Polarization of Luminescence in NaCl:Pb2+and KCl:Pb2+

The polarized luminescence of ${\mathrm{Pb}}^{2+}$ impurity-vacancy complexes in NaCl and KCl induced by excitation with polarized light has been studied. A single emission band at 340 nm of predominantly tetragonal symmetry was found in KCl: Pb; this emission decreased and shifted in wave-length during aging. Three emission bands were found in NaCl: Pb at 315, 380, and 460 nm. The 315-nm emission was at its maximum intensity after quenching and the 380-nm band was the first to appear upon aging with the 460-nm band appearing later. The 315-nm emission was found to originate from centers of both tetragonal and orthorhombic symmetry. Both the 380- and 460-nm bands are due to centers of trigonal symmetry. We have assigned the 340-nm band in quenched KCl and the 315-nm band in NaCl to isolated Pb-vacancy dipoles. We conclude that in KCl the vacancies are predominantly next-nearest neighbors to the impurity while in NaCl both nearest- and next-nearest-neighbor sites are comparable. The 380-nm emission band appears to be due to trimers of dipoles in a hexagon arrangement in {111} plane in agreement with the model of Dryden and Harvey. That the origin of the 460-nm band is due to higher aggregates is not well established.