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-µm) 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 µm) 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.