IR optical properties of thin H2O, NH3, and CO2 cryofilms

The infrared spectral transmittance of cryofilms formed by H2O, NH3, CO2, and mixtures of these constituents in an N2 matrix were measured. These 0.25 to 14 micrometer thick films were cryopumped onto 20 and 77 K germanium substrates; the deposition pressure for the films deposited at 20 K and that for the 77 K films was approximately 2 times 10 to the minus 6 torr. Transmission spectra were obtained for the 500 to 1/3,700 cm wave number range with a Fourier transform spectrometer. Values of the complex index of refraction for the H2O, NH3, and CO2 cryodeposits were derived from the experimental data using an analytical model and the nonlinear least-squares method. The analytical model treats the germanium as a thick noninterfering film and the deposit as a thin film. Results from the least-squares method are compared with a Kramers-Kronig determination of the real part of the index of refraction. The optical properties (n, k) of such cryofilms are essential for predicting the degradation of contaminated cryocooled optical surfaces.