On the Nature of the Colour Centres of Ruby

An investigation is made of the spectral dependence of the luminescence quantum yield ratio (η/η0) of coloured and non‐coloured ruby and of the yield (ηadd) calculated for additional absorption. It is shown that in the additional absorption (Δk) are bands of different nature. Those with ηadd = 0 are called “active”, the others, with ηadd = 0, are called “inactive”. The active absorption connected with an electron transition to the conduction band and a following recombination with R‐line radiation, is attributed to Cr2+ ions, the inactive to Cr4+ ions. Experiments of monochromatic light action on coloured ruby have confirmed this conclusion; light absorbed in the region of active bands bleaches the crystal, whereas bleaching does not occur in the region of inactive bands. The results obtained allow to suggest a level scheme of corundum with a Cr2+ centre.