The thermally stimulated depolarization currents and the temperature depend
ences of optical absorption are investigated. Interaction processes between
defects are observed. A model consisting of association-dissociation of qu
asidipoles in the form of donor-acceptor pairs with a distribution of activ
ation energies and relaxation times is proposed. (C) 1999 American Institut
e of Physics. [S1063-7834(99)01209-5].