A study was made of the absorption and luminescence spectra of color centers, in a thin crystalline surface layer, which were produced by low-energy electrons from a secondary electron emission microwave discharge. The intensity ratio of the absorption band due to the F2 color center, to the band due to the F center, was observed to be much higher than the ratio of these color centers when produced in a bulk crystal. It was found that the F2+ center was produced in a thin layer by the selective 2-step photo-ionization of F2 color centers by irradiation with the 514.5nm Ar+ laser line.

V.V.Ter-Mikirtychev, T.Tsuboi, M.E.Konyzhev, V.P.Danilov: Physica Status Solidi B, 1996, 196[1], 269-74