Kimber, HJ; (2016) The Interactions and Reactions of Atoms and Molecules on the Surfaces of Model Interstellar Dust Grains. Doctoral thesis , UCL (University College London).

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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. In the case of the surface reaction of O(3P) 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.

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