An alternative explanation which overcomes this difficulty is that the fluorescence arises from de-excited OH radicals formed by radiation excitation of water which are themselves subsequently excited. Both the radiation emission and the H202 production would then be directly related to the rate of formation of OH radicals, but there would be no direct conversion of one to the other. Excitation of OH itself would imply a relatively high concentration of these species, and therefore such a mechanism would suggest that OH* is formed in an excited state by a reaction involving another (long-lived) species.In addition to the above features, the band at 4600 Á. requires comment. It is not possible to explain this emission in terms of any known excitation levels of water or OH, or other species likely to be present. If, however, we accept the "polaron" theory of Weiss,22 (22) J. Weiss, Nature, 186, 751 (1960).this emission could be associated with some reaction of the hydrated electron, such as recombination with the holes. Using Smith and Symons28 and Kuper24 models the emission energy of the hydrated electron is cal-culated26 to correspond to 4700 ± 200 Á. This accords with our observations of emission at 4600 Á. It does not, however, predict absorption at other valuesforwhich a more refined treatment is necessary.V. Acknowledgments.-D. N. S. wishes to thank the Anti-Cancer Council of Victoria (Australia) for the award of a Post-doctoral Fellowship during the tenure of which this work was done. We also thank the Gamma Sterilization Pty. Ltd., Dandenong (Victoria), for allowing access to and the use of the 500,000 curie cobalt-60 -irradiation source in the thermoluminescence studies.
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