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Etude de la conversion de spin nucléaire du méthane en matrices d'Argon et de Krypton

Anica Lekic 1
1 Astrophysique
LPMAA - Laboratoire de Physique Moleculaire pour l'Atmosphere et l'Astrophysique
Abstract : Molecules presenting identical atoms can exist in several varieties called "nuclear spin configurations", which can be differentiated by the symmetry of their rovibrational wavefunction and by their total nuclear spin value. In the case of hydrogenated molecules that can be found in interstellar or cometary media, the measured population ratio for the nuclear spin configurations are not matching the thermodynamical equilibrium expected at these media's temperature. The origin of this disequilibrium is still under debate, and studies concerning the nuclear spin conversion (NSC) in the solid phase, and at the solid-gas interface, are needed to get a better understanding of the observational data. This experimental thesis deals with the study of the methane NSC in argon and krypton matrices. CH4 presents three different spin configurations that can be distinguished by means of Fourier transform infrared absorption spectroscopy. First, this work has been dedicated to the spectroscopic study of CH4 in matrices (peaks attributions, crystalline environment influence...). Then, the NSC dynamics has been studied via measuring the re-equilibration of matrices presenting an excess in one given spin configuration. The role played by (i) the nature, (ii) the CH4 concentration and (iii) the temperature (4.2 K - 10 K) of the matrices on the spin conversion rates has been subsequently studied, allowing us to clearly identify each different mechanism at the origin of the nuclear spin modification of the methane. This work suggests that, in the case of astrophysically relevant ices, the equilibrium between each nuclear spin species of CH4 should be achieved in the condensed phase. These results question the usual hypothesis of spin conservation in interstellar and cometary ices.
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Anica Lekic. Etude de la conversion de spin nucléaire du méthane en matrices d'Argon et de Krypton. Physique Quantique [quant-ph]. Université Pierre et Marie Curie - Paris VI, 2011. Français. ⟨tel-00674704⟩

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