Spectroscopic study of photon, ion and electron stimulated molecular synthesis in astrophysical ices

Dawes, Anita (2003). Spectroscopic study of photon, ion and electron stimulated molecular synthesis in astrophysical ices. PhD thesis University College London (University of London).

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The universe is a vast chemical and physical factory consisting of large collections of stars, gas and dust. Energetic processing of ices and subsequent molecular synthesis in astrophysical environments, including icy Solar System bodies and grain mantles in the interstellar medium, are responsible for the observation of some of the molecular species found in space. Gas phase processes alone cannot explain the reaction mechanisms and the observed abundances of some of these molecules.

This thesis reviews the current state of knowledge on solid state molecular synthesis in astrophysical ices and highlights the relevance of this work to understanding the chemical origins of life. The nature and origin of astrophysical ices and their environments is discussed to provide a background for the design and implementation of a new apparatus built to simulate astrophysical environments. An outline is given of the relevant collisional and chemical processes associated with interactions between radiation and matter pertinent to astrochemistry, e.g. ion, photon and electron processing of ices.

A detailed description of the design and construction of the new apparatus is given and the theory and instrumentation in the spectroscopic techniques used are discussed. This is followed by a detailed explanation of the experimental procedures implemented at both ion and synchrotron radiation sources. The first results of ion and photon irradiation of H2O and CO2 ices (both pure and binary) using this apparatus are presented and discussed in detail. Ion irradiation is carried out using both reactive and unreactive ions. Reactive carbon ion implantation in pure H2O is investigated and the production of CO and CO2 monitored. Experiments involving both high (100 keV) and low (1 – 5 keV) energy ion irradiation of mixed H2O:CO2 ices are described and the production of H2CO3 (carbonic acid) and CO investigated. The production of CO and CO3 is described in UV irradiation experiments of mixed H2O:CO2 ices. Particular attention is paid to the infrared band profile of CO in each of the irradiation experiments. The CO band profile shows a great degree of complexity, particularly in the carbon ion implantation experiments in H2O. Differences in the types and yields of molecular products formed and their infrared band profiles in ion and photon irradiated ices suggest different chemical and physical processes taking place.

Trial experiments of VUV synchrotron photoabsorption experiments of H2O and NH3 ices are also described. Results reveal a blueshift of 20 – 25 nm in the peak absorption of the first excited electronic state, observed in both species, and is attributed to the effect of hydrogen bonding. The thesis ends with suggestions of the possible modifications to the apparatus and plans for future work.

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