SPECTROSCOPIC INVESTIGATION AND POLARIZED LUMINESCENCE OF SILVER PAIR CENTERS IN SODIUM-CHLORIDE

Absorption, emission, and excitation spectra of heavily Ag** plus doped NaCl single crystals are investigated at various temperatures. Besides the ultra-violet emission due to Ag** plus single ions and the violet emission already observed, the crystal displays a new green fluorescence. It is shown that the fundamental emitters in the visible range are (Ag** plus )//2 pair centers. The polarization properties of luminescence of dimers are carefully analyzed. The measurement of the magnitude of the degree of polarization of fluorescence and its azimuthal dependence suggest that the two (Ag** plus )//2 luminescent centers are oriented along the C//4 axes of the crystal. Models for the paired (Ag** plus )//2 centers are proposed and some of their spectral properties discussed.