Far-infrared ro-vibrational spectroscopy of coriolis coupled molecules of interstellar importance.
thesisposted on 17.02.2017 by Bane, Michael Kenneth
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
One of the fundamental limiting factors in ro-vibrational spectroscopy has been the lack of high brilliance sources in the far-infrared region of the electromagnetic spectrum. Synchrotron sources are somewhat able to fill this gap, and spectroscopy of the usually weak lowest energy vibrational modes is becoming more accessible. A detailed knowledge of the molecular parameters of low energy vibrational modes is important for interstellar detection, since it is these modes that are most likely to be thermally populated in these systems. The first chapter of this thesis outlines the fundamental theory associated with the high- resolution vibration-rotation spectroscopy of molecules. Firstly, a theory of the allowable energies of molecules is presented, with careful consideration of the quantum mechanical nature of the entity. Secondly, a theory of the interaction with light is discussed, which relates to the observable absorbance and selection rules. Finally, the theory of the Fourier transform is discussed in relation to the collection and attributes of the recorded spectra. The second chapter discusses the instrumentation and analytical basics of the project. Detailed aspects of the synchrotron source and spectrometer are first outlined and give insight into the experimental work done during this project. For context, a discussion of complementary high-resolution techniques is included regarding the recording of both pure rotational and ro-vibrational spectra. The majority of this project involved detailed spectral analysis, and thus an explanation of the most useful tools and techniques is included. Finally a section justifying the selection of ketenimine, 1-phosphapropyne, thiirane and chlorodifluoromethane as target molecules is included. This also serves as a summary of the work done on each molecule, and elaborates slightly on what is presented in the publications. Chapter 3 contains the work published during this project. This work mainly relates to the analysis of high-resolution far-infrared spectra of molecules which are either predicted to be, or have been detected as, a part of the interstellar medium. Chapter 4 discusses the implications of this research, and suggests possible future extension. A major theme of this project is the de-perturbation of modes which exhibit strong coupling mechanics, highlighted by the characterization of an intensity stealing mechanism which allows effectively infrared inactive modes to be enhanced and become observable. This effect was found, by chance, to be crucial in explaining the spectra of two of the three molecules studied in this thesis. This leads to the tantalizing prospect that perhaps this mechanism may be important in the study of the ro-vibrational spectra of other molecules.