V. Pirronello

Molecular hydrogen formation on astrophysically relevant surfaces

Recent experimental results about the formation of molecular hydrogen on astrophysically relevant surfaces under conditions close to those encountered in the interstellar medium are analyzed using rate equations. The parameters of the rate equation model are fitted to temperature-programmed desorption curves obtained in the laboratory. These parameters are the activation energy barriers for atomic hydrogen diffusion and desorption, the barrier for molecular hydrogen desorption, and the probability of spontaneous desorption of a hydrogen molecule upon recombination. The model is a generalization of the Polanyi-Wigner equation and provides a description of both first- and second-order kinetic processes within a single model. Using the values of the parameters that best fit the experimental results, the efficiency of hydrogen recombination on olivine and amorphous carbon surfaces is obtained for a range of hydrogen flux and surface temperature pertinent to a wide range of interstellar conditions.

Master Equation for Hydrogen Recombination on Grain Surfaces

Recent experimental results on the formation of molecular hydrogen on astrophysically relevant surfaces under conditions similar to those encountered in the interstellar medium provided useful quantitative information about these processes. Rate equation analysis of experiments on olivine and amorphous carbon surfaces provided the activation energy barriers for the diffusion and desorption processes relevant to hydrogen recombination on these surfaces. However, the suitability of rate equations for the simulation of hydrogen recombination on interstellar grains, where there might be very few atoms on a grain at any given time, has been questioned. To resolve this problem, we introduce a master equation that takes into account both the discrete nature of the H atoms and the fluctuations in the number of atoms on a grain. The hydrogen recombination rate on microscopic grains, as a function of grain size and temperature, is then calculated using the master equation. The results are compared to those obtained from the rate equations, and the conditions under which the master equation is required are identified.

Efficiency of Molecular Hydrogen Formation on Silicates

We report on laboratory measurements of molecular hydrogen formation and recombination on an olivine slab as a function of surface temperature under conditions relevant to those encountered in the interstellar medium. On the basis of our experimental evidence, we recognize that there are two main regimes of H coverage that are of astrophysical importance; for each of them we provide an expression giving the production rate of molecular hydrogen in interstellar clouds.

Laboratory Synthesis of Molecular Hydrogen on Surfaces of Astrophysical Interest

We report on the first results of experiments to measure the recombination rate of hydrogen on surfaces of astrophysical interest. Our measurements give lower values for the recombination efficiency (sticking probability S times the probability of recombination upon H-H encounter, ?) than model-based estimates. We propose that our results can be reconciled with average estimates of the recombination rate [(1/2)nHngvHAS?] from astronomical observations, if the actual surface of an average grain is rougher, and its area bigger, than the one considered in models.

Experimental evidence for water formation on interstellar dust grains by hydrogen and oxygen atoms

Context. The synthesis of water is one necessary step in the origin and development of life. It is believed that pristine water is formed and grows on the surface of icy dust grains in dark interstellar clouds. Until now, there has been no experimental evidence whether this scenario is feasible or not on an astrophysically relevant template and by hydrogen and oxygen atom reactions. Aims. We present here the first experimental evidence of water synthesis by such a process on a realistic analogue of grain surface in dense clouds, i.e., amorphous water ice. Methods. Atomic beams of oxygen and deuterium are aimed at a porous water ice substrate (H2O) held at 10 K. Products are analyzed by the temperature-programmed desorption technique. Results. We observe the production of HDO and D2O, indicating that water is formed under conditions of the dense interstellar medium from hydrogen and oxygen atoms. This experiment opens up the field of a little explored complex chemistry that could occur on dust grains,which is believed to be the site where key processes lead to the molecular diversity and complexity observed in the Universe.

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.

Laboratory measurements of molecular hydrogen formation on amorphous water ice

We report on an experimental study of the formation of hydrogen molecules by surface recombination of adsorbed H atoms on amorphous water ice under conditions closely simulating those encountered in astrophysical environments. Our results show that hydrogen recombination via surface reactions on icy mantles on grains is able to account for H2 formation in dense cloud environments.

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.

EXPERIMENTAL STUDY OF CO2 FORMATION BY SURFACE REACTIONS OF NON-ENERGETIC OH RADICALS WITH CO MOLECULES

Surface reactions between carbon monoxide and non-energetic hydroxyl radicals were carried out at 10 K and 20 K in order to investigate possible reaction pathways to yield carbon dioxide in dense molecular clouds. Hydroxyl radicals, produced by dissociating water molecules in microwave-induced plasma, were cooled down to 100 K prior to the introduction of CO. The abundances of species were monitored in situ using a Fourier transform infrared spectrometer. Formation of CO2 was clearly observed, even at 10 K, suggesting that reactions of CO with OH proceed with little or no activation barrier. The present results indicate that CO2 formation, due to reactions between CO and OH, occurs in tandem with H2O formation, and this may lead to the formation of CO2 ice in polar environments, as typically observed in molecular clouds.

Direct Measurements of Hydrogen Atom Diffusion and the Spin Temperature of Nascent H2 Molecule on Amorphous Solid Water

Physicochemical processes (H-atom sticking, diffusion, recombination, and the nuclear spin temperature of nascent H2 molecules) important in the formation of molecular hydrogen have been experimentally investigated on amorphous solid water (ASW). A new type of experiment is performed to shed light on a longstanding dispute. The diffusion rate of H atom is directly measured at 8 K and is found to consist of a fast and a slow component due to the presence of at least two types of potential sites with the energy depths of ~20 and >50 meV, respectively. The fast diffusion at the shallow sites enables efficient H2 formation on interstellar ice dust even at 8 K, while H atoms trapped in the deeper sites hardly migrate. The spin temperature of nascent H2 formed by recombination on ASW has been obtained for the first time and is higher than approximately 200 K. After formation, H2 molecules are trapped and their spin temperature decreases due to the conversion of spin states on ASW.