Infrared optical constants of CO and CO2 thin icy films

We derived the optical constants (n, k) of carbon monoxide (CO) and carbon dioxide (CO2) cryofilms (T = 12 K) in the 4400-400-cm(-1) (2.27-25-mu m) infrared spectral range. Icy films were accreted on a silicon substrate inclined at an angle of 45 deg with respect to the infrared beam of the spectrometer. Transmittance spectra were taken at different thicknesses (0.05-3 mu m) with the electric vector parallel (p polarized) and perpendicular (s polarized) to the incidence plane. Optical constants were derived, with a nonlinear least-squares method, in a way such that they satisfy the Kramers-Kronig relation and give the best agreement with the transmittance spectra at oblique incidence for p and s polarizations measured at different thicknesses. We showed that this procedure can clearly separate the contribution of n and k to the spectrum. This is essential for deriving optical constants of ices where strong transitions occur. The optical constants derived can be found at the website http://www.ct.astro.it/lasp/optico.html. (C) 1998 Optical Society of America [S0740-3232(98)00612-7].