Color center accumulation in LiF crystals under irradiation with MeV ions: Optical spectroscopy and modeling

Lithium fluoride crystals irradiated at room temperature with energetic 14N and 84Kr ions were studied by absorption and luminescence spectroscopy. Dependence of the average F-center concentration on the fluence was found to be different for the lighter and the heavier projectiles. In particular, the maximal concentration of the F centers reached a notably higher value in the case of irradiation with 14N than that for 84Kr ions. This result is compared with our model of track overlapping and defect accumulation.The luminescence studies allowed distinguishing F3+ and F2 centers, having very similar absorption spectra. The luminescence corresponding to both centers was observed starting from the minimal fluencies. It means that the aggregation processes take place in single tracks in agreement with our model, which predicts the coagulation radius of about five lattice parameters. Anion vacancies va+, having higher mobility than F centers, should play an important role in the aggregation.