1997A&A...328..649E

Apolar ices have been observed in several regions in dense clouds and are likely dominated by molecules such as CO, CO₂ and the infrared inactive molecules O₂ and N₂. Interstellar solid CO has been well characterized by ground-based high resolution measurements. Recent ISO results showed the ubiquitous presence of abundant CO₂ ice and the presence of CO₂-rich ice mantles towards several molecular clouds. CO and CO₂ have sharp bands in the infrared and their band shape depends strongly on the ice composition. The profiles of the strong CO and CO₂ bands can therefore provide important information on the composition, temperature and thermal history of interstellar and precometary ices. We address, in this paper, the infrared spectra of 70 apolar ice mixtures of pure, binary, and multicomponent type. We studied their spectral properties at 10K, during warm up and UV photolysis, and derived the optical constants. We discuss the importance of particle shape calculations for strong transitions such as CO and CO₂. In the laboratory context, we investigate the formation of CO₂ in the interstellar medium by UV photolysis of interstellar ices. Together with astronomical spectra taken by the ISO satellite these laboratory data will be extremely valuable for the determination of the grain mantle composition in dense clouds.