Défense de thèse de doctorat en sciences physiques, par Tuong Le Cong

Isolated line shape study of methane and acetylene

Jury

Laurence REGALIA (Université de Reims), Ha TRAN, co-promoteur (Université Paris VI), Jean VANDER AUWERA (ULB), Philippe LAMBIN, président (UNamur), Muriel LEPERE, promoteur (UNamur)

Résumé

This work is devoted to the study of isolated line shapes of methane and acetylene, two important species of the atmospheres of various planets and satellites in the solar system. In the first part of this thesis, we focus on the experimental studies of the infrared absorption spectra of methane and acetylene. Using a high resolution tunable diode laser spectrometer, the infrared absorption spectra of these molecules diluted in various perturbers such as N₂, O₂, He and Ar are recorded for a wide pressure range at room temperature. The measured spectra have been then analyzed with different line-shape models, from the simple Voigt to the recently recommended Hartmann-Tran profiles. The obtained results confirm that the Voigt profile is only an approximated one and the non-Voigt effects (i.e., the Dicke narrowing and the speed dependences of the line widths and shifts as well as the correlation between velocity- and phase-changing collisions) should be taken into account in order to correctly represent the measured spectra. The second part of this work is devoted to a theoretical study of the spectral shape of nitrogen-broadened methane lines, based on re-Quantized Classical Molecular Dynamics Simulations (rCMDS). Using rCMDS, ab initio spectra of isolated methane transitions are calculated for the first time. The calculated spectra have been then compared with the experimental ones recorded with two different laser spectrometers for a large pressure range. The Voigt-fit comparisons show that ab initio calculated spectra reproduce with very high fidelity the non-Voigt effects observed in the measurements and that rCMDS can be used to predict spectral shapes of isolated lines of methane perturbed by nitrogen. Similar studies could be realized for CH₄ with other collision partners and at different temperatures but also for other interesting atmospheric species such as N₂O as well as C₂H₂.

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