Żak, Emil J.; (2017) Theoretical rotational-vibrational and rotational-vibrational-electronic spectroscopy of triatomic molecules. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

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.

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