Infrared spectroscopy of solid CO-CO sub(2) mixtures and layers

The spectra of pure, mixed and layered CO and CO sub(2) ices have been studied systematically under laboratory conditions using Transmission-Absorption Fourier Transform infrared spectroscopy. This work provides improved resolution spectra (0.5 cm super(-1)) of the CO sub(2) bending and asymmetric stretching mode, as well as the CO stretching mode, extending the existing Leiden database super(a) of laboratory spectra to match the spectral resolution reached by modern telescopes and to support the interpretation of the most recent data from the Spitzer Space Telescope. It is shown that mixed and layered CO and CO sub(2) ices exhibit very different spectral characteristics, which depend critically on thermal annealing and can be used to distinguish between mixed, layered and thermally annealed CO-CO sub(2) ices. CO only affects the CO sub(2) bending mode spectra in mixed ices below 50 K under the current experimental conditions, where it exhibits a single asymmetric band profile in intimate mixtures. In all other ice morphologies the CO sub(2) bending mode shows a double peaked profile, similar to that observed for pure solid CO sub(2). Conversely, CO sub(2) induces a blue-shift in the peak-position of the CO stretching vibration, to a maximum of 2142 cm super(1) in mixed ices, and 2140-2146 cm super(1)in layered ices. As such, the CO sub(2) bending mode puts clear constraints on the ice morphology below 50 K, whereas beyond this temperature the CO sub(2) stretching vibration can distinguish between initially mixed and layered ices. This is illustrated for the low-mass young stellar object HH 46, where the laboratory spectra are used to analyse the observed CO and CO sub(2) band profiles and try to constrain the formation scenarios of CO sub(2).