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The Interactions and Reactions of Atoms and Molecules on the Surfaces of Model Interstellar Dust Grains

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The Interactions and Reactions of Atoms and Molecules on the Surfaces of Model Interstellar Dust Grains A thesis submitted for the degree of Doctor of Philosophy Helen Jessica Kimber Department of Chemistry University College London 2016 -I, Helen Jessica Kimber, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. Signed, i Abstract The elemental composition of the known universe comprises almost exclusively light atoms (~99.9% hydrogen and helium). However, to date, close to 200 different molecules have been detected in the interstellar medium (ISM) where their distribution is far from uniform. The vast majority of these molecules are contained within vast clouds of gas and dust referred to as interstellar clouds. Within these interstellar clouds, many of the molecules present are formed via gas-phase ion-neutral reactions. However, there are several molecules for which known gas-phase kinetics cannot account for observed gas-phase abundances. As a result, reactions occurring on the surface of interstellar dust grains are invoked to account for the observed abundances of some of these molecules. This thesis presents results of experimental investigations into the interaction and reactions of atoms and molecules on the surface of model interstellar dust grains. Chapters three and four present results for the reaction of (3P)O on molecular ices. Specifically, the reaction of (3P)O and propyne or acrylonitrile. After a one hour dosing period, temperature programmed desorption (TPD), coupled with time-of-flight mass spectrometry (TOFMS), are used to identify (3P)O addition products. Each reaction is studied at a series of fixed surface temperatures between 15 K and 100 K and a kinetic model is used to determine desorption energies of reactants and reaction probabilities of reaction. In the case of the reaction of (3P)O and propyne two products with the empirical formulae C3H4O and C3H4O2 are observed. Transmission infrared spectroscopy (TIR) identified two isomers of the single addition product (propenal and methyl ketene) whilst only one isomer of the double addition product (glycidaldehyde) was observed. Furthermore, the TIR data reveals a product which remains on the surface under vacuum at room temperature which is assigned to the polymer β-propiolactone. 3 In the case of the surface reaction of O( P) and acrylonitrile (C3H3N) on a acrylonitrile/O2 matrix a single addition product is identified as cyanoethylene oxide. Finally, Chapter five presents results for the desorption characteristics of O2 and CO2 from amorphous porous fullerene-like carbonaceous dust grains produced in laser ablation experiments. In these experiments O2 and CO2 are found to have significantly larger binding energies to the surface when compared to the traditional carbonaceous dust grain analogue graphite. The effect of these large binding energies on the desorption characteristics under interstellar conditions is modelled. ii Acknowledgements Firstly I would like to thank my supervisor Professor Stephen Price. His support and encouragement throughout my PhD has been paramount especially whilst writing this thesis. I am also grateful to the present and past members of the Price group who have allowed me to discuss, sometimes at great lengths, the results presented in this thesis. In particular Felicity Gossan, Catherine Higgins, Dr Courtney Ennis and Dr Michael Parkes. During my PhD I visited the Friedrich-Schiller-University in Jena to collaborate with the Jäger group. I am particularly grateful to Dr Cornelia Jäger, Dr Daniele Fulvio and Hagen Walter for their help performing these experiments. I am also grateful to my family and friends who have listened to, and supported, me throughout my PhD. In particular my mum, Julia Callender, and my sister, Kristy Kimber. Finally, I would like to thank my husband Hamish Mcdougall, who has taken the brunt of my thesis writing, I intend to be a more present wife from now on. Thank you for being understanding. iii List of Publications Listed below are previous and expected future publications of the work presented in this thesis. Single and double addition of oxygen atoms to propyne on surfaces at low temperatures H. J. Kimber, C. P. Ennis and S. D. Price, Faraday Discuss., 2014, 168, 167-184. Addition of oxygen atoms to acrylonitrile on surfaces at low temperatures does not lead to saturation In preparation Contributions to interstellar dust grains from the addition of oxygen atoms to propyne on surfaces at low temperatures In preparation The desorption of O2 and CO2 from fullerene-like amorphous porous carbonaceous interstellar dust grain analogues In preparation iv Table of Contents Abstract ................................................................................................................................ ii Acknowledgements ..................................................................................................................... iii List of Publications ..................................................................................................................... iv List of Acronyms and Abbreviations ...................................................................................... viii List of Figures .............................................................................................................................. ix List of Tables ............................................................................................................................. xv Chapter 1 – Introduction .............................................................................................................. 1 1.1 Overview ......................................................................................................................... 1 1.2 A Brief Chemical History of the Universe ........................................................................ 2 1.3 Interstellar Clouds ........................................................................................................... 5 1.4 Interstellar Dust .............................................................................................................. 5 1.5 Interstellar Ice Composition ............................................................................................ 9 1.6 Interstellar Ice Reactivity .............................................................................................. 10 1.7 Hydrogen Atom Surface Reactivity ............................................................................... 11 1.7.1 H2 Formation .......................................................................................................... 11 1.7.2 H2O Formation ....................................................................................................... 12 1.7.3 CH3OH Formation ................................................................................................... 18 1.8 Oxygen Atom Surface Reactivity ................................................................................... 19 1.8.1 O3 Formation .......................................................................................................... 21 1.8.2 CO2 Formation ........................................................................................................ 24 1.8.3 Oxygen Atom Addition at UCL ............................................................................... 26 1.9 Astrochemical Models................................................................................................... 27 1.10 Summary ..................................................................................................................... 29 1.10 References .................................................................................................................. 32 Chapter 2 – Experimental Methodology and Data Analysis ..................................................... 37 2.1 Overview ....................................................................................................................... 37 2.2 Vacuum Set-up .............................................................................................................. 38 2.3 Oxygen Atom Generation ............................................................................................. 40 2.4 Transport of Reactants .................................................................................................. 42 2.5 Coldhead and Target Mount ......................................................................................... 43 2.6 Electron Gun ................................................................................................................. 46 2.7 Time-of-Flight Mass Spectrometer ............................................................................... 47 v 2.8 Data Acquisition ............................................................................................................. 50 2.9 Ion Signal Normalisation ................................................................................................ 52 2.10 Kinetic Model ............................................................................................................... 53 2.11 Experimental Developments ....................................................................................... 63 2.12 Summary .....................................................................................................................
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