ONE-PHOTON AND 2-PHOTON SPECTROSCOPY OF CE3+ IONS IN LAF3-CEF3 MIXED-CRYSTALS

Absorption, excitation and emission spectra and lifetime measurements in cerium-doped lanthanum fluoride single crystals were obtained using both one- and two-photon spectroscopy techniques. It is shown that two kinds of fluorescent centres are present. In weakly doped systems, a rapid fluorescence around 300 nm dominates, due to Ce3+ ions at normal sites. In heavily doped systems, an additional longer fluorescence appears at higher wavelengths due to Ce3+ ions occupying perturbed sites and acting as traps. In CeF3, the energy migrates among Ce3+ ions and is finally trapped so that only the trap emission is detected at room temperature. Significant energy losses in the fluorescence of Ce3+ at normal sites are explained by up-conversion and ionization processes and by non-radiative surface recombination phenomena for the case of strong absorption. Two-photon excitation spectra in the region of trap absorption confirm the role of traps in emission mechanisms and show narrow lines which could be vibronic structures of various kinds of traps existing in these materials.