La Delfa, S.; (2009) Preparatory work for C 3 line-list calculation. Doctoral thesis , UCL (University College London).

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Abstract

The goal of this dissertation is to study the infrared absorption spectrum of the C 3 (carbon three) molecule. In particular, the aim is to investigate its ground electronic state up to 12:500 cm-1, as this affects the atmospheres of cool C-rich stars. The linear C 3 molecule shows very unusual properties for a linear molecule: a high degree of floppiness, no permanent dipole moment and a strong bentstretch interaction. Consequently, the C 3 spectrum presents particular features such as overtones, hot bands, and, as has been recently detected in Carbon stars and molecular clouds, a quite low fundamental bending frequency (63 cm-1) when in the ground electronic state. This dissertation aims to address each of these features. The first section discusses the context for this work: the stars. It provides a brief introduction about the Astrophysics related to this research project. A review at the recent literature is provided and the experimental results which provide the goal for the results of the theoretical work in the rest of the dissertation are set out. The second section introduces the C 3 molecule and outlines its properties. Previous C 3 studies are discussed and the theoretical approach used to study ro-vibrational spectra of triatomic molecules is set out. Preparatory tests and calculations are carried out to allow a theoretical reproduction of C 3 roto vibrational spectrum in the infrared region to be produced. The third part of the thesis expands on the nuclear motion calculations of section 2 and presents the results of the large scale calculations performed using the DVR3D suit programs written by Tennyson et al. [1]. This program allows the calculation of energy levels, wave-functions, expectation values and Einstein Coefficients. It takes as input the Potential Energy Surface (PES) and a Dipole Moment Surface (DMS) constructed a priori (in section 2) by solving the electronic problem within the Born-Oppenheimer's approximation. Because the quality of the PES sets the accuracy of the ro-vibrational calculations tests on different C 3 PESs and DMSs are performed. To reproduce accurate spectra of cool stars atmosphere in the temperature range of 2000 - 4000 K it was necessary perform calculations with high rotational quantum number. For this reason, tests with J>>0 were necessary to optimize the DVR3DRJZ parameters in order to guarantee a certain degree of accuracy and energy levels convergence. The results of these calculations and associated C 3 line-lists should be very useful to support the observations and model atmospheric studies. This work was generously supported by the QUASAAR Marie Curie Network.

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