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The Interactions of Small Molecules on Astrochemically Relevant Surfaces

Higgins, Catherine; (2020) The Interactions of Small Molecules on Astrochemically Relevant Surfaces. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

The known Universe is composed of 99.9% hydrogen and helium. The remaining 0.1 % constitutes the abundance of the rest of the elements. Despite their low abundance relative to the lighter elements, the reactions of these heavier elements account for the complexity of species found in space. In particular, more than 200 complex organic species have been detected in the interstellar medium. They, along with carbonaceous and silicate dust grains, accumulate together in interstellar clouds. This thesis focusses on the reactions taking place within dense interstellar dust clouds, where the inner core is shielded from radiation emitted by nearby stellar objects by a dense, opaque outer layer of gas and dust. Within the cores of these dense clouds, molecular lifetimes are extended which allows for the formation of complex molecules through further reactions. The species observed within the interstellar medium can be formed via ion-neutral reactions in the gas-phase. However, for many molecules, the observed abundance cannot be accounted for by gas-phase chemistry alone. Therefore, reactions must also be occurring on the cold surfaces of the interstellar dust grains. This thesis will focus on the reactions taking place on these grains, presenting investigations into the interactions with, and reactions of, various species upon astrochemically relevant surfaces. The investigation into the reaction of acrolein with atomic oxygen on a graphitic surface is discussed in Chapter 4. Multilayer doses of both species were co-dosed onto a graphite substrate held at temperatures ranging from 20 K to 140 K. A product with m/z 72 is observed for this reaction, suggesting the addition of oxygen across the double bond of the organic molecule: C3H4O2. The kinetics of this reaction were modelled to gain a deeper understanding of the mechanisms occurring on the cold surface. Specifically, the contribution of the two prototypical surface mechanisms, Langmuir-Hinshelwood and Eley-Rideal mechanisms, was assessed at each surface temperature. Additionally, the desorption characteristics of various astrochemically relevant molecules are outlined in Chapter 5. The behaviour of each species on a graphite surface was investigated and compared to the desorption characteristics previously investigated in the literature. The behaviour of the species observed in our desorption experiments is in good agreement with the behaviour observed in previous studies. The cohesion between our data and interactions observed previously, validates our experimental technique as a method to model the interactions of molecules under conditions similar to those in the ISM. In Chapter 6 the desorption characteristics of molecular oxygen, acrolein and carbon dioxide from an amorphous carbonaceous surface are investigated. The amorphous surface is composed of porous, broken fullerene-like cage structures. Through analysis of the desorption spectra of the gases, it was found that each molecule had significantly larger binding energies to this amorphous surface than previously observed on graphite. This is consistent with previous studies into the desorption characteristics of various gases from the surface of single walled carbon nanotubes (SWCNTs) by Ulbricht et al. where the gas molecules showed enhanced binding to the SWCNTs compared to graphite surfaces. The implications of the discovery of these higher energy binding sites to astrochemistry is discussed. Finally, in Chapter 7, results concerning the reaction of atomic oxygen with propene on a different amorphous carbonaceous surface are presented. The new surface is hydrogenated and consists of a large proportion of sp3 hybridised carbon. Interestingly, the expected product, showing the addition of oxygen across the double bond of the propene was not observed for the reaction on this surface. On the other hand, a product with m/z 90 was detected, suggesting the addition of ozone across the double bond to form a product with the general formula C3H6O3. The ozonolysis of an alkene is a well-known reaction in gas-phase chemistry but has not yet been observed under astrochemically relevant conditions. Additionally, in the gas phase, the C3H6O3 product is known to undergo a rearrangement via the Criegee mechanism before breaking down into various oxygen-containing molecules, such as aldehydes, ketones, carboxylic acids and alcohols. If this mechanism can occur on the surfaces of interstellar dust grains it could help to account for the rich variety of complex organic molecules observed in the ISM.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: The Interactions of Small Molecules on Astrochemically Relevant Surfaces
Event: UCL
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2020. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Chemistry
URI: https://discovery.ucl.ac.uk/id/eprint/10107195
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