Infrared optical properties of thin H_2O, NH_3, and CO_2 cryofilms

To identify and account for the effects from cryocontamination, the infrared spectral transmittances of cryofilms formed by CO2, H2O, and NH3 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 × 10−6 Torr. Transmittance spectra were obtained for the 500–3700-cm−1 range with a Fourier-transform spectrometer. Values of the optical properties (n, k) for the CO2, H2O, and NH3 cryofilms were derived from the experimental data using a thin-film-transmittance analytical model and the nonlinear least-squares method. Results from the least-squares method are compared with a Kramers–Kronig determination of the refractive index (n). The optical properties (n, k) of such cryofilms are essential for predicting the degradation of contaminated cryocooled optical surfaces.