Inversion Symmetry ofMandRCenters

The energy states of color centers lacking inversion symmetry will generally possess permanent dipole moments. Consequently the optical absorption bands of such centers will exhibit linear stark effects and should become dichroic in large electric fields. No dichroism for the $M$ or $R$ bands in NaCl or KCl was observed at 78°K as a result of applied fields of 3×${10}^5$ v/cm. Since the sensitivity of the measurement was several hundred times greater than that required to detect the field-induced dichroism calculated for a model lacking inversion symmetry, it is tentatively concluded that $M$ and $R$ centers possess inversion symmetry.