Hydroperoxyl Radical Generation by γ-Irradiation of Glassy Water at 77 K

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 105 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 HO2 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 HO2 radicals to the ESR spectra of glassy water do not depend on irradiation dose in the range 0.8−18 kGy. We stress that HO2 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 HO2 radical formation in the early stages of water radiolysis in the glassy state are discussed.