Two-photon polarization spectroscopy of F3+ and F2 color centers in LiF crystals

We identified energy levels, and their corresponding symmetries, of F3+ and F2 color centers in LiF crystals, using the Feofilov method of polarized luminescence as applied to two-photon excitation. For this purpose, we measured the azimuthal dependencies of the polarization degree, P(ϕ), and the intensity, I(ϕ), of the F3+ and F2 color center luminescence in LiF crystals under two-photon excitation. To increase the reliability of the results we measured the polarization dependencies of three LiF crystal plates cut parallel to the three main crystallographic planes (1 0 0), (1 1 0) and (1 1 1), respectively. The measured polarization dependencies were seen to be in good agreement with the theoretical polarization dependencies derived for P(ϕ) and I(ϕ). We established that the new energy level of the F2 center has the same parity as the ground state. Consequently, the one-photon electric-dipole transitions are forbidden between the states. The symmetry of the new electronic state of the F3+center was found to be 1A1.