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Conference Programme

Heterogeneous Chemical Processes in the Astronomical Environment
15th June - 17th June 2009
Department of Chemistry, University College London

Download the programme and abstract book.

Oral Contributors:

Ices in Star Forming Regions - the ultimate spectroscopic tracer of interstellar chemistry
Helen J Fraser, J. A. Noble, A. Craigon, W. F. Thi, K. Pontoppidan, Y. Aikawa, J. Blum, R.W. Dawson, B. Dent, D. Heisselmann and I. Sakon
University of Strathclyde

In interstellar regions the greatest reservoir of molecular material is stored in icy-grain mantles. These molecular nano-factories play a key role in governing, replenishing and dictating the prevailing chemical and physical conditions in star-forming regions, right from the earliest stages of the stellar processes, in dense star-less cores, through young stellar objects and their proto-stellar disks, to more mature solar systems. Despite our "understanding" of this evolutionary process of the interstellar molecular soup, we are limited to theory and conjecture when it comes to proving the links between the atomic / molecular gas and solid state species, or the formation of complex organic molecules and their potential links with pre-biotic chemistry.
To use solid-state observations as anything more than just detections of chemical functional groups in the condensed material, it is vital that we can use ice-spectroscopy as both a tracer of star-formation parameters and a probe of the prevailing astrophysics. The generation of 'ice-maps' showing the distribution of key solid species (e.g. H₂O CO and CO₂) has the potential to do this. Using a combination of data from AKARI, JCMT and IRAM, I will present new gas-solid combined maps that constrain our understanding of star-formation chemistry. Key results will be supplemented (where possible) by laboratory data, utilising recent results from many different groups. I will challenge some long-held beliefs in the astrochemistry community, e.g. is there a critical-extinction at which the onset of ice formation occurs? Is CO-ice really a precursor to CO₂ ice formation?

Are PAHs a viable route of dust formation in carbon rich environments?
Harpreet Dhanoa
University College London

The method of dust formation is still unclear. The main sources of dust production are evolved AGB stars, novae and supernovae. Polycyclic aromatic hydrocarbons have been detected in the circumstellar of carbon rich AGB stars and nova ejecta. It has been proposed that PAHs are key intermediates in carbon dust formation and act as nucleation sites for the dust grains. Acetylene is a key molecule in the formation of PAH molecules. The aim of our study is to ascertain the viability of dust production through PAH molecules via acetylene. An investigation into three different carbon rich environments (AGB stars, novae and carbon rich main sequence stars) is presented.

Chemical evolution of warm cores around low-mass protostellar objects.
Zainab Awad, Serena Viti and David A. Williams
University College London

In the last decade, observations towards low-mass prestellar cores showed that there are warm, chemically rich, dense regions in the inner envelopes around Class 0 objects. These regions are believed to be the analogous of the well known hot cores around massive stars. Warm cores are characterised by a multitude of complex organic molecules. The first warm core around a solar-type protostar was discovered toward the Class 0 source IRAS16293-2422 which shows high abundances of hydrides (e.g. CH₃OH, H₂CO, H₂O), high deuteration levels (~ >10 %), and complex molecules (HCOOCH₃, HCOOH, CH₃OCH₃, CH₃CN, C₂H₅CN). It is believed that such complex molecules are the second generation of simpler large molecules such as H₂CO and CH₃OH which are suggested to be formed on grains and then returned to the gas-phase by some thermal desorption.
In this work, we explore the chemical evolution of such warm cores adopting with a chemical model adapted from studies of hot cores. We find that warm cores show a rich chemistry quite similar to that found in hot cores, and hence the suggestion for warm cores to be the analogous of hot cores is plausible. Our results are in fair agreement with observations toward the IRAS 16293-2422 source and in strong agreement with other chemical models.

Gas grain modelling at Queen's University Belfast
T. J. Millar
Queen's University Belfast

Over the past decade, observations of star forming regions have led to the conclusion that molecular abundances are mediated by processes on the surface of interstellar grains, in particular through chemical reactions in molecular ices. The Astrochemistry Group at Queen's University Belfast is developing new models of solid-state reactions and the interaction between gas-phase and solid-state molecules with particular applications to hot molecular cores and regions of low-mass star formation. We are involved in the JCMT Spectral Line Survey and will make particular models to compare to observations of sources contained in the survey. In this talk, I will outline the research underway, its astronomical context and the need for better-determined fundamental data.

Interstellar chemistry driven by energetic processing
Maria Elisabetta Palumbo
INAF

Lately a large number of complex molecules have been detected towards star forming regions with abundances higher than those predicted by gas phase chemical models. This has reinvigorated the debate on their origin and it has been suggested that molecules observed in the gas phase could be formed in the solid phase and released to the gas phase after desorption of icy grain mantles. Suggested mechanisms are grain surface reactions, and energetic processing (i.e. ion irradiation and UV photolysis) of icy mantles.
Our knowledge on the effects of energetic processing of icy samples is mainly based on laboratory experiments performed at low temperature (10-100 K). Experimental results show that after ion irradiation and UV photolysis the chemical composition and the structure of the target is modified. Both more volatile and less volatile species are formed and if a C-bearing species is present in the original sample a refractory residue is left over after warm-up to room temperature.
Here I will present the results of some recent ion irradiation experiments performed in the Laboratory of Experimental Astrophysics in Catania, will describe the experimental procedures and discuss their astrophysical relevance.

Electrons vs. Photons: Chemical Control at Surfaces
Jan Hendrik Bredehoeft and P. Swiderek
University of Bremen

In the past experiments regarding the chemical processes occurring within the icy mantles of inter-and circumstellar dust grains have been conducted using either UV light or particulate beams (protons, light ions) as an energy source. Cases in which the ions are directly involved in chemical reactions aside, chemical change is probably induced not by the beam itself, but rather by secondary electrons. In experiments using electron beams as sources of energy, a number of different processes occurring at different energies can be observed. At energies above the ionization threshold, molecules lose an electron by Electron Impact ionization. This process results in a cation. At lower energies an electron can be attached, causing formation of an anion. This process usually has some few sharp resonance energies at which it can occur. Alternatively an electron can also polarize a molecule in passing, resulting in activation of the neutral molecule. These various activated states are normally not stable and result in conversion or dissociation of the molecule.

Chemical processes of electrons with biomolecules inside helium droplets.
Slywia Ptasinka, Filipe Ferreira da Silva, Achim Edtbauer, Stephan Denifl, Tilmann D. Märk, and Paul Scheier
Universität Innsbruck

The interaction of photons and particulate radiation (i.e. electrons and ions) with organic compounds in interstellar medium is one of current research interests.
So far many studies on low energy electron impact to biomolecules in the gas phase have been carried in our laboratory. In these studies it was possible to investigate the fundamental processes driven by free electrons without any perturbation by a surrounding medium. The vast number of fragmentation pathways was observed including dehydrogenation, complex bond breaking and intermolecular rearrangement of molecules. Our recent experiment on biomolecules embedded in superfluid helium droplets showed that many fragmentation channels are frozen in a cold environment.
The gas-phase studies were performed by means of a crossed electron/molecule beams-instrument consisting of a double-focusing two sector mass spectrometer. While helium droplet studies were carried out by a He cluster source and a pick-up cell containing the vapour of biomolecules. In both experiments, the ions formed via electron collision were detected by mass spectrometry.
Our experimental results exhibit several interesting features. In contrary to the gas-phase studies, the stabilization of transient negative parent ions in He droplet via solvation was observed. In addition, the yield for formation of dehydrogenated anions in helium droplets was detected at higher energies, above electronic excitation states, while in the gas phase this process was observed below 3 eV. Moreover, besides the formation of homogenous clusters of biomolecules the synthesis of new products was observed, e.g. anhydride acetic from acetic acid in cold environment.

Formation of Molecules on Amorphous Silicates
Gianfranco Vidali, L.Li, H.Perets, V.Pirronello and O.Biham
Syracuse University

I'll present a selection of results from our laboratory program on the formation of hydrogen molecules on amorphous silicates. These are: 1. Efficiency of formation of H2 on amorphous silicates of composition (MgxFe(1-x))2SiO₄ (0<x<1); 2. Energies of adsorption, diffusion and desorption of hydrogen atoms and molecules; 3. Connection between morphology and efficiency of H₂ formation; 4. Application of experimentally derived quantities of elementary steps of molecule formation to the determination of the efficiency of H₂ formation in actual ISM conditions.

Mobility of cold H-atoms on icy interstellar dust grains
Elie Matar, E.Congiu, F.Dulieu, A.Momeni, J.-L.Lemaire
Observatory of Paris

The mobility of H atoms on the surface of interstellar dust grains at low temperature is still a matter of debate. In dense clouds, the hydrogenation of adsorbed species (i.e., CO) depends on the mobility of H atoms on water ice. Astrochemical models widely assume that H atoms are mobile on the surface of dust grains even if controversy still exists. We present here direct experimental evidence of the mobility of H atoms on porous water ice surfaces at 10 K. In a UHV chamber, O₂ is deposited on a porous amorphous water ice substrate. Then D atoms are deposited onto the surface held at 10 K. TPD technique is used and desorptions of O₂ and D₂ are simultaneously monitored. We find that the amount of O₂ that desorbs during the TPD diminishes if we increase the deposition time of D atoms. O₂ is thus destroyed by D atoms even though these molecules have previously diffused inside the pores of thick water ice. Our results can be easily interpreted if D is mobile at 10 K on the water ice surface. A simple rate equation model fits our experimental data and best fit curves were obtained for a D atom diffusion barrier of 22 ± 2 meV.

Interaction of UV and Soft-X-rays with Nanoparticles
James Brian Mitchell and J.L. LeGarrec
Universtité de Rennes

In previous experiments at the ESRF it has been found that hard x-rays produce heavy ionization of soot nanoparticles in a hydrocarbon flame. This can be explained by multi-electron emission model which predicts the coulomb explosion of the aggregated particle structure. A new project has been launched in collaboration with SOLEIL, the CEA and the Freiuniversitat de Berlin to investigate the interaction of UV and soft X-rays with mass spectrometric analysis of the resulting products. First experiments are due in early 2010.

Laboratory surface astrochemistry in the UK
Martin McCoustra
Heriot-Watt University

Over the last decade the UK has taken significant strides in coordinating research activity in heterogeneous aspects of laboratory astrochemistry; first through the UCL Centre for Cosmic Chemistry and subsequently through the AstroSurf Network. This presentation will attempt to review activity within the UK and attempt to outline how our coordinated and cooperative today grew out of these earlier local and national networking activities.

Modelling complex chemistry in the ISM
Robin Garrod
Cornell University

Organic molecules of increasing complexity are being detected in star-forming regions. While dust-grain chemistry has long been recognized as being a part of the story, the apparent failure of the traditional gas-phase mechanisms makes understanding grain-surface chemistry, and its interaction with the gas phase, a priority.
I will discuss our most recent gas-grain chemical models of hot-cores, and show how the changing physical conditions of the star-formation process can lead to chemical enrichment, and to the formation of highly complex organics. I will discuss how the models could be made more accurate through improved laboratory measurements of certain key quantities.
I will also discuss our recent detections of ethyl formate and n-propyl cyanide, the most complex molecules yet discovered in Sgr B2N, and show how the observational data allow us to place strong constraints on the formation mechanisms of these and other molecules.

Surface processes on interstellar grains: linking laboratory data with models
Herma Cuppen, Sergio Ioppolo, Claire Romanzin, Guido Fuchs, Suzanne Bisschop, Ewine F. van Dishoeck and Harold Linnartz
Leiden University

Many important molecules such as H₂O and CH₃OH do not have efficient gas phase formation routes under the cold, dilute conditions of the interstellar medium and it has been realised that they form on the surfaces of dust. The exact chemical networks are in most cases still unclear. Laboratory experiments are a powerful way to explore the chemical reactions that can lead to abundant interstellar molecules in a well-defined and controllable environment. Recently, the formation of H₂O and CH₃OH has been studied by hydrogenating CO and O₂ ices, respectively. Both systems exhibit very different results, showing first order destruction behaviour of CO and zeroth order destruction behaviour of O₂ ice. Furthermore the yields of H₂O and CH₃OH as well as the intermediates H₂O₂ and H₂CO are very different.
These experiments are not straightforward to interpret and are probably the result of several different formation and destruction mechanisms competing with each other. Monte Carlo simulations are a tool to help with the interpretation by disentangling different formation mechanisms. We will show how this method can be successfully applied to model the formation of H₂O and CH₃OH over a range of different temperatures and pressures, both under laboratory and interstellar conditions. The specific properties of the CO and O₂ ice are responsible for the difference in reaction order. It further gives more inside into the actual chemical reaction network itself.

A stochastic model for diffusive heterogeneous chemical reactions in interstellar space
Christiane Maria Losert-Valiente Kroon and Ian J Ford
UCL

Traditional rate equations employed in population dynamics do not, in general, account for statistical fluctuations in a physical system. When abundances are low the mean-field approach might not accurately determine the average population. As an example we consider heterogeneous chemical reactions in interstellar space. In conditions where reactions occur significantly faster than the rate at which species are adsorbed onto the surface of a grain the traditional mean-field approach fails to predict the correct density of reactants and reaction products. We investigate the effects that the diffusive processes have on the chemical reaction taking place on a one dimensional grain. As an alternative to the traditional rate equations, we make use of the Doi-Peliti-Cardy formalism [1] which ---starting from a master equation--- gives us the means to compute average populations via a path integral expression and corresponding constraint equations that are stochastic partial differential equations. We explore the parameter space numerically and determine the threshold between the deterministic regime where the results of the stochastic model coincide with the predictions of the mean-field approach and the stochastic regime where this agreement fails [2].

[1] U.C. Taeuber, M. Howard & B.P. Vollmayr-Lee, J.Phys.A: Math.Gen.38, R79-R131, 2005
[2] C.M. Losert-Valiente-Kroon & I.J. Ford, arXiv:0710.5540, 2007

Incorporation of stochastic chemistry on dust grains in the PDR code using moment equations
Evelyne Roueff, F. Le Petit, J. Le Bourlot, B. Barzel and O. Biham
Observatoire de Paris

Unlike gas-phase reactions, chemical reactions taking place on interstellar dust grain surfaces cannot always be modelled by rate equations. Due to the small grain sizes and low flux, these reactions may exhibit large fluctuations and thus require stochastic methods such as the moment equations.
We evaluate the formation rates of H₂, HD and D₂ molecules on dust grain surfaces and their abundances in the gas phase under interstellar conditions. We incorporate the moment equations into the Meudon PDR code and compare the results with those obtained from the rate equations. We find that within the experimental constraints on the energy barriers for diffusion and desorption and for the density of adsorption sites on the grain surface, H₂, HD and D₂ molecules can be formed efficiently on dust grains. Under a broad range of conditions, the moment equation results coincide with those obtained from the rate equations. However, in a range of relatively high grain temperatures, there are significant deviations. In this range, the rate equations fail while the moment equations provide accurate results. The incorporation of the moment equations into the PDR code can be extended to other reactions taking place on grain surfaces.

Probing the structure of star forming clouds using Herbig Haro jets.
Helen Christie
UCL

Several recent studies model the effects of Herbig Haro objects illuminating quiescent clumps in a surrounding molecular cloud. Some have included a complex chemistry but, so far, these complicated models have all treated the radiation source as static. Herbig Haro jets travel at speeds of several hundred kilometres per second, hence the assumption of a static field is unphysical. In this work the chemical code of Viti and Williams (1999) is adapted to include a moving source. As well as facilitating a comparison between the effects of a moving and static source, it may help to pick out species for use as observational probes in clumps ahead of Herbig Haro objects. Viti et al. (2006) recently completed a survey of molecules in clumps ahead of Herbig Haro objects. A comparison of results from the survey with our model seems to support the idea that observed abundances in the clumps are a result of frozen out material returning to the gas phase when illuminated by radiation from the nearby shock front. Using a moving source in the model changes the interpretation of the observations slightly in that the clumps appear older than in the static case by approximately 500 years. In addition some elements can survive a lot longer after the jet has passed than they could if the radiation source remained close. These results are encouraging since they allow the distinctive chemistry of the clumps to survive for longer helping to reconcile the large number of clumps so far observed with the models.

Water radiolysis by carbon ions from first principles
Jorge Kohanoff and Emilio Artacho
Queen's University Belfast

Water radiolysis by low-energy carbon projectiles is studied by first-principles molecular dynamics simulations. The most abundant products are H and OH fragments. Surprisingly, the spatial density of radiolysis products is larger for slower projectiles, reaching one H every 5 Ang at the lowest speed studied (1 Bohr/fs). We observe the generation of new species such as hydrogen peroxide and formic acid. The former occurs when an O radical created in the collision process attacks a water molecule. The latter when the C projectile is completely stopped and reacts with two water molecules. These results are relevant for water ice.

Thermal and photoinduced processes at interstellar ices: a computational chemist's view
Stefan Andersson
SINTEF

Processes at interstellar ice surfaces form a challenging field of study, well suited for methods of computational chemistry. To date the number of such computational studies remains rather limited. Computer simulations are a valuable complement to experiments on astrochemically relevant systems. An overview will be given of suitable computational tools to study thermal and UV induced processes. Some examples will be given from simulations on the photodesorption of water ice and the hydrogenation of CO ice.

Computational study of astrochemical mechanisms on model dust grains
Davy Adriaens, T P M Goumans, C R A Catlow, S Viti and W A Brown
UCL

At the very low temperature-pressure combination found in the interstellar medium (ISM), reactions are unlikely to happen unless their activation barriers are sufficiently low. Some important molecules are too abundant to be exclusively formed in gas-phase reactions; these processes must therefore be catalysed. Catalysis by dust grains has already been shown to be important for H₂ formation. Dust grains are composed of carbonaceous and siliceous materials possibly covered in water ice. In addition to this, other molecules are also thought to be formed on the grain surface and experiments on models of these grains are currently being performed.
Here we will show theoretical results for the formation of methanol on a carbonaceous surface. Density Functional Theory has been used to study the effects of this surface as a model for a carbonaceous dust grain. Methanol is assumed to be formed from CO which is sequentially hydrogenated. The calculations show that some activation barriers for reaction are reduced on the surface, when compared to the gas phase.

Computational work on molecules and metal clusters adsorbed on grafine and metal surfaces
Lauri Halonen and Elina Sälli
University of Helsinki

Density functional theory (DFT) with periodic boundary conditions has been used to investigate adsorption properties of molecules and metals clusters on solid surfaces. We have been interested in vibrational properties of adsorbed molecules. Methods commonly used to describe vibrational motion of isolated molecules beyond the harmonic approximation have been applied to the adsorbed species. Although the absolute band positions cannot be reproduced accurately with the DFT methods, relative positions, i.e. experimental shifts of the adsorbed data compared with gas phase values, can often be computed accurately. For astrochemical purposes, grafine surfaces could provide an interesting model system for molecular adsorption. We have used he grafine surface to investigate adsorption energetics of gold and silver clusters.

Advanced Kinetic Monte Carlo Simulation
Jean-Claude Berthet and Hannes Jónsson
University of Iceland

The Kinetic Monte Carlo method is used to perform simulation of the time evolutions over time scale that is not accessible by classical dynamics simulation. Though this method has been successfully used in many cases, it does not perform well with systems where there are regions of low barriers surrounded by significantly higher barriers. The algorithm being much more likely to select a low barrier as the next step, the simulation gets stuck in those regions. We are working on an Advanced Kinetic Monte Carlo algorithm to deal with the issue. The states of the regions where the simulation is stuck are merged and then treated by the Monte Carlo algorithm as one state. This allows the simulation to move more rapidly out of the region.
The algorithm has been tested on one and two-dimension models to calculate the diffusion coefficient and the average time required to move between two basins. We are now testing the algorithm on a system of water molecules on a platinum surface to study the diffusion of the molecules, the formation of islands and over-layer growth.

Theoretical Study of the Formation of the Aminoacetonitrile Precursor of Glycine on Icy
Celine Toubin, Denise M. Koch, Gilles H. Peslherbe, and James T. Hynes
Universite Lille

Electronic structure calculations have been carried out for one of the key reactions in a Strecker synthesis route to the amino acid glycine, in connection with amino acid production in the interstellar medium (ISM). Density functional calculations at the B3LYP/6-31+G(d,p) level have been performed for the reaction between methanimine, CH₂NH, and the two isomers HNC/HCN, leading to aminoacetonitriles a known precursor of glycines in both the gas phase and on a model icy grain surface. All of the reaction paths have quite high barriers, but on a model interstellar grain icy surface, very considerable barrier reduction results due to a concerted proton relay mechanism. The significance of these results for glycine production in the ISM is discussed.

Poster Contributors:

The adsorption of C₆H₆ on surfaces of astrophysical relevance
John D Thrower, M. P. Collings and M. R. S. McCoustra
Heriot-Watt University

Laboratory investigations have shown that the surface of an interstellar dust grain is likely to provide a wide range of possible adsorption sites. Some of these may result in significantly stronger binding between adsorbed species and bare grain surfaces. We have studied the thermal desorption of benzene (C₆H₆) from an amorphous silica grain mimic and extracted the associated adsorption energy distribution. This reveals a significant population of sites that result in the C₆H₆ adsorption energy being increased by as much as 20 kJ mol^-1. Simulations performed at astrophysical heating rates indicate retention of molecules to significantly later times than if a single binding energy model is considered. Such effects are likely to impact on the thermal desorption of a wide range of species from grain surfaces. The desorption of C₆H₆ from a pre-adsorbed water ice film is also considered.

The irradiation of ammonia ice studied by NEXAFS spectroscopy.
Philippe Parent, F. Bournel, J. Lasne, C. Laffon, S. Lacombe, G. Strazzulla, S. Gardonio and S. Lizzit
Université Pierre et Marie Curie

Frozen ammonia and its hydrates are present in many icy objects of the outer Solar system and in the interstellar space medium. These ices are exposed to various radiation environments, and studying their photolysis is important to understand their role in astrochemistry. We have investigated the photolysis of pure ammonia ice films irradiated at 20 K with XUV photons, using Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy at the nitrogen K-edge. We have already applied this technique on a related system, the water ice (at the oxygen K-edge), where we obtained detailed insights on the bulk and the surface structures of H₂O ice [1] its heterogeneous chemistry [2] and its bulk photochemistry [3,4]. For ammonia, we have identified - with the help of quantum calculations - most of the byproducts, which allows us to propose a possible reaction scheme for this photochemistry [5].

[1] Ph. Parent, C. Laffon, C. Mangeney, F. Bournel, and M. Tronc, J. Chem. Phys. 117, 10842 (2002).
[2) Ph. Parent and C. Laffon, J. Phys. Chem. B 109, 1547 (2005).
[3) S. Lacombe, F. Bournel, C. Laffon, and Ph. Parent, Angew. Chem. Int. Ed. 45, 4159 (2006).
[4) C. Laffon, S. Lacombe, F. Bournel, and Ph. Parent, J. Chem. Phys. 125, 204714 (2006).
[5] Ph. Parent, F. Bournel, J. Lasne, S. Lacombe, G. Strazzulla, S. Gardonio, S. Lizzit, C. Laffon and S. Carniato, submitted.

Potential metallicity tracers in extragalactic dark clouds
Nadya Kunawicz, A. J. Markwick and P.J. Woods
University of Manchester

A time-dependent model of a dark cloud was constructed, in an attempt to model these clouds in low metallicity environments such as external galaxies. The models were calculated with varying initial elemental abundances of carbon, oxygen, nitrogen, sulphur and the heavy metals Fe, Mg and Na (henceforth, M). These abundances were taken from observations of HII regions in the LMC and SMC. The results were used to identify potential metallicity tracer species in dark clouds. The most useful tracers were ratios of two species, notably CO/OH and HCO⁺/CO, which trace the underlying carbon abundance and the underlying M abundance respectively. In the future, these results can potentially be extrapolated, so that tracers observed in galaxies at high redshift will allow the calculation of the underlying metallicity of an extragalactic dark cloud.

Electron Driven Processes In Ices: Decomposition & Synthesis Reactions
Anne Lafosse, L. Amiaud and R. Azria
University Paris-Sud

The release of secondary low-energy electrons (Ei < 30 eV) is one of the physical processes resulting from the irradiation with light or high-energy ions of icy grain mantles. Upon interaction with condensed molecules, low-energy electrons efficiently drive bond cleavages, thus generating a population of very reactive species in the medium. These species interact further within the volume, which leads to molecular decomposition and synthesis reactions. In the case of high-energy particles the chemical selectivity is low due to the large number of dissociative open channels. In contrast electrons, whose energy is smaller than the ionization and excitation thresholds of the considered molecular systems, can support the Dissociative Electron Attachment mechanism, known to be efficient and selective.
Electron induced chemistry within pure or binary ices was studied by High Resolution Electron Energy Loss Spectroscopy (HREELS) as a function of the irradiation energy, in order to determine the electron-induced primary reaction steps and thereby to get insight in the mechanisms involved in the overall reactions. In particular electron induced decarboxylation in condensed films of pure organic acids RCOOH (R = H, CH₃, C₂H₅, CF₃) occurs either via the formation of a resonant state [RCOOH]- around 1 eV, or via overlapping electron-induced dissociative processes above an onset located in the range 6-9 eV, depending on the carboxylic acid under consideration. Carbamic acid formation in CO₂:NH₃ binary mixtures was studied and the observed formation threshold suggests that radicals play a key role in this electron-induced synthesis reaction.

Studies of Hydrogen Formation on Interstellar Dust Grains Analogues
Elspeth Latimer, Farahjabeen Islam, Michael Ward and Stephen Price
UCL

Much of the matter in the Universe exists in the form of stars and planets, however, the region between the stars is far from empty. This area is known as the Interstellar Medium (ISM) and it contains vast areas of gas and dust grains, called interstellar clouds. The conditions of low temperature and pressure prevent the either the H + H gas phase or 3-body reactions from occurring, so there must be another reaction pathway to account for the high abundance of molecular hydrogen. It is now believed that the formation of H₂ in the ISM takes place on the surface of dust grains.
The experiment, presented in this poster, is designed to probe the ro-vibrational excitation of HD formed on an interstellar dust grain analogue. Nascent HD formed on our HOPG surface at 15 K has been detected in the ro-vibrationally excited states v? = 1 – 7. Two different scaling methods were employed to place the rotational distributions for each vibrational level on a common scale. One methodology simply using the original data scaled with the relevant Franck-Condon factors. A second approach involved deriving scaling factors from a series of rapid consecutive scans over one particular J? state from each vibrational level for v? = 3 – 7. Both scaling methodologies indicate that v? = 4 is the most populated vibrational state for HD formed on the surface.

Ortho/Para spin conversion of D₂ on a porous water ice surface at 10 K in the presence of O₂ traces
Jean-Hugues Fillion, M. Cherouri, A. Lekic, F. Dulieu, X. Michaut, H. Chanbouni, H. Mokrane and J.L. Lemaire
UPMC-LPMAA

Molecular hydrogen is the most abundant molecule in the universe. It is at the center of several fundamental questions in astrophysics, and in particular the physics and chemistry of the interstellar medium (ISM). The relative proportions of ortho- and para-populations of H₂ are of particular importance for establishing the nature of interstellar shocks, the energy budget of molecular clouds and may contain information on the history of the grains.
We present results on the nuclear spin conversion of D₂ adsorbed on a porous water ice surface at 10 K Experiments have been performed with the FORMOLISM (Formation of Molecules in the ISM) setup available at the LAMAp/LERMA laboratory. Using laser multi-photon ionization (REMPI) and time of flight (TOF) mass spectrometry, it is possible to achieve TPD experiments (Thermally Programmed Desorption) with rovibrational quantum states resolution. The transitions E,F (v'=0,J) - X (v '' =0,J) were selected to detect separately the molecules in the ortho state (J=0) or in the para state (J=1).
Our results derive from relative population measurements of the molecules just after desorption from the cold surface. A very small amount of molecular oxygen deposited on the ice surface induces a conversion from the para to the ortho state. The conversion times have been measured at several O₂ and D₂ coverage. The general trend of these experimental results can be understood considering the mobility of the D₂ molecules on the surface at 10 K together with the catalytic role of paramagnetic O₂.

Towards Understanding the Formation of Water on Interstellar Dust Grains
Victoria Frankland, M. P. Collings and M. R. S. McCoustra
Heriot-Watt University

Exposed to the harsh radiation fields of interstellar space, few molecules can escape photodestruction. However, the vast clouds of gas and dust that accumulate in the gulfs of space between the stars (known as the interstellar medium) have been observed to contain more than 120 different molecular species [1]. The low temperature (10-100 K) and pressure (10-14 mbar) conditions within the interstellar medium limit the range of viable gas-phase reactions resulting in the gas-phase chemistry alone being insufficient to explain the observed abundances of some key chemical species (for example, H₂ and H₂O). Dust grains provide a surface on which adsorbed species can react [2] and hence an alternative pathway to key interstellar molecules. Indeed, this has been proven for the efficient formation of H₂ both experimentally [3-6] and computationally [7-9].
Surface chemistry within an ultrahigh vacuum chamber is being used to explore the surface irradiation reactions of O on a range of astrophysically relevant surfaces using atomic beam methods. In these experiments, the products are identified using temperature programmed desorption. The results of the experiments are interpreted using kinetic analysis of a simple surface mechanism. Analogous experiments will be conducted using an atomic H beam. The ultimate aim of this research will be to combine the two beams to study in situ H₂O formation on a grain surface.

[1] from “129 reported interstellar and circumstellar molecules” http://www.cv.nrao.edu/%7Eawootten/allmols.html (assessed 7th May 2009)
[2] R. Gould, E.Salpeter, Astrophys. J., 1963, 138, 393
[3] G. Vidali, J. Roser, G. Manico, V. Pirronello, Adv. Spa. Res., 2004, 33, 6
[4] L. Hornekaer, A. Baurichter, V. Petrunin, A. Luntz, B. Kay, A. Al-Halabi, J. Chem. Phys., 2005, 122, 124701
[5] F. Dulieu, L. Amiaud, S. Baouche, A. Momeni, J. Fillion, J. Lemaire, Chem. Phys. Lett., 2005, 404, 187
[6] J. Perry, J. Gingell, K. Newson, J. To, J. Watanabe, S. Price, Meas. Sci. Technol., 2002, 13, 1414
[7] O. Biham, A. Lipshtat, Phys. Rev. E, 2002, 66, 56103
[8] N. Katz, I. Furman, O. Biham, V. Pirronello, G. Vidali, Astrophys. J., 1999, 522, 305
[9] H. Cuppen, E. Herbst, Mon. Not. Roy. Astron. Soc., 2005, 361, 565

Adsorption and Desorption of Carbon Dioxide from Polar Interstellar Ice Analogues
John Edridge, K. Freimann, D. J. Burke and W. A. Brown
UCL

Carbon dioxide (CO₂) has been detected in the interstellar medium within ices accreted on the surface of dust grains. With typically less than 5% of interstellar CO₂ being detected in the gas phase, it has been shown to be a major constituent of icy mantles with the solid phase abundance of CO₂ found to be around 10 - 23% relative to H₂O. CO₂ has been detected in both polar (H₂O and CH₃OH rich) and apolar (CO, CO₂ and O₂ rich) icy mantles. Understanding the behaviour of CO₂ in polar ices is therefore of importance in understanding the chemistry of dense molecular clouds and hot core regions, where its behaviour varies depending on its local environment.
Reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) experiments have been undertaken to investigate pure CO₂, CO₂ and H₂O layers, CO₂:H₂O mixtures and CO₂ adsorbed on CH₃OH:H₂O mixtures all adsorbed on a highly oriented pyrolytic graphite (HOPG) surface at 28 K. Multilayers of crystalline CO₂ ice form on a bare HOPG surface at 28 K, desorbing around 80 K upon annealing. On a film of pre-adsorbed amorphous H₂O, or a CH₃OH:H₂O mixture, a thin layer of H₂O associated CO₂ forms, shown by the presence of a new CO₂ infrared band and OH dangling bonds in the RAIR spectra. H₂O associated CO₂ becomes trapped within pores of the H₂O film, desorbing with the amorphous to crystalline phase transition of the H₂O at 160 K. In a CO₂:H₂O mixture, multilayers of crystalline CO₂ cannot form. Upon annealing, a small amount of CO₂ is able to diffuse through the mixture to desorb at 90 K. The remaining CO₂ becomes trapped within the H₂O matrix. The amorphous nature of the H₂O surface therefore plays an important role in the behaviour of the CO₂ ice and can be used to identify the nature and thermal history of the ice.

Potential Energy and Morphology Effects in Low Energy Ion Irradiation of Water Ices
Adam Hunniford, A. Dawes, D. Fulvio, B. Sivaraman, T.L. Merrigan, R. W. McCullough, N. J. Mason and M.E. Palumbo
Queen's University Belfast

Ion processing plays an important role in the chemical and physical modification of ice surfaces in astrophysical environments. Magnetospheric ions surrounding the Gas Giants in the outer Solar System impinge upon and modify icy satellite surfaces creating new chemical species, incorporating elements not originally present in the local ice composition.
¹³C⁺ and ¹³C²⁺ ions were produced by an Electron Cyclotron Resonance ion source, accelerated to several keV and were incident upon pure water ice samples. The most significant modifications observed within were the appearance of features corresponding to ¹³CO₂. The yield is highest in more porous ice formed at 30 K (compared to that formed at 90 K) which is assigned to the larger internal surface area. This has been observed elsewhere [Icarus 180 265 2006] and demonstrates the importance of porosity in determining molecular formation.
Another set of experiments were carried out where the morphology was kept the same and irradiation temperature varied. At 30 K, no difference was observed between C⁺ and C²⁺ ion irradiation. However, at 90 K, a significantly greater yield was observed with C²⁺ ions. This may be a result of the additional potential energy released by C²⁺ upon single electron capture (sufficient to access dissociative ionisation channels). Products of this dissociation are then able to migrate within the ice and react yielding further CO₂. Thus, at higher temperatures, where the mobility is higher, the CO₂ yield is greater [Phys. Chem. Chem. Phys. 9 2886 2007].

Experimental evidence for water formation via ozone hydrogenation on a water ice covered surface under interstellar conditions
Henda Chaabouni, H. Mokrane, M. Accolla, E. Congiu, F. Dulieu, M. Chehrouri and J. L. Lemaire
l'Université de Cergy Pontoise

In dense cold interstellar clouds, dust grains are covered with ice mantle mainly composed of water H₂O ice [1]. The formation of water in the gas phase is not efficient to reproduce the observed abundance. Water ice in dark clouds is likely to be formed directly on grain surfaces. Several reaction schemes for water formation on cold grain surfaces, predicted by astrochemical models are likely to occur via the hydrogenation of O, O₂ and O₃ [2,3]. Although O3 molecules have not so far been detected on interstellar dust grains, their presence is presumed.
Experiments are performed with the FORMOLISM setup using the Temperature Programmed Desorption (TPD) diagnostics. This work presents the first experimental evidence of water formation from O₃ reaction with D-atoms on non-porous amorphous solid water ice under conditions relevant to interstellar molecular clouds.

[1] Gibb, E. L., Whittet, D. C. B., Schutte, W. A., Boogert, A. C. A., Chiar, J. E., Ehrenfreund, P., Gerakines, P. A., Keane, J. V., Tielens, A. G. G. M., van Dishoeck, E. F., Kerkhof, O. 2000, ApJ, 536, 347
[2] Tielens, A. G. G. M., Hagen, W. 1982, AA, 114, 245.
[3] Cuppen. H. M., Herbst. E, 2007, ApJ, 668, 294

This page last modified 12 June, 2009 by John Edridge

University College London
Department of Chemistry
United Kingdom
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