Paraelectric alignment of NH 2 - centres in alkali halide crystals

The low temperature paraelectric alignment of substitutional NH ₂ ^- dipole centres in alkali halide crystals is studied measuring the electric field induced variation of the UV absorption and the electro-birefringence (Kerr-effect). We obtain the magnitude of the electric dipole moment and of the anisotropy of the oscillator strength. The reorientation of the dipoles between their equilibrium orientations is investigated by measuring the time dependence of the electro-birefringence after rapid changes of the applied electric field. The results indicate the predominance of one-phonon assisted tunneling processes.