The diffusion of atoms and molecules in ices covering dust grains in dense clouds in interstellar space is an important but poorly characterized step in the formation of complex molecules in space.Here we report the measurement of diffusion of simple molecules in amorphous solid water (ASW), an analog of interstellar ices, which are amorphous and made mostly of water molecules. The new approach that we used relies on measuring in situ the change in band strength and position of midinfrared features of OH dangling bonds as molecules move through pores and channels of ASW. We obtained the Arrhenius pre-exponents and activation energies for diffusion of CO, O 2 , N 2 , CH 4 , and Ar in ASW. The diffusion energy barrier of H 2 and D 2 were also measured, but only upper limits were obtained. These values constitute the first comprehensive set of diffusion parameters of simple molecules on the pore surface of ASW, and can be used in simulations of the chemical evolution of ISM environments, thus replacing unsupported estimates. We also present a set of argon temperature programmed desorption experiments to determine the desorption energy distribution of argon on nonporous ASW. K) for a long time, is clear. However, except for a few cases, the processes leading to the formation of these complex molecules in ices are either unknown or still insufficiently characterized. One poorly understood step in molecule formation is the diffusion of reactants in/on ices. In the Langmuir-Hinshelwood mechanism, which is the most important mechanism in gas-grain astrochemical modeling, the rate of reactions is largely determined by the diffusion rate of reactants on the surface. After gas phase radicals and molecules accrete on the ice mantle, they diffuse on the surface or penetrate into the ice to react with each other. The rate of diffusion governs how fast chemical reactions take place in the solid state and the abundance of ICOMs in the ice mantle. Yet, the process of diffusion under typical dense cloud conditions in the ISM is poorly characterized.The most common molecule that has been detected in interstellar ices is water, followed by CO, CO 2 , CH 3 OH, CH 4 , and NH 3 . N 2 and O 2 should also be present, but it is hard to establish their abundance from the infrared

The need to characterize ices coating dust grains in dense interstellar clouds arises from the importance of ice morphology in facilitating the diffusion and storage of radicals and reaction products in ices, a well-known place for the formation of complex molecules. Yet, there is considerable uncertainty about the structure of ISM ices, their ability to store volatiles and under what conditions. We measured the infrared absorption spectra of CO on the pore surface of porous amorphous solid water (ASW), and quantified the effective pore surface area of ASW. Additionally, we present results obtained from a Monte Carlo model of ASW in which the morphology of the ice is directly visualized and quantified. We found that 200 ML of ASW annealed to 20 K has a total pore surface area that is equivalent to 46 ML. This surface area decreases linearly with temperature to about 120 K. We also found that (1) dangling OH bonds only exist on the surface of pores; (2) almost all of the pores in the ASW are connected to the vacuum-ice interface, and are accessible for adsorption of volatiles from the gas phase; there are few closed cavities inside ASW at least up to a thickness of 200 ML; (3) the total pore surface area is proportional to the total 3-coordinated water molecules in the ASW in the temperature range 60-120 K. We also discuss the implications on the structure of ASW and surface reactions in the ice mantle in dense clouds.

Analyses of infrared signatures of CO 2 in water dominated ices in the ISM can give information on the physical state of CO 2 in icy grains and on the thermal history of the ices themselves. In many sources, CO 2 was found in the "pure" crystalline form, as signatured by the splitting in the bending mode absorption profile. To a large extent, pure CO 2 is likely to have formed from segregation of CO 2 from a CO 2 :H 2 O mixture during thermal processing. Previous laboratory studies quantified the temperature dependence of segregation, but no systematic measurement of the concentration dependence of segregation is available. In this study, we measured both the temperature dependence and concentration dependence of CO 2 segregation in CO 2 :H 2 O mixtures, and found that no pure crystalline CO 2 forms if the CO 2 :H 2 O ratio is less than 23%. Therefore the segregation of CO 2 is not always a good thermal tracer of the ice mantle. We found that the position and width of the broad component of the asymmetric stretching vibrational mode of 13 CO 2 change linearly with the temperature of CO 2 :H 2 O mixtures, but are insensitive to the concentration of CO 2 . We recommend using this mode, which will be observable towards low mass protostellar envelopes and dense clouds with the James Webb Space Telescope, to trace the thermal history of the ice mantle, especially when segregated CO 2 is unavailable. We used the laboratory measured 13 CO 2 profile to analyze the ISO-SWS observations of ice mantles towards Young Stellar Objects, and the astrophysical implications are discussed.