EPR and Optical Absorption Studies of Radiation-Induced Radicals in Biphenyl Single Crystals

Biphenyl single crystals irradiated with γ rays at liquid nitrogen temperature have been investigated by electron paramagnetic resonance and optical absorption experiments. EPR spectra at room temperature exhibit a triplet characteristic separated by 39 gauss and each line of the triplet further splits to a triplet. The main triplet has been interpreted as being caused by hydrogen added biphenyl radicals. The splitting of each line of the main triplet is consistent with the calculated splittings of 2-hydro-biphenyl radicals and 4-hydro-biphenyl radicals. The presence of 3-hydro-biphenyl radicals could not be judged from the EPR spectra due to the broadness of the lines. In the optical absorption spectra at low temperature, obtained with the light polarized to the a-, b- and c'-axes of the crystals, nine bands were detected. Transition energies were calculated using SCF-C1 molecular orbital theory for the three kinds of the hydrogen added biphenyl radicals. By comparing the experimental spectra with the theoretical transitions of the three kinds of radicals, the bands at 666 nm and 424 nm have been assigned to the transitions of 2-hydro-biphenyl radicals, the 660 nm band to that of 3-hydro-biphenyl radicals, and the bands at 604 nm and 598 nm to the ones of 4-hydro-biphenyl radicals. The decays of these radicals at room temperature are very similar and upon annealing a new absorption band appears at 462 nm.