Absorption Spectrum of Optically Pumped ${\mathrm{Al}}_2$${\mathrm{O}}_3$:${\mathrm{Cr}}^{3+}$

The absorption cross sections of the ${\mathrm{Cr}}^{3+}$ ion in the metastable states ${}_{}{}^2{}_{}{}^{}E \left(\overline{E} \mathrm{and} 2\overline{A}\right)$ of lightly doped ruby (∼0.01%) have been measured as a function of polarization at temperatures of 30, 105, and 296°K in the region from 2300 to 7000 Å. The lifetime of these states was measured by observing the decay rate of the excited-state absorption, and agreed with the value of the measured fluorescence decay rate at various temperatures. The population of the excited states was measured by the change of the area under the first two vibronic sidebands at low temperatures. At room temperature the population was measured by the change of transmitted intensity of a ruby laser.