Étude théorique et expérimentale des spectres de vibration-rotation des molécules tétraédriques de silane et méthane

This thesis reports, in its theoretical part, a method to compute vibration and vibrorotation spectra of tetrahedral molecules. The local mode model describes the stretch vibrations by means of anharmonic oscillators localized on each bond. The nearly local molecules are characterized by weak couplings between these oscillators. These couplings are treated perturbatively and the computation of the vibrational and rovibrational energies of the first excited levels is then easier. The vibration - rotation problem, more complicated than the pure vibrational one, is solved by coupling, in the Td group, a local vibrational operator with a rotational one, both having well-defined symmetries. The transition intensities are computed in the same way and by using a bond separable dipole moment function. The experimental part reports the results obtained on methane and silane by IR IR double resonance. The vibrational structure of the last unknown component (of F1 symmetry) of the 3ʋ3 level has been observed as well as those of the polyads (1100) (3000) and (2100) of silane. In the case of methane, the vibrational energy of the Ft level as well as the experimental values of the rotational and Coriolis constants are close to the calculated ones. For silane, the agreement is better both for the vibrational energies and for the rovibrational and rotational parameters. The stretch-bend Fermi resonances, not taken into account in this study, are weaker for silane than for methane, and are probably responsible for this.