Brady, Ryan P.; (2025) Beyond Born-Oppenheimer: non-adiabatic interactions of diatomic molecules for high resolution spectroscopy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Spectroscopy is essential for understanding a wide range of scientific phenomena, making the production of accurate spectroscopic data crucial. The demand for such data has grown significantly, particularly in astrophysics, where the recent deployment of the James Webb Space Telescope requires accurate and comprehensive line lists for effective data analysis. To meet this need, the thesis introduces SOLIS, an infrared and visible line list for the sulfur monoxide (SO) radical. Generated using variational calculations and refined with empirically determined energy levels and experimental uncertainties, SOLIS is expected to be valuable for studying exoplanetary atmospheres, stellar formation regions, the interstellar medium, and shock zones. Notably, it has already been applied in the analysis of JWST data, with the results discussed in this thesis. The ultraviolet spectra of molecules are especially important for studying astrophysical environments like planetary atmospheres. Accurately describing the highly excited electronic states of molecules is vital. However, many molecules exhibit complex electronic structures due to non-adiabatic interactions from the breakdown of the Born-Oppenheimer approximation. The limitations of this approximation are evident in an increasing number of molecules, where non-adiabatic effects play a significant role. To address this challenge, this thesis reformulates existing diabatisation theory in a rovibronic context, implementing non-adiabatic modules within the variational rovibronic code Duo. This allows the full incorporation of non-adiabatic effects in generating molecular line lists. Additionally, novel diabatisation methodologies are developed to construct accurate and practical diabatic spectroscopic representations, where non-adiabatic couplings vanish, facilitating the construction of accurate contracted vibronic basis sets. This thesis also presents the first demonstration of adiabatic-diabatic rovibronic equivalence for an arbitrary number of coupled electronic states, applied to YO, CH, N₂, and a synthetic 10-state system. Duo serves as a powerful tool to benchmark the importance of different non-adiabatic effects on computed spectroscopy.

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