F. Dulieu

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.

Thermal desorption characteristics of CO, O2 and CO2 on non-porous water, crystalline water and silicate surfaces at submonolayer and multilayer coverages

The desorption characteristics of molecules on interstellar dust grains are important for modelling the behaviour of molecules in icy mantles and, critically, in describing the solid–gas interface. In this study, a series of laboratory experiments exploring the desorption of three small molecules from three astrophysically relevant surfaces is presented. The desorption of CO, O2 and CO2 at both submonolayer and multilayer coverages was investigated from non-porous water, crystalline water and silicate surfaces. Experimental data were modelled using the Polanyi–Wigner equation to produce a mathematical description of the desorption of each molecular species from each type of surface, uniquely describing both the monolayer and multilayer desorption in a single combined model. The implications of desorption behaviour over astrophysically relevant time-scales are discussed.

EXPERIMENTAL EVIDENCE FOR WATER FORMATION VIA OZONE HYDROGENATION ON DUST GRAINS AT 10 K

The formation of water molecules from the reaction between ozone (O3) and D-atoms is studied experimentally for the first time. Ozone is deposited on non-porous amorphous solid water ice (H2O), and D-atoms are then sent onto the sample held at 10 K. HDO molecules are detected during the desorption of the whole substrate where isotope mixing takes place, indicating that water synthesis has occurred. The efficiency of water formation via hydrogenation of ozone is of the same order of magnitude as that found for reactions involving O-atoms or O2 molecules and exhibits no apparent activation barrier. These experiments validate the assumption made by models using ozone as one of the precursors of water formation via solid-state chemistry on interstellar dust grains.

CO2 FORMATION IN QUIESCENT CLOUDS: AN EXPERIMENTAL STUDY OF THE CO + OH PATHWAY

The formation of CO2 in quiescent regions of molecular clouds is not yet fully understood, despite CO2 having an abundance of around 10%-34% H2O. We present a study of the formation of CO2 via the nonenergetic route CO + OH on nonporous H2O and amorphous silicate surfaces. Our results are in the form of temperature-programmed desorption spectra of CO2 produced via two experimental routes: O-2 + CO + H and O-3 + CO + H. The maximum yield of CO2 is around 8% with respect to the starting quantity of CO, suggesting a barrier to CO + OH. The rate of reaction, based on modeling results, is 24 times slower than O-2 + H. Our model suggests that competition between CO2 formation via CO + OH and other surface reactions of OH is a key factor in the low yields of CO2 obtained experimentally, with relative reaction rates of k(CO+H) << k(CO+OH) < k(H2O2+H) < k(OH+H), k(O2+H). Astrophysically, the presence of CO2 in low AV regions of molecular clouds could be explained by the reaction CO + OH occurring concurrently with the formation of H2O via the route OH + H.

Interaction of D2 with H2O amorphous ice studied by temperature- programed desorption experiments

The gas-surface interaction of molecular hydrogen D2 with a thin film of porous amorphous solid water (ASW) grown at 10 K by slow vapor deposition has been studied by temperature-programmed-desorption (TPD) experiments. Molecular hydrogen diffuses rapidly into the porous network of the ice. The D2 desorption occurring between 10 and 30 K is considered here as a good probe of the effective surface of ASW interacting with the gas. The desorption kinetics have been systematically measured at various coverages. A careful analysis based on the Arrhenius plot method has provided the D2 binding energies as a function of the coverage. Asymmetric and broad distributions of binding energies were found, with a maximum population peaking at low energy. We propose a model for the desorption kinetics that assumes a complete thermal equilibrium of the molecules with the ice film. The sample is characterized by a distribution of adsorption sites that are filled according to a Fermi-Dirac statistic law. The TPD curves can be simulated and fitted to provide the parameters describing the distribution of the molecules as a function of their binding energy. This approach contributes to a correct description of the interaction of molecular hydrogen with the surface of possibly porous grain mantles in the interstellar medium.

How micron-sized dust particles determine the chemistry of our Universe

In the environments where stars and planets form, about one percent of the mass is in the form of micro-meter sized particles known as dust. However small and insignificant these dust grains may seem, they are responsible for the production of the simplest (H2) to the most complex (amino-acids) molecules observed in our Universe. Dust particles are recognized as powerful nano-factories that produce chemical species. However, the mechanism that converts species on dust to gas species remains elusive. Here we report experimental evidence that species forming on interstellar dust analogs can be directly released into the gas. This process, entitled chemical desorption (fig. 1), can dominate over the chemistry due to the gas phase by more than ten orders of magnitude. It also determines which species remain on the surface and are available to participate in the subsequent complex chemistry that forms the molecules necessary for the emergence of life.

Dust as interstellar catalyst - I. Quantifying the chemical desorption process

Context. The presence of dust in the interstellar medium has profound consequences on the chemical composition of regions where stars are forming. Recent observations show that many species formed onto dust are populating the gas phase, especially in cold environments where UV and CR induced photons do not account for such processes. Aims. The aim of this paper is to understand and quantify the process that releases solid species into the gas phase, the so-called chemical desorption process, so that an explicit formula can be derived that can be included into astrochemical models. Methods. We present a collection of experimental results of more than 10 reactive systems. For each reaction, different substrates such as oxidized graphite and compact amorphous water ice are used. We derive a formula to reproduce the efficiencies of the chemical desorption process, which considers the equipartition of the energy of newly formed products, followed by classical bounce on the surface. In part II we extend these results to astrophysical conditions. Results. The equipartition of energy describes correctly the chemical desorption process on bare surfaces. On icy surfaces, the chemical desorption process is much less efficient and a better description of the interaction with the surface is still needed. Conclusions. We show that the mechanism that directly transforms solid species to gas phase species is efficient for many reactions.

NO ICE HYDROGENATION: A SOLID PATHWAY TO NH2OH FORMATION IN SPACE

Icy dust grains in space act as catalytic surfaces onto which complex molecules form. These molecules are synthesized through exothermic reactions from precursor radicals and, mostly, hydrogen atom additions. Among the resulting products are species of biological relevance, such as hydroxylamine—NH2OH—a precursor molecule in the formation of amino acids. In this Letter, laboratory experiments are described that demonstrate NH2OH formation in interstellar ice analogs for astronomically relevant temperatures via successive hydrogenation reactions of solid nitric oxide (NO). Inclusion of the experimental results in an astrochemical gas–grain model proves the importance of a solid-state NO + H reaction channel as a starting point for prebiotic species in dark interstellar clouds and adds a new perspective to the way molecules of biological importance may form in space.

The desorption of H2CO from interstellar grains analogues

Context. Much of the formaldehyde (H2CO) is formed from the hydrogenation of CO on interstellar dust grains, and is released in the gas phase in hot core regions. Radio-astronomical observations in these regions are directly related to its desorption from grains. Aims. We study experimentally the thermal desorption of H2CO from bare silicate surfaces, from water ice surfaces and from bulk water ice in order to model its desorption from interstellar grains. Methods. Temperature-programmed desorption experiments, monitored by mass spectrometry, and Fourier transform infrared spectroscopy are performed in the laboratory to determine the thermal desorption energies in: (i.) the multilayer regime where H2CO is bound to other H2CO molecules; (ii.) the submonolayer regime where H2CO is bound on top of a water ice surface; (iii.) the mixed submonolayer regime where H2CO is bound to a silicate surface; and (iv.) the multilayer regime in water ice, where H2CO is embedded within a H2O matrix. Results. In the submonolayer regime, we find the zeroth-order desorption kinetic parameters nu(0) = 10(28) mol cm(-2) s(-1) and E = 31.0 +/- 0.9 kJmol(-1) for desorption from an olivine surface. The zeroth-order desorption kinetic parameters are nu(0) = 10(28) mol cm(-2) s(-1) and E = 27.1 +/- 0.5 kJmol(-1) for desorption from a water ice surface in the submonolayer regime. In a H2CO:H2O mixture, the desorption is in competition with the H2CO + H2O reaction, which produces polyoxymethylene, the polymer of H2CO. This polymerization reaction prevents the volcano desorption and co-desorption from happening. Conclusions. H2CO is only desorbed from interstellar ices via a dominant sub-monolayer desorption process (E = 27.1 +/- 0.5 kJmol-1). The H2CO which has not desorbed during this sub-monolayer desorption polymerises upon reaction with H2O, and does not desorb as H2CO at higher temperature.

Laboratory evidence for the non‐detection of excited nascent H2 in dark clouds

There has always been a great deal of interest in the formation of H2 as well as in the binding energy released upon its formation on the surface of dust grains. The present work aims at collecting experimental evidence for how the bond energy budget of H2 is distributed between the reaction site and the internal energy of the molecule. So far, the non-detection of excited nascent H2 in dense quiescent clouds could be a sign that either predictions of emission line intensities are not correct or the de-excitation of the newly formed molecules proceeds rapidly on the grain surface itself. In this Letter, we present experimental evidence that interstellar molecular hydrogen is formed and then rapidly de-excited on the surface of porous water ice mantles. In addition, although we detect ro-vibrationally excited nascent molecules desorbing from a bare non-porous (compact) water ice film, we demonstrate that the amount of excited nascent hydrogen molecules is significantly reduced no matter the morphology of the water ice substrate at 10 K (both on non-porous and on porous water ice) in a regime of high molecular coverage as is the case in dark molecular clouds.