Bowesman, Charles; (2024) Spectroscopic networks and variational models for hyperfine-resolved molecular line lists. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Spectroscopic data in the form of line lists underpin the study of molecules in many astrophysical regions of interest and recent advances in observational techniques have necessitated the production of these data to a high-resolution standard. Line lists for the refractory species aluminium monoxide (AlO) and vanadium monoxide (VO) which are of interest in stellar and exoplanetary atmospheres, as well as H₃⁺ which is known to exist in ionospheres of giant planets and the interstellar medium are presented. These data are computed using variational nuclear motion calculations and are further augmented through several post-processing steps to prepare them for use in high-resolution spectroscopy. Four such methods for adjusting calculated term energies and providing uncertainty estimates are detailed, outlining the impetus behind the construction of so-called hybrid line lists. The primary source of high-resolution term energies and associated uncertainties is the Marvel (measured active vibration-rotation energy levels) procedure which obtains empirical energy values from spectroscopic networks of measured transition data. Hyperfine-unresolved Marvel networks for ²⁷Al¹⁶O, H₃⁺, H₂D⁺, D₂H⁺ and ⁵¹V¹⁶O are constructed, in addition to a hyperfine-resolved ⁵¹V¹⁶O network. These empirical energies are used to replace their calculated counterparts and high-accuracy machine-learned predicted energy shifts are computed for levels not observed through experiment. The refined energies of ²⁷Al¹⁶O are used to inform corrections to line lists for three isotopically substituted species of AlO. Existing ExoMol line lists for H₃⁺ and H₂D⁺ are updated and new line lists for the other deuterated isotopologues, D₂H⁺ and D₃⁺, are produced. Previously recorded spectra of VO intercombination bands are assigned and facilitate the construction of a new, 15 electronic state spectroscopic model for ⁵¹V¹⁶O with the novel inclusion of six different hyperfine coupling terms. This is used to produce a line list comprising fully hyperfine-resolved term energies and transition intensities for VO.

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