The first direct observation of radiative spectral transfer within an inhomogeneously broadened line (the R1 line of ruby) is reported. As opposed to nonradiative transfer, the temperature dependence of the radiative process is consistent with exponential, in agreement with recent predictions. A temperature-independent, resonant process is also directly observed. The implication of these results for the concept of microscopic strain broadening is discussed.
P. M. Selzer and W. M. Yen, "Radiative spectral transfer in ruby," Opt. Lett. 1, 90-92 (1977)