Magnetic resonance investigation of the dynamics of F centers in LiF

Damage caused in lithium fluoride crystals by γ-rays and electrons includes, besides other point defects, F centers and their agglomerates. We have studied the paramagnetic F centers created by radiation doses that vary by several orders of magnitude. We measured the electronic spin relaxation time T1e of the F centers at low temperatures by the recovery of the magnetic circular dichroism of absorption (MCDA) as well as the temperature dependence of the 19F and 7Li nuclear spin relaxation (NSR) times, T1n. Our results indicate that the nuclear spin relaxation is dominated by spin-diffusion limited paramagnetic relaxation. The electron spin correlation function is determined by the electronic spin–lattice relaxation. In the studied temperature range from 4 to 300 K, the electron spin–lattice relaxation time T1e is long compared to the nuclear Larmor period, ωLT1e≫1 (ωL: nuclear Larmor frequency). The temperature variation of T1n indicates that the electron spin–lattice relaxation is dominated by phonon-F center interactions. At temperatures T greater than 360 K, an annealing process takes place that eliminates the F centers and their agglomerates.