Two-photon polarization spectroscopy of F' and F 2 color centers in LiF crystals

We identi"ed energy levels, and their corresponding symmetries, of F‘ and F 2 color centers in LiF crystals, using the Feo"lov method of polarized luminescence as applied to two-photon excitation. For this purpose, we measured the azimuthal dependencies of the polarization degree, P(u), and the intensity, I(u), of the F‘ and F 2 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(u) and I(u). We established that the new energy level of the F 2 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 F‘ center was found to be 1A 1 . ( 2000 Elsevier Science B.V. All rights reserved.