Electrodichroism and paraelectric alignment of〈110〉-oriented OH−centers in KI

The uv absorption of O${\mathrm{H}}^{-}$ centers in KI has been studied at low temperatures under applied fields of various symmetries and in different polarization geometries. The field-induced absorption changes do not follow the spectral shape of the O${\mathrm{H}}^{-}$ band and depend drastically on field and polarization geometries. This complex behavior can be accounted for by a model involving field alignment of $〈110〉$-oriented dipoles which are characterized by three different spectrally partially overlapping uv transitions, one polarized parallel and two polarized along the two inequivalent directions perpendicular to the $〈110〉$ dipole axis. Strength and spectral shape of these transitions can be determined from saturation alignment experiments under $〈110〉$ fields. With these optical parameters, the measured electro-optical behavior can be fitted rather well to a simple paraelectric model, yielding for the only free-fitting parameter an uncorrected (for local fields) electric dipole moment of 1.35±0.1 e Å.