Comparative studies of O-2 and N-2 in pure, mixed and layered CO ices

We present laboratory data on pure, layered and mixed CO and O-2 ices relevant for understanding the absence of gaseous O-2 in space. Experiments have been performed on interstellar ice analogues under ultra high vacuum conditions by molecular deposition at 14 K on a gold surface. A combination of reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) is used to derive spectroscopic and thermodynamic properties of the ices. It is found that for pure ices the desorption energy of O-2 is larger than that of CO and N-2. TPD spectra reveal similar desorption processes for all examined CO - O-2 ice morphologies. The different amorphous and crystalline components of pure (CO)-C-13 RAIR spectra are analyzed. The RAIRS data of the (CO)-C-13 stretching vibration show a significant difference between layered and mixed CO - O-2 ices: layered CO - O-2 ices resemble that of pure (CO)-C-13 whereas the spectra of mixed ices are broadened. The experiments also show that the sticking probabilities of O-2 on CO and O-2 on O-2 are close to unity. These new results are compared with recently analyzed data of CO - N-2 ices.(1,2) The differences in the TPD and RAIRS spectra of the CO - N-2 and CO - O-2 ice systems are explained by differences in quadrupole intermolecular interactions and by different crystallization processes of these ices.