Absorption of 6328-Å radiation by Cr+3 ions in the 2E level is used to determine the population density of this level in a ruby crystal under optical pumping. This method allows a much more accurate and simple determination of the excited state population than has been available previously.
Arthur N. Rosen, "Measurement of the Population Density of the 2E Levels of Cr+3 in Ruby via Excited State Absorption," Appl. Opt. 7, 1555-1557 (1968)