The luminescence and excitation spectra of electronic excitations which were localized near to a single anion vacancy, a di-vacancy, or a vacancy quartet were investigated at 4.2K in deformed crystals. It was shown that the dissociation of a vacancy quartet into VK and an electron-vacancy complex occurred together with their radiative decay in NaBr, KBr and RbI. On the basis of the spectral proximity of the optical characteristics of vacancy quartets and Ex luminescence on the one hand, and of single anion vacancy and  luminescence on the other hand, it was concluded that vacancy quartet excitations reflected weak off-center configurations and single anion vacancy excitations reflected strong off-center configurations.

E.Vasilchenko, E.Sarmukhanov, A.Elango: Physica Status Solidi B, 1994, 185[1], 189-97