Synchrotron FTIR spectroscopy of interstellar and atmospherically relevant molecules

Abstract

A recurring theme throughout this thesis is the analysis of Fourier transform infrared (FTIR) spectra of molecules that exist in planetary atmospheres and interstellar media. FTIR spectroscopy enables the accurate characterisation and identification of molecules based on the shapes and positions of vibrational bands. Unless stated otherwise, all of the experimental work has been performed at the Australian Synchrotron THz/far-IR beamline utilising a synchrotron source which is several magnitudes brighter than conventional thermal sources. This is crucial for the far-IR region, as there are no efficient thermal emitters of far-IR radiation. Furthermore, spectroscopy within this region is most applicable to monitoring atmospheric and interstellar molecules. Chapter 1 introduces some of the key concepts associated with FTIR spectroscopy as well as the instrumentation used for the experiments. Quantum theory is also introduced where needed as part of the concepts that are discussed. Chapter 2 focuses on the spectra of molecules recorded using high resolution FTIR spectroscopy and the fundamental theory is introduced in Chapter 2.1 as it is a prerequisite to understanding the rotational Hamiltonian, transition assignment and fitting processes used throughout Chapters 2.2 - 2.5. Chapter 2.2 describes an ongoing analysis of the first high resolution FTIR spectrum of propynethial which is predicted to exist in interstellar media. The v5 band near 1100 cm-1 is the focus of this work as it was predicted to be the most intense band based on B3LYP/cc-pVTZ calculations. Due to inaccurate ground state rotational and centrifugal distortion constants, it was not possible to accurately assign the ro-vibrational transitions of the v5 band. Thus, it was necessary to record millimetre-wave spectra in order to assign transitions with a wider range of J″ and Ka″ quantum number so that more accurate ground state constants could be determined before re-assigning the v5 infrared transitions. This work will hopefully lead to the identification of propynethial in the interstellar media. Chapter 2.3 contains the first published paper on the analysis of the far-IR bands of 1,1-difluoroethane. This molecule is relevant to atmospheric applications due to its high global warming potential and will help facilitate the analysis of higher energy vibrational bands that are present within the greenhouse window. Chapter 2.4 contains a publication on one of the far-IR bands of 1,1,1,2-tetrafluoroethane. Also known as R134a, this molecule is commonly used in industry and needs to be continuously monitored due to its high global warming potential and long atmospheric lifetime. iv Chapter 2.5 is a submitted paper on the analysis of the four lowest IR-active fundamental bands of trans-d2-ethylene. The work completed here is part of a chemical-education collaboration between Dr. C. Thompson (Monash University) and Prof. T. L. Tan (Nanyang Technological University). Chapter 3 progresses into the analysis of low resolution particulate ices that were formed in a collisional cooling cell that has been installed onto the Australian Synchrotron THz/far-IR beamline. These molecular ices are important to interstellar chemistry, as they can provide reaction sites for building more complex molecules from smaller molecules; and atmospheric chemistry by having a large influence on the radiative forcing of planetary atmospheres. Some key aspects of condensed phase spectroscopy are introduced in Chapter 3.1 Chapter 3.2 is an accepted paper detailing the analysis of the mid-IR spectra of isotope mixed H2O crystalline ice particles. The effect of isotopic mixing using D2O is explored over a range of temperatures and concentrations, giving new insights to the behaviour of inter- and intramolecular bonding. Chapter 3.3 contains a manuscript that reports the mid- and far-IR spectra of particulate crystalline ethylene. Ethylene is one of the most abundant species found in Titan’s atmosphere (largest moon orbiting Saturn) and is formed from the photodissociation of methane. An accurate characterisation of these ethylene particles in situ will provide a more direct relationship with what is observed from passing satellites and help profile the temperature of Titan’s atmosphere at different altitudes. Chapter 4 describes the application of a chemometric technique: band target entropy minimisation (BTEM) to gas phase Fourier transform microwave (FTMW) and FTIR spectra. Chemometrics is a widely used technique to help extract additional information from chemical, physical and biological systems that may not be readily discernible through conventional methods. The preliminary results from the analysis of FTMW and FTIR spectra, using BTEM, highlights both the potential application of chemometrics to gas phase spectroscopy as well as its shortcomings. Chapter 5 introduces preliminary data on the application of the EFC cell to transient spectroscopy. Many of the gaseous molecules that are found in interstellar media often exist as short-lived species under laboratory conditions. This work is aimed at increasing the lifetime of transient molecules so that some of the experimental burdens can be reduced when performing high resolution spectroscopy.