Ice mantles on dust grains are important sites for chemistry in cold interstellar regions.  This chemistry depend on the ability of molecules to migrate around in the ice mantle. Water is the dominant ice component, followed by CO or CO2 dependent on the line of sight, and diffusion barriers and mechanisms within water ice are generally poorly understood.  For this reason, diffusion barriers in astrochemical models are set to 50-70% of the energy barrier for desorption. We have carried out laboratory studies of CO diffusion into water ice to quantitatively characterize the diffusion processes for a key ice system.        In each experiment, 20-200 ML water ice is deposited on top of a thin CO ice at 12 K.  Diffusion of CO into the water layer is monitored between 12 and 23 K using infrared spectroscopy to measure the changes in the CO and water spectral features resulting from ice mixing.  The data are used to calculate the amount of CO that diffused into the water layer over time, and the diffusion barrier. In addition to temperature, we varied CO and water ice thicknesses, isotopic composition, and water ice porosity. We also explored the effects of adding CO2 to the water ice.        Diffusion of CO into the water layer is clearly observed starting at 15K. The diffusion rate increases with increasing temperature and with increasing porosity. The water ice thickness regulates the fraction of CO that mixes with water. Finally adding CO2 to the water ices surprisingly slows down diffusion of CO into the water ice. These results are used in concert to quantify CO diffusion in water ice and in astrochemically relevant water ice mixtures, as well as to put constraints on the mechanisms through which molecules diffuse in amorphous water ice maintained at cryogenic temperatures.

CO diffusion in water ice

247th National Meeting (2014)

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