Fluorescence and Reflection Spectra of NaI Single Crystals

Abstract

The fluorescence emission and excitation, reflection, and transmission spectra of 22 different NaI crystal samples grown in crucibles of Pt or quartz under atmospheres of ${\mathrm{N}}_2$ or ${\mathrm{H}}_2$ or ${\mathrm{H}}_2$ plus ${\mathrm{I}}_2$ and in some cases containing stoichiometric excess of Na or ${\mathrm{I}}_2$ or containing various concentrations of Tl, have been explored from 2.5 to 8 ev. At -190°C an emission band centered at 295 mμ (4.20 ev) was observed in all but the one sample which contained the highest Tl concentration, 0.1 mole%. Another emission band at 375 mμ (3.31 ev) was found to be strongest in crystals containing excess ${\mathrm{I}}_2$. A 425 mμ (2.92 ev) emission appeared strongly in all crystals containing Tl, but a 325 mμ (3.82 ev) band appeared only in the one crystal with 0.1 mole% Tl. With excitation energy below 5.5 ev the different emission bands arise from discrete excitation bands. With excitation energies above 5.5 ev up to 8 ev the emission was primarily in the 295-mμ band until Tl concentration reached 0.05 mole% where the 425-mμ band and 295-mμ band became approximately equal. Maxima in reflection spectra coincide closely but not exactly with published maxima in the absorption spectra of NaI evaporated films. An inflation, not seen in the published absorption spectra, appears at 5.62 ev in the reflection spectrum at -190°C. Coincidental with the lowest energy maximum in the reflection spectrum at 5.59 ev the excitation spectra for both 295-mμ and 425-mμ emission exhibit minima too strong to be accounted for by reflection loss. At excitation energies of 5.7 ev and above, luminescence quantum efficiency is greatest for an illuminating angle of incidence of 45° whereas at lower energies no definitely preferable incidence angle was found. Also with higher exciting energies it was found that the emission was strongest on the axis of illumination. With exciting energies below 5.5 ev the emission was isotropic.