We measured the binding energy of N 2 , CO, O 2 , CH 4 , and CO 2 on non-porous (compact) amorphous solid water (np-ASW), of N 2 and CO on porous amorphous solid water (p-ASW), and of NH 3 on crystalline water ice. We were able to measure binding energies down to a fraction of 1% of a layer, thus making these measurements more appropriate for astrochemistry than the existing values. We found that CO 2 forms clusters on np-ASW surface even at very low coverages. The binding energies of N 2 , CO, O 2 , and CH 4 decrease with coverage in the submonolayer regime. Their values at the low coverage limit are much higher than what is commonly used in gas-grain models. An empirical formula was used to describe the coverage dependence of the binding energies. We used the newly determined binding energy distributions in a simulation of gas-grain chemistry for cold cloud and hot core models. We found that owing to the higher value of desorption energy in the sub-monlayer regime a fraction of all these ices stays much longer and up to higher temperature on the grain surface compared to the single value energies currently used in the astrochemical models.

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

Accurate modeling of physical and chemical processes in the interstellar medium (ISM) requires detailed knowledge of how atoms and molecules adsorb on dust grains. However, the sticking coefficient, a number between 0 and 1 that measures the first step in the interaction of a particle with a surface, is usually assumed in simulations of ISM environments to be either 0.5 or 1. Here we report on the determination of the sticking coefficient of H 2 , D 2 , N 2 , O 2 , CO, CH 4 , and CO 2 on nonporous amorphous solid water. The sticking coefficient was measured over a wide range of surface temperatures using a highly collimated molecular beam. We showed that the standard way of measuring the sticking coefficient-the King-Wells method-leads to the underestimation of trapping events in which there is incomplete energy accommodation of the molecule on the surface. Surface scattering experiments with the use of a pulsed molecular beam are used instead to measure the sticking coefficient. Based on the values of the measured sticking coefficient, we suggest a useful general formula of the sticking coefficient as a function of grain temperature and molecule-surface binding energy. We use this formula in a simulation of ISM gas-grain chemistry to find the effect of sticking on the abundance of key molecules both on grains and in the gas phase.

In the interstellar medium (ISM), an important channel of water formation is the reaction of atoms on the surface of dust grains. Here, we report on a laboratory study of the formation of water via the O+D reaction network. While prior studies were done on ices, as appropriate to the formation of water in dense clouds, we explored how water formation occurs on bare surfaces, i.e., in conditions mimicking the transition from diffuse to dense clouds (Av ∼ 1-5). Reaction products were detected during deposition and afterward when the sample is brought to a high temperature. We quantified the formation of water and intermediary products, such as D 2 O 2 , over a range of surface temperatures (15-25 K). The detection of OD on the surface signals the importance of this reactant in the overall scheme of water formation in the ISM.

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.

We have studied how the formation of molecular hydrogen on silicates at low temperature is influenced by surface morphology. At low temperature (<30 K), the formation of molecular hydrogen occurs chiefly through weak physical adsorption processes. Morphology then plays a role in facilitating or hindering the formation of molecular hydrogen. We studied the formation of molecular hydrogen on a single crystal forsterite and on thin films of amorphous silicate of general composition (Fe(x)Mg((x-1)))(2)SiO(4), 0 < x < 1. The samples were studied ex situ by Atom Force Microscopy (AFM), and in situ using Thermal Programmed Desorption (TPD). The data were analysed using a rate equation model. The main outcome of the experiments is that TPD features of HD desorbing from an amorphous silicate after its formation are much wider than the ones from a single crystal; correspondingly typical energy barriers for diffusion and desorption of H, H(2) are larger as well. The results of our model can be used in chemical evolution codes of space environments, where both amorphous and crystalline silicates have been detected.

The quest to detect prebiotic molecules in space, notably amino acids, requires an understanding of the chemistry involving nitrogen atoms. Hydroxylamine (NH 2 OH) is considered a precursor to the amino acid glycine. Although not yet detected, NH 2 OH is considered a likely target of detection with ALMA. We report on an experimental investigation of the formation of hydroxylamine on an amorphous silicate surface via the oxidation of ammonia. The experimental data are then fed into a simulation of the formation of NH 2 OH in dense cloud conditions. On ices at 14 K and with a modest activation energy barrier, NH 2 OH is found to be formed with an abundance that never falls below a factor 10 with respect to NH 3 . Suggestions of conditions for future observations are provided.

Methane is one of the simplest stable molecules that is both abundant and widely distributed across space. It is thought to have partial origin from interstellar molecular clouds, which are near the beginning of the star formation cycle. Observational surveys of CH 4 ice towards low-and high-mass young stellar objects showed that much of the CH 4 is expected to be formed by the hydrogenation of C on dust grains, and that CH 4 ice is strongly correlated with solid H 2 O. Yet, this has not been investigated under controlled laboratory conditions, as carbon-atom chemistry of interstellar ice analogues has not been experimentally realized. In this study, we successfully demonstrate with a C-atom beam implemented in an ultrahigh vacuum apparatus the formation of CH 4 ice in two separate co-deposition experiments: C + H on a 10 K surface to mimic CH 4 formation right before H 2 O ice is formed on the dust grain, and C + H + H 2 O on a 10 K surface to mimic CH 4 formed simultaneously with H 2 O ice. We confirm that CH 4 can be formed by the reaction of atomic C and H, and that the CH 4 formation rate is 2 times greater when CH 4 is formed within a H 2 O-rich ice. This is in agreement with the observational finding that interstellar CH 4 and H 2 O form together in the polar ice phase, i.e., when C-and H-atoms simultaneously accrete with O-atoms on dust grains. For the first time, the conditions that lead to interstellar CH 4 (and CD 4 ) ice formation are reported, and can be incorporated into astrochemical models to further constrain CH 4 chemistry in the interstellar medium and in other regions where CH 4 is inherited.