2-PHOTON EXCITATION OF THE 4F1 -] 5D1 TRANSITIONS OF CE3+ IN LUPO4 AND YPO4

Two-photon excitation (TPE) spectra of the 4f1 --> 5d1 transitions of Ce3+ in LuPO4 and YPO4 have been investigated. For Ce3+ in LuPO4, tran sitions to four out of the five 5d1 levels are observed as zero-phonon lines. The symmetry properties of the levels were obtained from the p olarization dependence of the TPE signals, Measurements on Ce3+ in YPO 4 support the given assignments. Although a crystal-field fit yields a satisfactory rms energy deviation, an unrealistic value of the spin-o rbit coupling parameter, zeta5d1, is obtained. The vibronic coupling a ppears to be smaller than for the single-photon absorption spectra, an d the vibronic bands show a rich structure. There are striking differe nces between the energies and the relative intensities of the vibronic lines and the associated zero-phonon lines. The vibronic coupling is also found to be strongly dependent on the Ce3+ concentration.