Radiation-induced color centers in p a r a-terphenyl

Abstract

P a r a-terphenyl single crystals irradiated with g rays at liquid nitrogen temperature have been investigated by electron paramagnetic resonance and optical absorption experiments. In the optical absorption spectra at 77 K, obtained with the light polarized to the a, b, and c′ axes of the crystals, four absorption bands were detected both in the longer wavelength region and in the shorter one. These absorptions were compared with the transition energies and moments of 2-,3-,4-, and 9-hydroterphenyl radicals calculated using SCF–CI molecular orbital theory. The 707 and 637 nm bands which are polarized along the c′ axis and stable at room temperature were assigned to 2- and 9-hydroterphenyl radicals, respectively. The very weak transitions of 716 and 604 nm were ascribed to the transitions of 4- and 3-hydroterphenyl radicals, respectively. The four bands in the shorter wavelength region were assigned to the four kinds of hydroterphenyl radicals considering the results of the thermal and optical annealing experiments. Upon annealing at room temperature, growths of four c′-polarized clear absorption bands at 497, 536, 567, and 583 nm were observed. These bands were attributed to the absorptions of four kinds of hydrogen added terphenyl dimers judging from the different annealing behaviors and theoretical results. The EPR spectrum at room temperature exhibits triplet–triplet characteristics with splittings of 41 and 8.5 G at a rotation angle of 15 °. This spectrum was attributed to the signals arised from 2- and 9-hydroterphyl radicals.