uv laser and x-ray radiation damage to l-ascorbic acid single crystals from 770K to 3000K: ESR analysis

Single crystals of l-ascorbic acid and partially deuterated l-ascorbic acid were irradiated in the temperature range of 770K to 3000K by either x-rays from a 3 MeV Van de Graaff electron accelerator or uv radiation (337.1 nm) from a pulsed nitrogen gas laser having a peak power of 240 kW. Several types of free radicals were produced by the radiation and were studied by X-band and Q-band ESR from 4.20K to 3000K. The kinds of radicals produced by the uv and x-irradiation were the same. The relative intensities of the spectral lines of the different types of radicals depend upon both the temperature and the type of radiation. Four radicals were identified: two oxidation products (radicals I and IV), one neutralized anion (radical III), and one adduct (radical II). Radicals I, III, and IV are present at 770K after irradiation at 770K. Upon annealing to 1400K, radical I changes to dehydroascorbic acid and proton adduction creates radical II. Upon annealing to 3000K, radicals II, III, and IV remain and are stable at 3000K for at least several months. All four radicals are carbon based radicals and are located on the furan ring. All hyperfine couplings are to β-protons. No carbon-13 hyperfine couplings were observed. ENDOR was tried at 4.20K, 770K, and 3000K after x-irradiation at 770K or 3000K. In all cases a matrix ENDOR line was observed, but no hyperfine ENDOR lines were observed. uv bleaching was attempted at 3000K on a deuterated crystal which had been x-irradiated previously. The uv laser radiation had no observable effect on the ESR spectra in which radicals II, III, and IV were already present