Description:
<jats:title>Abstract</jats:title><jats:p>An investigation is made of the absorption spectra, the spectral dependence of the luminescence quantum output, glow curves, and the radiation spectra of the glow curves of ruby crystals. These crystals are coloured by intense light flashes or by γ‐rays from Co<jats:sup>60</jats:sup>. It is found that the absorption is greater in coloured crystals while the luminescence quantum output is smaller. The thermal decoloration of crystals is accompanied by a luminescence in the R‐line region. A decrease in the Cr<jats:sup>3+</jats:sup> ion concentration is detected in coloured crystals by ESR measurements. The similarity of all the characteristics obtained allows a conclusion about the identity of the colour centres arising under optical and γ‐irradiation of the crystals. The results obtained here and published data suggest that Cr<jats:sup>2+</jats:sup> and Cr<jats:sup>4+</jats:sup> act as colour centres, and that their formation under optical coloration is duo to Cr<jats:sup>3+</jats:sup> ion ionization via a two‐step process. It is shown that the gradual increase of the ruby generation threshold observed experimentally, which gives laser operation, is due to the formation of colour centres which cause a parasitic absorption of the pumping light.</jats:p>