Optical Absorption and Fluorescence of Oxygen in Alkali Halide Crystals

Single crystals of NaCl, KCl, and KBr were grown from the melt in an oxygen atmosphere, and their optical absorption, fluorescence excitation, and fluorescence emission spectra were measured at 300°, 77°, and 4.2°K. The weak absorption band caused by oxygen was the same in all three crystals, and did not vary with temperature. The band had maximum absorption at 5.0 ev and a half-width of 1.0 ev. All fluorescence excitation spectra contained a component identical to this absorption band. The fluorescence emission spectra consisted of a series of peaks in the wavelength range 4000-10 000 A, with an approximately equal energy separation of 1000 ${\mathrm{cm}}^{-1\mathrm{}}$. At 300°K, 12 to 15 peaks were resolved, and at 4.2°K each of these peaks split into 4 to 6 components. From these optical results and from paramagnetic resonance experiments, it is concluded that ${\mathrm{O}}_2^{-}$ molecule-ions located in anion sites in the crystal are responsible for the absorption and fluorescence.