Two-photon polarization spectroscopy of F-3(+) and F-2 color centers in LiF crystals

We identified energy levels, and their corresponding symmetries, of F-3(+) and F-2 color centers in LiF crystals, using the Feofilov method of polariz ed luminescence as applied to two-photon excitation. For this purpose, we m easured the azimuthal dependencies of the polarization degree, P(phi), and the intensity, I(phi), of the F-3(+) and F-2 color center luminescence in L iF crystals under two-photon excitation. To increase the reliability of the results we measured the polarization dependencies of three LiF crystal pla tes cut parallel to the three main crystallographic planes (100), (110) and (111), respectively, The measured polarization dependencies were seen to b e in good agreement with the theoretical polarization dependencies derived for P(phi) and I(phi). We established that the new energy level of the F-2 center has the same parity as the ground state. Consequently, the one-photo n electric-dipole transitions are forbidden between the states. The symmetr y of the new electronic state of the F-3(+) center was found to be (1)A(1). (C) 2000 Elsevier Science B.V. All rights reserved.