 3–9 orders of magnitude smaller than the previously accepted value measured by Van Wieringen and Warmoltz in 1956. Here we point out several items often overlooked in the analysis of profiling measurements. A limited flux model is proposed to explain the observed results. Predictions by the model are supported by further experiments.

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