Excited-state photoionization of Ce3+ ions in Ce3+:CaF2

Abstract

The formation of stable color centers in ${\mathrm{Ce}}^{3+}$:${\mathrm{CaF}}_2$ is found to result from the optical pumping of the lowest 4f→5d absorption band of the ${\mathrm{Ce}}^{3+}$ ions at $C_{4v}$ symmetry sites with a 308-nm excimer laser. Absorption spectra of these color centers indicate that they are ${\mathrm{Ce}}^{2+}$ ions located at sites having $O_h$ symmetry. The color centers are photochromic since they can be optically bleached. Measurements of the dependence of the number of photochromic color centers on the incident laser intensity and length of exposure are presented. The data is interpreted in terms of a two-step photoionization of the ${\mathrm{Ce}}^{3+}$ ions and the subsequent electron capture at a ${\mathrm{Ce}}^{3+}$ ion having $O_h$ symmetry. Rate equations incorporating a two-photon creation term and a one-photon bleaching term are formulated and their solutions are consistent with the observed color-center growth kinetics. The implications of this work for other potential vibronic laser materials are also discussed.