Radiation Imprisonment and Magnetic Field Effects on Luminescence in Pink Ruby

Abstract

The theory of imprisoned radiation developed by Holstein and recently extended by Scherr is adapted to apply to the luminescence decay in a solid. In addition to including a correction for losses from the excited level, it is necessary to consider reflections at the sample surfaces. Reflections are calculated approximately for a slab. The single Gaussian absorption function used in the reabsorption theory in lieu of an actual absorption function for the imprisoned radiation has the same integrated absorption $I$ as the actual function and has a height equal to $\sqrt{2}I$ times the integral of the square of the actual function. Calculations are described for the ruby $R$ lines, including the case where the Zeeman components are separated in a magnetic field. Experimental measurements of decay times at 77\ifmmode^\circ\else\textdegree\fi{}K of the ruby $R$ lines are presented for samples having a ${\mathrm{Cr}}^{+3}$ concentration of approximately ${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and thickness from 0.6 to 13 mm. Magnetic fields up to 20 kG are used. The various decay times, lying between 5 and 12 msec, are predicted within about 5% by the theory, with the help of the approximate absorption corrections and of Zeeman component intensities given by Sugano and Tanabe. The ${R}_{1}$ line is used for most comparisons. Intensity increases of up to 100% in the steady-state $R$-line emission in a 20-kG magnetic field are shown to be accounted for by approximate treatments related to the decay-time theory. No change of transition probabilities with magnetic field is indicated by the foregoing results. Further observations suggest that the channels leading from the excitation levels for both single ions and pairs are affected by magnetic field, however. These observations include changes in the relative intensities of various pair lines when the field is applied, and a field-dependent difference in the emission intensities produced with blue and green excitation. Field-induced changes in pumping light absorption, if present, are insufficient to account for these variations.