Comparative studies of O2 and N2 in pure, mixed and layered CO ices.

We present laboratory data on pure, layered and mixed CO and O2 ices relevant for understanding the absence of gaseous O2 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 O2 is larger than that of CO and N2. TPD spectra reveal similar desorption processes for all examined CO-O2 ice morphologies. The different amorphous and crystalline components of pure 13CO RAIR spectra are analyzed. The RAIRS data of the 13CO stretching vibration show a significant difference between layered and mixed CO-O2 ices: layered CO-O2 ices resemble that of pure 13CO whereas the spectra of mixed ices are broadened. The experiments also show that the sticking probabilities of O2 on CO and O2 on O2 are close to unity. These new results are compared with recently analyzed data of CO-N2 ices. The differences in the TPD and RAIRS spectra of the CO-N2 and CO-O2 ice systems are explained by differences in quadrupole intermolecular interactions and by different crystallization processes of these ices.