Apolar ices

Apolar ices have been observed in several regions in dense clouds and are likely dominated by molecules such as CO and the IR inactive molecules O(2) and N(2). Recent ISO results showed the ubiquitous presence of abundant CO(2) ice and the presence of CO(2)-rich ice mantles as well as strong CO(2) outgassing from Comet Hale-Bopp. CO and CO(2) have strong, sharp bands in the IR and their position and band width indicate not only the polar and apolar character of the ice, but also give an indication of the temperature and radiation history of grains. The study of CO and CO(2) band profiles provides a powerful tool to determine the grain mantle composition, in particular the presence of solid O(2) and N(2), and the grain evolution in the cycle of interstellar clouds. We consider here laboratory IR spectra of apolar ice mixtures of pure, binary and multicomponent type. We try to investigate the formation of apolar ices in the interstellar medium. We present new laboratory results which provide evidence for ice segregation in the line-of-sight towards protostars. Together with astronomical spectra taken by the ISO satellite, we show how laboratory data are currently used for the identification and abundance determination of interstellar and solar system ices