TWO-PHOTON LASER SPECTROSCOPY OF LASER-ACTIVE F_3~+ COLOR CENTERS IN LiF CRYSTALS

摘要

We studied the energy level structure of laser-active F_3~+ color centers in LiF crystals using two-photon spectroscopy and stepwise laser excitation. This allowed us to obtain for the first time the complete system of spin-triplet electronic states of the F_3~+ center. Also, we were able to measure the triplet-triplet absorption spectra from the lowest triplet state, ~3E, to higher lying states, ~3Γ_α, ~3Γ_β, the triplet-triplet, ~3Γ_α,→~3E, fluorescence spectra and the spin-forbidden triplet-singlet, ~3E→~1A_1, phosphorescence spectra. In addition, the temperature behavior of the nonradiative intersystem singlet-triplet ~1E→~3E and triplet-singlet ~3E→~1A_1 transition probabilities could be measured. The dependencies of both polarization and fluorescence intensity of F_3~+ centers in LiF crystals were measured as function of the polarization of strong IR laser excitation used to produce two-photon excitation of the F_3~+centers. Also, both dependencies were derived analytically. Their comparison suggests that the two-photon transition leads to the excitation of a previously unknown spin-singlet electronic state possessing ~1A_1 symmetry. As a result of these spectroscopic investigations, we could construct a comprehensive energy level diagram of the F_3~+ center. Finally, the influence of spin-triplet states on the LiF:F_3~+ color center laser characteristics can be evaluated.