Giulio Manico

Formation of Carbon Dioxide by Surface Reactions on Ices in the Interstellar Medium

The formation of carbon dioxide by surface reactions has been investigated experimentally in conditions close to those encountered in the interstellar medium. Carbon monoxide and oxygen atoms have been concurrently deposited on a copper substrate at 5 K. The formation and release in the gas phase of carbon dioxide have been monitored by a mass spectrometer during a programmed desorption. Our measured rates and energy barrier show that it is possible to make CO2 efficiently in ice mantles on grains without the intervention of energizing (UV or energetic particles) agents.

Molecular Hydrogen Formation on Ice Under Interstellar Conditions

The results of experiments on the formation of molecular hydrogen on low-density and high-density amorphous ice surfaces are analyzed using a rate equation model. The activation energy barriers for the relevant diffusion and desorption processes are obtained. The more porous morphology of the low-density ice gives rise to a broader spectrum of energy barriers compared to the high-density ice. Inserting these parameters into the rate equation model under steady-state conditions, we evaluate the production rate of molecular hydrogen on ice-coated interstellar dust grains.

Solid state astrochemistry

Preface. The interstellar medium: an overview D.A. Williams. Important open questions in astrochemistry: how can dust help? D.A. Williams. In dust we trust: an overview of observations and theories of interstellar dust A.Li, J.M. Greenberg. Cosmic silicates: a review T. Henning. Interstellar chemistry in the gas and on the surfaces of dust particles E, Herbst. How to identify diffuse band carriers J. Krelowski. Matrix and gas-phase spectroscopic studies of possible DIB carriers J. Fulara. Chemical reactions on solid surfaces of astrophysical interest O. Biham, V. Pirronello, G. Vidali. From interstellar polycyclic aromatic hydrocarbons and ice to astrobiology L.J. Allamandola, D.M. Hudgins. Ice chemistry in space P. Ehrenfreund, H. Fraser. Ion interactions with solids: astrophysical applications E.M. Bringa, R.E. Johnson. Trans-Neptunian objects M.A. Barucci, J. Romon. Chemical models: where to start from? O.M. Shalabiea. The chemical evolution of TMC-1 M.S. El-Nawawy. The model of chemical composition of Halley dust particles B.M. Andreichikov, G.G. Dolnikov, Yu.P. Dikov. List of participants. Index.

Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions

Experimental results on the formation of molecular hydrogen on amorphous silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers, we evaluate the efficiency of molecular hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H2 formation within a temperature range that is relevant to diffuse interstellar clouds. The results also indicate that the hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states.

Measurement of the Kinetic Energy of Hydrogen Molecules Desorbing from Amorphous Water Ice

A hydrogen molecule that is formed on an interstellar grain might retain some of the 4.48 eV of energy that is released in the recombination reaction of two hydrogen atoms. We set up an experiment to measure the translational (kinetic) energy of hydrogen molecules after they are formed on and are ejected from the surface of an interstellar dust grain analog. Here we report the first measurements of the kinetic energy of molecular deuterium as it leaves the surface of an amorphous water sample. The astrophysical implications of such measurements are discussed.

Analysis of Molecular Hydrogen Formation on Low-Temperature Surfaces in Temperature Programmed Desorption Experiments

The study of the formation of molecular hydrogen on low-temperature surfaces is of interest both because it enables the exploration of elementary steps in the heterogeneous catalysis of a simple molecule and because of its applications in astrochemistry. Here, we report results of experiments of molecular hydrogen formation on amorphous silicate surfaces using temperature-programmed desorption (TPD). In these experiments, beams of H and D atoms are irradiated on the surface of an amorphous silicate sample. The desorption rate of HD molecules is monitored using a mass spectrometer during a subsequent TPD run. The results are analyzed using rate equations, and the energy barriers of the processes leading to molecular hydrogen formation are obtained from the TPD data. We show that a model based on a single isotope provides the correct results for the activation energies for diffusion and desorption of H atoms. These results are used in order to evaluate the formation rate of H2 on dust grains under the actual conditions present in interstellar clouds. It is found that, under typical conditions in diffuse interstellar clouds, amorphous silicate grains are efficient catalysts of H2 formation when the grain temperatures are between 9 and 14 K. This temperature window is within the typical range of grain temperatures in diffuse clouds. It is thus concluded that amorphous silicates are good candidates to be efficient catalysts of H2 formation in diffuse clouds.

Quantum Tunneling of Oxygen Atoms on Very Cold Surfaces

Any evolving system can change state via thermal mechanisms (hopping a barrier) or via quantum tunneling. Most of the time, efficient classical mechanisms dominate at high temperatures. This is why an increase of the temperature can initiate the chemistry. We present here an experimental investigation of O-atom diffusion and reactivity on water ice. We explore the 6-25 K temperature range at submonolayer surface coverages. We derive the diffusion temperature law and observe the transition from quantum to classical diffusion. Despite the high mass of O, quantum tunneling is efficient even at 6 K. As a consequence, the solid-state astrochemistry of cold regions should be reconsidered and should include the possibility of forming larger organic molecules than previously expected.

Formation of molecular hydrogen on analogues of interstellar dust grains: experiments and modelling

Molecular hydrogen has an important role in the early stages of star formation as well as in the production of many other molecules that have been detected in the interstellar medium. In this review we show that it is now possible to study the formation of molecular hydrogen in simulated astrophysical environments. Since the formation of molecular hydrogen is believed to take place on dust grains, we show that surface science techniques such as thermal desorption and time-of-flight can be used to measure the recombination efficiency, the kinetics of reaction and the dynamics of desorption. The analysis of the experimental results using rate equations gives useful insight on the mechanisms of reaction and yields values of parameters that are used in theoretical models of interstellar cloud chemistry.

Efficient diffusive mechanisms of O atoms at very low temperatures on surfaces of astrophysical interest

At the low temperatures of interstellar dust grains, it is well established that surface chemistry proceeds via diffusive mechanisms of H atoms weakly bound (physisorbed) to the surface. Until recently, however, it was unknown whether atoms heavier than hydrogen could diffuse rapidly enough on interstellar grains to react with other accreted species. In addition, models still require simple reduction as well as oxidation reactions to occur on grains to explain the abundances of various molecules. In this paper we investigate O-atom diffusion and reactivity on a variety of astrophysically relevant surfaces (water ice of three different morphologies, silicate, and graphite) in the 6.5-25 K temperature range. Experimental values were used to derive a diffusion law that emphasizes that O atoms diffuse by quantum mechanical tunnelling at temperatures as low as 6.5 K. The rates of diffusion on each surface, based on modelling results, were calculated and an empirical law is given as a function of the surface temperature. The relative diffusion rates are k(H2Oice) > k(sil) > k(graph) >> k(expected). The implications of efficient O-atom diffusion over astrophysically relevant time-scales are discussed. Our findings show that O atoms can scan any available reaction partners (e.g., either another H atom, if available, or a surface radical like O or OH) at a faster rate than that of accretion. Also, as dense clouds mature, H2 becomes far more abundant than H and the O : H ratio grows, and the reactivity of O atoms on grains is such that O becomes one of the dominant reactive partners together with H.

A Summary of Experimental Results on Molecular Hydrogen Formation on Dust Grain Analogues

We review the main laboratory results of investigations of processes of molecular hydrogen formation on surfaces. The problem of the formation of molecular hydrogen is a fundamental issue in astrophysics/astrochemistry, because of the great importance that molecular hydrogen has for the structure and evolution of our Universe. Such experiments are done using ultra-high vacuum, low temperature, and atomic/molecular beam techniques to study the formation of molecular hydrogen on dust grain analogues in conditions as close as technically feasible to the ones present in relevant ISM environments. In experiments conducted at Syracuse University, we studied H 2 formation on the three most ISM-relevant classes of surfaces: silicates, carbonaceous materials and amorphous water ice. Our experimental investigations range from the evaluation of the catalytic efficiency of the studied surfaces to the energetics of the reaction, i.e. the partition of the formation energy between the grain and the nascent molecule. Such measurements have been done by changing various parameters such as: the temperature of the interstellar dust analogue, the kinetic temperature of the atoms, the morphology of the surface and, to be completed soon, the composition of the solid. Quantitative and qualitative information on the processes of H 2 formation is then fed in theoretical models to extract results that pertain to desired ISM environments.