Temperature dependence of the luminescence from heavily doped ruby

Ruby, with a high concentration of chromium ions, emits sharp line emission from single ions and from exchange-coupled ion pairs. Large variations in the intensities and lifetimes of these sharp lines are observed, when the temperature is varied (X.X. Zhang et al., J. Lumin. 48 & 49 (1991) 248). These variations can be understood if one takes into account that at high dopant levels clusters of chromium ions are formed, efficient excitation transfer can occur between the clusters and the single ions, and these clusters can act as a reservoir of optical excitation in the ruby crystal.