Variationally Computed Line Lists for SO2 and SO3

The work presented in this thesis concerns the production of high-temperature spectroscopic line lists for the SO2 and SO3 molecules, for the purposes of astrophysical, terrestrial and industrial applications. Both line lists are computed using ab initio computational methods to calculate rovibrational energy levels and dipole moment transition intensities. The hot SO2 line list is computed using the DVR3D program suite, optimised to work efficiently with the molecule, and by making use of high performance computing. The calculations are based on an empirically refined ab initio potential energy surface (PES), and a purely ab initio dipole moment surface (DMS). Results are compared to previous ab initio studies and available experimental data. The final line list can be used in spectroscopic models for temperatures up to and including 1500 K. A preliminary, room-temperature line list for SO3 is calculated using the TROVE program, in conjunction with a purely ab initio PES and DMS. The results are compared to available experimentally derived energy level data. These are then used to empirically refine the ab initio PES, which is subsequently employed in the calculation of the complete, hot line list, suitable for modelling SO3 spectra up to 773.15 K. Preliminary comparisons are made with experimental high-temperature measurements, and the quality of the ab initio DMS is discussed. In addition, the rotational behaviour of the SO3 molecule is investigated from a theoretical perspective using the synthetic SO3 line list, where the ‘forbidden’ rotational spectrum is analysed. The formation of so-called 6-fold rotational energy clusters at high rotational excitation is also predicted, the dynamics of which are analysed both quantum mechanically and semi-classically. The potential applications and limitations of both line lists is outlined, and implications for further work are also discussed

Theoretical rotational-vibrational and rotational-vibrational-electronic spectroscopy of triatomic molecules

The major part of this work is construction of 54 room-temperature infrared absorption line lists for isotopologues of carbon dioxide. In accurate nuclear motion calculations an exact nuclear kinetic energy operator is used in the Born-Oppenheimer approximation and three ab initio and semi-empirical potential energy surfaces for generation of rotational-vibrational wavefunctions and energy levels. Transition intensities are calculated with two different high quality ab initio dipole moment surfaces. The generated line lists are comprehensively compared to state-of-the-art measurements, spectroscopic databases and other theoretical studies. As a result, uncertainties in calculated transition intensities in several vibrational CO2 bands are shown below 1%, which is sufficient for use in remote sensing measurements of carbon dioxide in the Earth’s atmosphere. Results of the present calculations set a new state-of-the-art and have been included in the 2016 release of the HITRAN database. A theoretical procedure for estimating reliability of computed transition intensities is presented and applied to CO 2 line lists. As a result, each transition intensity received a reliability factor, a particularly useful descriptor for detecting resonance interactions between rotational-vibrational energy levels, as well as a good measure quantifying the strength of such interactions. The theoretical procedure used for CO 2 is extended to electronic transitions in the Born-Oppenheimer approximation. In this extended framework rotational- vibrational-electronic line lists for SO2 and CaOCa molecules are generated. For this purpose appropriate ab initio potential energy surfaces and a transition dipole moment surface are generated. Absolute transition intensities are then calculated both in the Franck-Condon approximation and with a full transition dipole moment surface. Resulting line lists are compared with available experimental and theoretical data. The unprecedented accuracy of the model used in these calculations and the rotational resolution of transition lines renders the present approach as promising for future uses in atmospheric science. Finally a theoretical framework for fully non-adiabatically coupled Hamiltonian is derived and discussed. A proposition for computer implementation of this theoretical scheme is also given