Time‐Resolved Polarization Spectroscopy and Off‐Centre Position of Excited Ga + Ions in NaCl:Ga Crystal

In the temperature region from 0.35 to 4.2 K the decay kinetics of the triplet A T and A x emission of NaCl:Ga crystals as well as the spectral and polarization charcteristics of the single decay components of the emission are investigated. An off‐centre displacement of the Ga + ion in the tetragonal (T) minima of the relaxed excited state (RES) is found. No manifestations of the off‐centre displacement of the Ga + ion in the trigonal (X) minima are observed. It is shown that fast tunnelling transitions between various equivalent off‐centre positions of an excited Ga + ion in the plane normal to the tetragonal Jahn‐Teller distortion axis lead to the tunnel splitting of the metastable as well as of the radiative T minima of the triplet RES of the Ga + centre. It is evident from the existence of two slow and two fast components in the low‐temperature decay kinetics of the A T emission. In case of NaCl:Ga the energy (δ) of tunnel splitting of the radiative minimum is assumed to be larger than the energy (2 E ) of the spin–orbit splitting of this minimum which is caused by the lowering of the RES symmetry due to the off‐centre displacements of the Ga + ion. As a result, off‐centre effects are not observable in the ODMR spectra of Ga + centres in NaCl:Ga crystals, unlike Ga + centres in other alkali halides, where the energy δ is probably smaller than 2 E .