Abstract

To identify and account for the effects from cryocontamination, the infrared spectral transmittances of cryofilms formed by CO<sub>2</sub>, H<sub>2</sub>O, and NH<sub>3</sub> were measured. These 0.25–14-µm-thick films were cryopumped onto 20-, 80-, and in one case 50-K germanium substrates; the deposition pressures for the films deposited at 20 K and that for 80-K films were approximately 2 × 10<sup>-6</sup> Torr. Transmittance spectra were obtained for the 500–3700-cm<sup>-1</sup> range with a Fourier-transform spectrometer. Values of the optical properties (<i>n</i>, <i>k</i>) for the CO<sub>2</sub>, H<sub>2</sub>O, and NH<sub>3</sub> cryofilms were derived from the experimental data using a thin-film-transmittance analytical model and the nonlinear leastsquares method. Results from the least-squares method are compared with a Kramers-Kronig determination ofthe refractive index (<i>n</i>). The optical properties (<i>n</i>, <i>k</i>) of such cryofilms are essential for predicting the degradation of contaminated cryocooled optical surfaces.

© 1982 Optical Society of America

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