Extrinsic absorption in 10.6-μm-laser-window materials due to molecular-ion impurities

It is proposed that molecular-ion impurities that substitute for the halide ion in alkali-halide crystals constitute an important class of impurities limiting infrared transmission in laser-window materials. A survey of the experimental literature indicates that concentrations of less than 0.1 ppm of NO2−, HCO3−, SO42−, and CrO42− will yield an absorption coefficient β>10−4 cm−1 at 10.6 μm. Predicted temperature dependences of impurity-limited β(10.6 μm) are T1.6, nearly temperature independent, and decreasing β with increasing temperature depending on the particular ionic impurity.