ESR spectroscopy has been used to identify the paramagnetic species trapped at 77 K upon γ-irradiation of glassy water prepared by hyperquenching of liquid water at cooling rates exceeding 10 5 K/s. Besides OH radicals, which are the only paramagnetic intermediates of water radiolysis stabilized at 77 K in hexagonal ice prepared by slow cooling of liquid water, comparable amounts of HO 2 radicals were found in glassy water. The ESR spectra for trapped OH radicals were typical of such radicals in a glass. Neither trapped electrons nor trapped hydrogen atoms were detected. The contributions of OH and HO 2 radicals to the ESR spectra of glassy water do not depend on irradiation dose in the range 0.8-18 kGy. We stress that HO 2 radicals are not detected in irradiated hexagonal ice under these conditions. They are formed as secondary products after annealing to destroy the OH radicals, but the yields are tiny relative to the remarkably high yields observed in the glassy water. Possible mechanisms for HO 2 radical formation in the early stages of water radiolysis in the glassy state are discussed.

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The effects of γ-irradiation at 77 K on two crystalline forms of water, namely, cubic and hexagonal ice, and on two amorphous forms of water, made by hyperquenching of liquid water or deposition of water vapors, have been studied by electron spin resonance (ESR) spectroscopy. Mainly OH radicals are formed from cubic ice directly made from liquid water. In contrast, comparable amounts of OH and HO 2 radicals are formed on irradiation of cubic ice made by heating glassy water, and their relative yields depend strongly on the irradiation dose. Comparable yields of OH and HO 2 radicals were observed on γ-irradiation of the two amorphous solids, and their ratio did not depend on irradiation dose. The formation of HO 2 radicals is attributed to the presence of shallow proton traps, or Bjerrum-like L-defects, in the two amorphous solids and in disordered regions of cubic ice.