(Received September 9, 1965)
Depopulation of ground-state Cr ions in ruby (DGS) under optical pumping with a Xe flash light has been measured as a function of time. Experiments have been made by means of measuring the decrease in absorbing power for a probe light (5790 Å or 5461 Å) from a Hg lamp. DGS reaches its maximum value after some delay from the peak of Xe light, and decreases after discontinuation of optical pumping with a time constant of 3.4 ms, which is the lifetime of metastable-state 2E. During laser oscillation mean value of DGS is forced to take a constant value, which varies with the condition of laser oscillation. So long as the input energy to the Xe lamp is not altered, the shorter the duration of Xe flash light is, the higher becomes the maximum value of DGS. On the basis of the rate equation, these experimental results have been explained.