We have measured the radiative efficiency (ratio of $R$-line photons to the total number of photons emitted) and absorption cross sections for the $R$ lines of dilute ruby, as a function of polarization and temperature between 20 and 373°K. We have also measured the fluorescent lifetime from 20 to 373°K in optically thin crystals, and from 20 to 273°K in optically thick crystals. The difference between the reciprocal lifetimes in thin and thick crystals is directly comparable (via the Einstein relations) with the integrated absorption in the $R$ lines. This comparison shows that for the $R$ lines at 20 and 77°K detailed balance holds to well within the experimental error of 5%, a precision never to our knowledge previously approached in solids. An upper limit of 0.002 ${\mathrm{cm}}^{-1\mathrm{}}$ was placed on any Stokes shift of the $R_1$ line, confirming that it is a no-phonon line. Comparison of $\sigma$ and $\alpha$ spectra confirms that the $R$ lines and their vibronic satellites are predominately electric-dipole in character. The radiative efficiency of the $R$ lines is strongly polarization dependent, a result which differs from that of previous workers. Its temperature dependence agrees well with that calculated from the observed vibronic spectrum at 77°K. The integrated absorption in the $R$ lines increases by about 20% between 20 and 373°K. The absorption in the vibronic satellites on the high-frequency side of the $R$ lines appears to be weaker than the corresponding emission on the low-frequency side. The temperature variation of the lifetime agrees well with that calculated from the absorption and radiative efficiency, but about 10% of the decay at 77°K and 30% at 373°K is either nonradiative or by emission at wavelengths outside the range of observation. The implications of our results for laser work are discussed.

• Received 1 September 1964

DOI:https://doi.org/10.1103/PhysRev.137.A1117