Surface generation of color centers in lithium fluoride by EUV-irradiation

A lithium fluoride (LiF) crystal was irradiated with extreme ultraviolet (EUV) radiation, which generated primary and aggregate color centers (CCs) within a surface layer. The optical properties of this layer were investigated by means of spectrophotometry in the wavelength range 210-800 nm. The photometric spectra were analyzed by means of two independent approaches, which gave comparable optical constants and thickness of the colored layer. Its thickness was 46.6 ± 0.7 nm, which is approximately the same as the estimated penetration depth of EUV radiation in LiF. The stable formation of CCs induced an increase of the complex refractive index of the layer by approximately 2.8% in the visible range. The volume concentrations of the main CCs were estimated from their dielectric-susceptibility partial contributions to be in the range ∼2 × 1019-2 × 1020 cm−3 within the layer.