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

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Abstract
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 NO 2 − , HCO 3 − , SO 4 2− , and CrO 4 2− 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 T 1.6, nearly temperature independent, and decreasing β with increasing temperature depending on the particular ionic impurity.