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Investigation of the adsorption, desorption and formation of molecules on the surface of cosmic dust grains
Current work is concerned with investigating the adsorption, desorption and formation of small molecules, such as methanol, water, carbon dioxide and carbonyl sulphide, on the surface of interstellar dust grains. These molecules have all been observed in interstellar regions frozen out in ices in abundances that cannot be accounted for by gas phase reactions. Astronomers have proposed that they may instead be formed by heterogeneous reactions which take place on the surface of dust grains (see figure). Cone nebula (NGC 2264). Copyright NASA, H. Ford (JHU), G. Illingworth (USCS/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA Our work uses the technique of surface infrared spectroscopy to probe the adsorbate species that are formed on a model dust grain surface, dosed with the relevant reactants. The experiments involve studying molecular synthesis on graphite, as well as on thin films of water, carbon and, in the future, SiO2. Recent experiments include temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) investigations of the adsorption/desorption of model interstellar ices containing carbon dioxide, carbonyl sulphide (see figure below), and sulphur dioxide. In all cases we have investigated pure ices, layered ices where these species are adsorbed on water and mixtures of these species in water. The latter systems are particularly relevant to the interstellar medium, as most ices usually consist of a mixture of molecules These experiments provide detailed kinetic parameters for desorption which can be incorporated directly into astronomical models. We are just beginning a new project to look at the adsorption, desorption, stability and formation of glycolaldehyde in the interstellar medium. Glycolaldehyde has recently been discovered in star-forming regions. It is the simplest of the monosaccharide sugars and it reacts with propenal to form ribose, a central constituent of RNA. Because of this, it is thought that glycolaldehyde may have a role to play in the origins of life in our universe. Gas phase reactions are thought to be too slow to produce detectable abundances of glycolaldehyde during the star and planet formation process. It is therefore thought that surface processes, involving thermal, UV and electron processing of interstellar ices, may be responsible for the formation of this species. This project will investigate glycolaldehyde formation, desorption and destruction from several fronts: experimental, theoretical, astrochemical modelling and observational, bringing together experts from astronomy, astrochemistry, surface chemistry and computational chemistry. To complement the experimental studies, we have also undertaken theoretical investigations of the formation of small molecules on the surface of model dust grains using DFT and QM/MM techniques. Our most recent results show that silica surfaces catalyse the formation of methanol, particularly when negatively charged defects are present. Other results also show that the formation of OCS can both take place on a coronene surface (see figure below). This work forms part of a wider investigation of the formation of molecules in interstellar space which is currently taking place in the UCL Centre for Cosmic Chemistry and Physics. This work also forms part of the LASSIE ITN network. Dr Brown is also a founding member of the AstroSurf network. This work was featured in the 2004 and 2006 Royal Society Science exhibition via an exhibit called Stars R Us!.
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