New Astronomical Challenges In Surface Science.

University College London, 18th - 19th April 2005.

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Monday 18th April 2005
1.30 pm WELCOME AND INTRODUCTION
2.00 pm Paola Caselli
  • Surface chemistry in interstellar clouds
    Surface chemistry in interstellar clouds
    Chemical processes happening on dust particles play a key role in the formation of molecular species in the interstellar medium. First of all, dust grains are required for the formation of H2, the most abundant species (by at least 10,000 times) in molecular clouds. it is well established that solid water quickly forms on grain surfaces in molecular clouds, as soon as the visual extinction overcomes a critical value. In denser clouds, CO and other neutral species disappear from the gas phase, the most likely explanation being that they are adsorbed onto dust particles, enhancing the thickness and likely chemical complexity of icy mantles. The freeze-out boosts deuterium fractionation in the gas phase, but for species such as H2CO and CH3OH, surface chemistry, followed by an efficient selective desorption mechanism, is need to reproduce the observations. In the earliest phases of newly born proto-stars, the surrounding gas and dust is heated by the stellar radiation field and the gas composition is enriched by complex molecules, which cannot be produced in the gas phase because of inefficient or absent chemical routes; surface chemistry is again invoked to explain the observed spectra. In this talk, I will review recent progress on surface chemical models, pointing out the importance of a correct mathematical treatment to take into account the discrete nature of dust particles. Recent results on the use of inhomogeneous grain surfaces, which allows efficient H2 formation, in a larger temperature range than previously thought, will also be shown. Finally, a comparison between current model predictions and observational results will be furnished, with the aim of highlighting the main problems still open in this field.
2.50 pm Jonathan Rawlings
  • The physical and chemical properties of dust grains in nova outflows
    The physical and chemical properties of dust grains in nova outflows
    Chemical processes happening on dust particles play a key role in the formation of molecular species in the interstellar medium. First of all, dust grains are required for the formation of H2, the most abundant species (by at least 10,000 times) in molecular clouds. it is well established that solid water quickly forms on grain surfaces in molecular clouds, as soon as the visual extinction overcomes a critical value. In denser clouds, CO and other neutral species disappear from the gas phase, the most likely explanation being that they are adsorbed onto dust particles, enhancing the thickness and likely chemical complexity of icy mantles. The freeze-out boosts deuterium fractionation in the gas phase, but for species such as H2CO and CH3OH, surface chemistry, followed by an efficient selective desorption mechanism, is need to reproduce the observations. In the earliest phases of newly born proto-stars, the surrounding gas and dust is heated by the stellar radiation field and the gas composition is enriched by complex molecules, which cannot be produced in the gas phase because of inefficient or absent chemical routes; surface chemistry is again invoked to explain the observed spectra. In this talk, I will review recent progress on surface chemical models, pointing out the importance of a correct mathematical treatment to take into account the discrete nature of dust particles. Recent results on the use of inhomogeneous grain surfaces, which allows efficient H2 formation, in a larger temperature range than previously thought, will also be shown. Finally, a comparison between current model predictions and observational results will be furnished, with the aim of highlighting the main problems still open in this field.
3.10 pm Helen Fraser
  • The Effects of CO and N2 Desorption on the Gas-phase Chemistry of Pre-stellar Dense Cores
    The Effects of CO and N2 Desorption on the Gas-phase Chemistry of Pre-stellar Dense Cores
    Millimeter observations of pre- and proto-stellar cores show that the abundances of the gas-phase tracer molecules C18O and N2H+ anti-correlate with each other and often exhibit “holes” where the density is greatest. These results are reasonably reproduced by astrochemical models which include differential freeze-out and desorption of the volatile species CO and N2, if the ratio between the binding energies of N2 and CO, RBE, is taken to be between 0.5 and 0.75. In this talk, I will present recent experimental data on the desorption of CO and N2 from layered and mixed ices at temperatures relevant to dense cores, studied under ultrahigh vacuum laboratory conditions using temperature programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS). From control experiments with pure ices, RBE = 0.923 ± 0.003, given E2 (N2-N2) = 790 ± 25 K and Eb (CO-CO) = 855 ± 25 K. In mixed (CO:N2 = 1:1) and layered ice systems (CO above and below N2), both molecules become mobile within the ice matrix at temperatures as low as 20 K, and appear miscible. Consequently, although a fraction of the deposited N2 desorbs at lower temperatures than CO, up to 100% of the N2 molecules leave the surface as the CO itself desorbs, a process not included in existing gas-grain models. This co-desorption suggests that for a fraction of the frozen-out molecules, RBE is unity; in the mixed ice Eb (CO-CO) is raised slightly, lowering RBE to 0893 ± 0.003. The relative difference between the CO and N2 binding energies as derived from these experiments is therefore significantly less than that currently adopted in astrochemical models.

    Of greater chemical interest it is clear that non-linear ice film growth has been observed during these experiments, with significant effects on the desorption kinetics of both species. The most recent kinetic models of these data will also be presented, illustrating how laboratory surface astrophysics can positively contribute to both astronomy and physical chemistry.
3.30 pm TEA/COFFEE
4.00 pm Mike Barlow
  • Hydrogenation Reactions on Carbon Grain Surfaces
    Hydrogenation Reactions on Carbon Grain Surfaces
    The direct chemical reaction of H-atoms with carbon atoms on carbon grain surfaces, to form hydrocarbons, has been suggested to be of astrophysical significance. I will discuss the process and its possible relevance to the origin of the mid-IR emission bands (UIBs) seens in the spectra of many astronomical sources.
4.20 pm June McCombie
  • Identifications of PAHs
    Identifications of PAHs
    The Unidentified Infrared Emission (UIR) Bands at 3.3, 6.2, 7.7, 8.6 and 11.3 microns are ubiquitous along many lines of sight and polycyclic aromatic hydrocarbons (PAHs) are thought to be good candidate carriers for these bands. Commonly it is proposed that PAHs are formed either by collisions between dust grains or from reactions between smaller unsaturated hydrocarbons in carbon-rich stars. However at present no single PAH has been identified observationally in the interstellar medium.

    It is clear that observation evidence of PAHs around carbon-rich AGB stars would aid hypotheses of PAH formation and this has led us to carry out spatially resolved observational studies on a morphologically well defined object, the Red Rectangle, in an attempt to probe the evolution of these bands as a function of distance from the central binary star HD 44179. The evolution of the spectral profiles provides useful insight into the nature of the carriers and suggests the appearance of different carriers along the biconical interfaces of this unusual protoplanetary nebula. The growth of a new 3.28 micron feature is reported, comparison with laboratory emission data suggesting that the new feature arises from acene structures whereas the main band is from condensed PAHs, and analysis of the mid-infared low-resolution spectra gives a key to the relative abundances of ionised and hydrogenated/dehydrogenated species in regions of the nebula.
4.40 pm POSTER SESSION

Tuesday 19th April 2005
9.00 am Theodore E. Madey
  • Far-out Surface Science - Relevance to Surface Processes in Space
    Far-out Surface Science - Relevance to Surface Processes in Space
    Radiation-induced surface processes are prevalent in the solar system. Solar photons, together with energetic ions and electrons from the solar wind and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, and asteroids. Many of these bodies exist in ultrahigh vacuum environments so that direct particle-surface collisions dominate the interactions.

    Two topics relating surface science to space science will be discussed. The first concerns "far-out" surface science over long length scales in interplanetary space. We discuss the role of thermal and radiation-induced surface processes on the origins of the tenuous atmospheres of Na and K atoms surrounding the Moon, the planet Mercury, and Jupiter's icy satellite Europa. In studies of model mineral oxides as well as a lunar basalt sample, we find evidence that photon-stimulated desorption by UV solar photons is a dominant source process for alkalis in the tenuous atmospheres.

    The second topic concerns radiation-induced processes in H2O ice, a nearly ubiquitous substance in space environments. We describe the use of a variety of surface spectroscopies (X-ray photoelectron spectroscopy, XPS, refection-absorption infrared, RAIR, temperature programmed desorption, TPD, electron-stimulated desorption, ESD, and laser techniques) to characterize radiation-induced chemistry of amorphous and crystalline ice films (a few nm thick), as well as molecules trapped in such ice films. Much of the chemistry is attributed to effects of low-energy electrons excited by higher energy radiation.
9.50 am Ian Sims
  • Low Temperature Molecular Collisions - Implications for Astrochemistry
    Low Temperature Molecular Collisions - Implications for Astrochemistry
    The importance of gas-phase molecular collisions - both reactive and inelastic - between neutral species will be discussed in relation to astrochemistry, in particular in cold environments such as dense or dark interstellar clouds. A review of current state-of-the-art experiments in low temperature chemical kinetics and energy transfer will be given, with an emphasis on the use of the CRESU (Reaction Kinetics in Uniform Supersonic Flow) technique for the study of gas phase collisional processes at temperatures down to ca. 10 K and below. Trends in reactivity will be discussed, and a selection of new and unpublished results will be presented.
10.10 am Chris Mayhew
  • Gas-Phase Ion-Molecule Reactions
    Gas-Phase Ion-Molecule Reactions
    The Molecular Physics Group at the University of Birmingham investigates the reactions of ions and electrons with molecules. The raw results of this research are the rate coefficients for the reactions and the identification of the ion products. The projects we pursue reveal the details of how elementary reactions between molecules and ions and electrons proceed. The laboratory techniques used by us to study ion-molecule and electron attachment processes will be described.
10.30 am Christine Joblin
  • Studying Interstellar Gas/Grain Interactions in the Laboratory: The PIRENEA Experiment
    Studying Interstellar Gas/Grain Interactions in the Laboratory: The PIRENEA Experiment
    One of the challenges for astrochemistry experiments is to approach the physical conditions that prevail in interstellar space: absence of collisions on long timescales, cold environment in the interior of molecular clouds (10-50 K) and presence of stellar UV photons (hn < 13.6 eV) at the surface of clouds. PIRENEA, Piège à Ions pour la Recherche et l'Etude de Nouvelles EspËces Astrochimiques, is an experimental set-up which has been developed according to these specifications. Using the trapping and mass spectrometry analysis of an ion cyclotron resonance cell which is connected to cryogenic shields, original studies can be performed on physical processes which occur on long timescales and which are the ones of interstellar relevance: radiative exchanges involving IR emission, photodissociation near threshold, radiative association.

    We will focus here on the study of the gas/grain interactions where the grain is an isolated system of nanometric size and the gas is a molecule such as H2, O2, H2O, CO and CH3OH. Our grain test case was the polycyclic aromatic hydrocarbon (PAH) molecule C24H12+ and its dehydrogenated derivatives. We will present some of the results and describe the experiments that are now planned on nano-grains: PAH clusters, PAH-Fe complexes, carbonaceous and silica nanoparticles.
10.50 am TEA/COFFEE
11.20 am Andrew Hodgson
  • Thermal Effects in Hydrogen Recombination Dynamics
    Thermal Effects in Hydrogen Recombination Dynamics
    Surface temperature has a direct influence on the recombination dynamics of hydrogen at surfaces, changing the energy available for reaction and hence the balance between steric and energetic constraints on the reaction channel. The behaviour seen for recombination at metal surfaces will be described and used to extrapolate to other interfaces.
11.40 am Steve Price
  • Laboratory Studies of H2 Formation on Graphite at 15 K
    Laboratory Studies of H2 Formation on Graphite at 15 K
    We will discuss the results from an experimental programme which measures the rovibrational distribution of molecular hydrogen formed, by recombination of a controlled beam of atomic hydrogen, on cold prototypical interstellar surfaces under ultra-high vacuum conditions. These laser spectroscopic results show that vibrationally excited hydrogen molecules, v =1 and v =2, are certainly formed in such heterogeneous processes. These excited vibrational levels possess moderate rotational excitation. The strengths, and weaknesses, of this experimental approach will be discussed and its relation to recent experiments to study H2 formation on ice surfaces. Recent results to study the formation of HD will also be presented.
12.00 pm Tom Govers
  • Recombination of Atomic Hydrogen Physisorbed on Low Temperature Surfaces
    Recombination of Atomic Hydrogen Physisorbed on Low Temperature Surfaces
    Molecular beam experiments have been conducted combining bolometry and mass spectrometry to investigate adsorption and recombination of atomic hydrogen on a low temperature surface. Hydrogen atoms physisorbed on polycrystalline water ice are shown to be sufficiently mobile to recombine efficiently even at surface temperatures as low as 3 K.

    Molecules are formed with substantial internal energy, probably of the order of 35000 K, and are immediately released when formed. Coverage by molecular hydrogen plays an important role in determining overall recombination efficiency and may self-regulate recombination in interstellar clouds: on hydrogen-free grains recombination is limited by the low sticking coefficient of hydrogen atoms, while on grains covered by molecular hydrogen the binding energy is reduced so that recombination is limited by the rapid evaporation of physisorbed atoms.
12.20 pm LUNCH
1.30 pm George Darling
  • Protons Interacting with Ice Surfaces: High Reflection and Water Induced Desorption
    Protons Interacting with Ice Surfaces: High Reflection and Water Induced Desorption
    We present results of classical trajectory calculations on the sticking of protons to the basal plane (0001) face of crystalline ice, for normal incidence at a surface temperature of 80 K. The calculations were performed for moderately low incidence energies (E i ) ranging from 0.05 to 4.0 eV. Surprisingly, significant reflection is predicted at low values of Ei (< 0.2 eV). The sticking probability increases with Ei and converges to unity for Ei > 0.8 eV. In the case of sticking, the proton is trapped in the ice forming a Zundel complex (H5O2+), with an average binding energy of 9.9 eV, independent of Ei . In nearly all sticking trajectories, the proton is implanted into the ice surface, with a penetration depth that increases with Ei . The strong interaction with the neighbouring water molecules leads to a local rupture of the hydrogen bond network, resulting in collision induced desorption of water (puffing). In nearly all trajectories in which water desorption occurs, a single 3-coordinated water molecule is desorbed from the topmost monolayer.
1.50 pm Maria Elisabetta Palumbo
  • The Effects of Ion Irradiation on Amorphous Solid Water
    The Effects of Ion Irradiation on Amorphous Solid Water
    I have studied by infrared absorption spectroscopy the effects of ion irradiation on the morphology of amorphous water ice. Both the behaviour of the OH dangling bond feature and the ability to trap carbon monoxide have been used to investigate the evolution of icy samples after irradiation. The results obtained indicate that the porosity of amorphous water ice decreases after ion irradiation. Here I will present the experimental results and discuss their relevance to our understanding of the properties of interstellar water ice.
2.10 pm Andrew Horn
  • Internal Reflection Methods for the Study of Condensed Ices
    Internal Reflection Methods for the Study of Condensed Ices
    Over the last 20 years, a number of methods have been applied to the study of reactive uptake on icy materials under laboratory conditions, aimed at probing the kinetics and mechanisms of heterogeneous reactions of atmospheric and astrophysical significance. Following on from initial work using reflection-absorption IR spectroscopy, we have developed new methods based upon attenuated total internal reflection (ATR). Using this methodology, we have been able to monitor a number of features of reactive uptake and transformation on ice and acidic hydrates, including direct measurements of diffusion in thin films.
2.30 pm David Clary
  • Calculations of Reactions on Surfaces of Astrophysical Interest
    Calculations of Reactions on Surfaces of Astrophysical Interest
    This talk will describe how electronic structure and dynamics calculations on chemical reactions on surfaces can be useful for astrophysics. Particular emphasis will be placed on a description of the various calculations that have been carried out on the formation of hydrogen molecules on graphite – the simplest model for an interstellar grain.
3.20 pm TEA/COFFEE
3.40 pm Inmaculada Gomez-Morilla
  • Ion Beam Analysis of Astronomical Samples at the Surrey Ion Beam Centre
    Ion Beam Analysis of Astronomical Samples at the Surrey Ion Beam Centre
    The facilities at the University of Surrey Ion Beam Centre offer a wide range of applications form implantation and lithography, to analysis. Some of the analytical techniques are Proton Induced X-ray Emission (PIXE), Rutherford Backscattering (RBS), Nuclear Reaction Analysis (NRA), and Scanning Transmission Ion Microscopy (STIM). Analysis can be performed either in vacuum with a focused ion beam of 1 m m of spatial resolution and energies of 1 – 3 MeV, or in air with a resolution of 50 m m.

    The applicability of two of these techniques, PIXE and STIM, for the analysis of comet dust samples, has been recently investigated. The NASA “STARDUST” spacecraft collected dust from Comet Wild 2 in January 2004 and is expected to deliver samples back to Earth in January 2006 (see http://stardust.jpl.nasa.gov/ ). The dust was collected in silica aerogel, a material with an ultra-low density, ranging from about 0.005 to 0.05 g/cc. The primary scientific objective of the mission is to reveal the physical and chemical composition of a comet, a body that is thought to be representative of the most primitive material in the Solar System. The returned samples should provide vital information on the dominant chemical and mineralogical processes that occurred in the early stages of solar system evolution, prior to planetary formation.

    Analogue samples have been prepared in terrestrial laboratories by hypervelocity (ca. 6 km s-1) implantation of appropriate mineral particles of known composition into blocks of silica aerogel. Several recent papers have described how impacted grains can be analysed with a suite of different techniques, to determine the best ways to examine the real samples. Here we present preliminary results from PIXE and STIM experiments in the United Kingdom , and assess the potential and limitations of these techniques for characterisation of extraterrestrial material.
4.00 pm David McPhail
  • Application of Secondary Ion Mass Spectrometry and Focused Ion Beam Milling to the Study of Micrometeorite Impacts
    Application of Secondary Ion Mass Spectrometry and Focused Ion Beam Milling to the Study of Micrometeorite Impacts
    Micrometeorites are a major issue in Low Earth Orbit (LEO) as they can cause damage to satellites. The micrometeorites can be in two forms, man-made or natural. The Surface Analysis Centre at Imperial College has been collaborating with the Natural History Museum , the Lawrence Livermore Labs and Berkeley University in projects where we have been studying various aspects of micrometeorites impacts into a variety of targets including aerogel, recovered solar cells from the Hubble telescope and a recovered titanium fuel tank.

    One of the main issues with the aerogel is how to to access the particles as they can be embedded to depths of tens of microns. We are using our FEI FIB 200 focused ion beam system to cut away the material around the particles and we are developing strategies for then removing the particles. Mass spectrometry and imaging of selected areas for metallic particles will then be conducted at high lateral resolution in the FIB-SIMS. An attempt will then be made to measure isotopic ratios of metallic elements found for comparison with natural abundance ratios with full assessment of the errors.

    The study of the impacts into silicon based solar cells and titanium are concerned with residue analysis, primarily, and the very high resolution and sensitivity of SIMS is a key aspect of this work.

    The ion beam based surface analysis techniques offer both analysis and ion beam based milling. SIMS has ppm sensitivity, and in different configurations can offer sub-nanometre depth resolution and 50nm lateral resolution of surfaces of interest.
4.20 pm CLOSE

Posters:

Thomas Bell Surface Chemistry in Photon-dominated Regions
Natacha Bruneleau Gas/Grain Interactions in Interstellar Chemistry Explored with the PIRENEA Set-up
Mark Collings Astrophysics in a Surface Science Laboratory
Ian Ford Kinetics of Heterogeneous Nucleation for very Small Adsorbed Populations
Antonio Hales Near-Infrared Scattered Light Models of the Disk Around TW Hya
Lauri Halonen A Density Functional Theory Study of the Adsorption Energetics and Stretching Vibrations of Ammonia Adsorbed on a Nickel Surface
Virginie Jantou Analyses of Micrometeorite Impact Features on the Salyut 7 Russian Space Station Titanium Tank
Mercedes Ramos Lerate High Spectral Resolution Far-IR ISO LWS Observations
Alastair Murray Booth Towards Fast Experimentation with NEXAFS under Control of Environmental Conditions: Possible: Applications in the Astrochemical Community?
Lidija Siller VUV Photoinitiated Reactions in Solid CO 2 at 20 K: Study by Photoemission Spectroscopy
Anthony Stone Adsorption of Water on Barium Fluoride (111)
Angela Wolff Mixed Methanol/Water Ice on Cosmic Dust Grain Analogues
Anita Dawes Photoabsorption studies of ice films: Part I
Michael Davis Photoabsorption studies of ice films: Part II

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