Optical Absorption by the Cu+ Center in NaI

The color center which exhibits absorption in NaI at 4.84 eV and emission at 3.30 eV when measured at liquid nitrogen temperature is found to be the Cu+ impurity center. The oscillator strength is determined by performing copper analysis on the same crystals which were measured optically. The value is found to be 0.24 ± 0.03 and to have a weak temperature dependence. The selection of the free ion transition responsible for the absorption is discussed with relation to the oscillator strength and its temperature dependence, the free ion energies of candidate transitions, and ionic size. It is felt that evidence for a choice between the allowed 3d10 → 3d94p and the parity forbidden 3d10 → 3d94s free ion transition is inconclusive at this time.