Abstract
The ultraviolet absorption spectra of Ce3+, 0.005%, in crystals of the alkaline-earth fluorides CaF2, SrF2, and BaF2 have been measured at both room and liquid-nitrogen temperatures. The general features in the spectra of all three hosts are similar. The ultraviolet spectra of Ce3+, 0.005% to 5%, in CaF2 show three regions of interconfigurational absorption which may be assigned as: (1) 4f5d bands from about 31 000 to 56 000 cm1, with half-width about 2000 cm1 at Ce3+ concentration ∼102%; (2) 4f6s weak and broad band from 60 000 to about 77 000 cm1 with maximal absorption at about 70 000 cm1; and (3) a charge-transfer absorption, F(2p6)Ce3+(6s), appearing as an apparent red shift of the absorption edge of the host CaF2 at approximately 80 000 cm1. The assignments of these three regions of absorption are based on the location, width, number, oscillator strength, and temperature dependence of the bands. The effect of large Ce3+ concentration, ≥0.1%, on the uv absorption spectra of Ce3+ is interpreted as the result of cluster formation from pairs of Ce3+-interstitial F. The vibronic structure in the lowest 4f5d band may be conceived in terms of pseudolocalized vibrations, as in the case of several divalent rare-earth ions in alkali halides treated by Wagner and Bron.