Recombination Luminescence fromVKCenters in Potassium Iodide

This paper reports experiments with KI to determine whether the near-uv emission characteristic of the "pure" crystal arises from the recombination of an electron with a $V_K$ center. KI crystals containing Tl or Eu, when irradiated at 77°K with 1.7-MeV electrons, exhibit absorption bands due to $V_K$ and $F$ centers. Illumination of an irradiated crystal in the $F$ band or near infrared results in attenuation of $V_K$ absorption bands and in emission of the 302- and 371-mμ bands characteristic of the pure crystal, as well as in ${\mathrm{Tl}}^{+}$ emission. Thermal or optical destruction of $V_K$ centers results in attenuation of the 302- and 371-mμ bands, while the ${\mathrm{Tl}}^{+}$ emission remains. Preferential orientation of $V_K$ centers by bleaching with polarized light at liquid-helium temperature or at 77°K results in a polarization of both these emission bands. Assuming that the orienting transition is $\sigma$ polarized, the 302-mμ band is polarized parallel to the $V_K$ axis, and the 371-mμ band perpendicular. It is concluded that these bands do arise from the radiative decay of a $V_K$-plus-electron center.