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Investigation of high overtone rotational vibrational spectra of small molecules using theoretical methods with primary focus on the water molecule

Conway, Eamon Kenneth; (2020) Investigation of high overtone rotational vibrational spectra of small molecules using theoretical methods with primary focus on the water molecule. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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This research is dedicated towards extending our fundamental knowledge and understanding of the principles surrounding theoretical spectra on small molecules. The motivation for this work is discussed together with the respective applications. The primary focus is on the water molecule, given both its importance in the terrestrial atmosphere and the atmosphere of exoplanets. Water is a well studied molecule, both theoretically and experimentally, however, little is known in the near ultraviolet. Calculating spectra for such an important molecule requires high accuracy in both line positions and transition intensities, which places high demands on the quality of both potential energy and dipole moment surfaces. In the pursuit of accuracy, this work exploits the use of large basis sets and ultimately, a significant number of CPU hours of computational time. In this undertaking, a new dipole moment surface is created which provides reliable and accurate dipoles extending into the near ultraviolet. A simple yet highly effective method of calculating high-quality ab initio dipoles for large molecules in reduced time was also discovered and should provide the means of improving calculated intensities for such molecules. The desire for accurate high-temperature spectra has lead to the creation of a new semi-empirical potential energy surface, which combined with the new dipole surface has led to a new H216O line list which extends into the ultraviolet and is valid for temperatures up to 5000 K. The resulting semi-empirical spectra, hot and cold, have been compared against a variety of experimental observations, together with the latest editions of the HITRAN and HITEMP databases for the purposes of validation. Improvements over previous models are also evident, particularly in the visible and near ultraviolet. By exploiting the properties of calculated wave functions, an accurate and reliable method of predicting asymmetric top quantum labels (Ka,Kc) is applied to the high-temperature line list. This has allowed the new line list to have significantly more assignments than would otherwise be possible. Reduced matrix elements play an important rule in line-shape calculations and up to now, it was not possible to calculate these with the DVR3D suite of programs. Modifications have been made to the transition moment calculation which now simultaneously allows for the calculation of these elements at the dipole, quadrupole and octupole orders. For water, comparisons have been made against other theoretical sources which includes variational and effective-Hamiltonian methods. This new edition is applicable for systems treated in a Radau frame with a bisector embedding. This work represents a global study on the spectroscopy of small molecules focusing on water vapour, covering past, current and future research.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Investigation of high overtone rotational vibrational spectra of small molecules using theoretical methods with primary focus on the water molecule
Event: UCL (University College London)
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.
Keywords: Spectroscopy, Theoretical Methods, Small molecules, Near-Ultraviolet, High-Temperature
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
URI: https://discovery.ucl.ac.uk/id/eprint/10110549
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