PhD Thesis

Group:             Molecular and Atmospheric Spectroscopy (Angers, France)

Supervisors:      Pr. Michel Chrysos, Dr. Florent Rachet

Title:                Light scattering spectroscopy by atomic gas mixtures

Context:

The Raman spectrum commonly recorded by a molecular fluid features the superposition of a standard contribution dependent on the permanent molecular polarizability alone, together with a far weaker contribution due to the interactions between different molecules, mostly two bodies, usually referred to as the "collision-induced" spectrum. This last contribution is therefore proportional to the square of the gas density and is generally highly anisotropic. In the late 60’s, the ‘anisotropic collision-induced’ Raman scattering has been proved in atomic gases thanks to a conventional right-angle Raman setup and using gas densities varying from 10 up to 150 amagats. In the late 70’s, Proffitt and Frommhold were the pioneers measuring the weak and delicate isotropic profiles by atomic gases and provided a procedure to calibrate the intensities on an absolute scale. Precise knowledge of these spectra is indispensable for the determining of macroscopic electro-optical properties of the medium, such as its refractive index, its Kerr constant, etc… and also, at a microscopic scale, for the understanding of the structure of the very short range inter-atomic interactions (polarizability tensor, energy potential surface) for which quantum effects mainly participate.

For the last two decades, the group in Angers has developed a Raman set-up particularly adapted to the detection of extremely low photon fluxes, enabling detection of intensities as low as a few photons per week (i.e. equal to the light flux we would receive on the Earth from the flame of a candle settled on the Moon). With this expertise, precise measurements of the very weak, far wings of Raman spectra of all pure rare gases were possible. My thesis presents the first experimental and theoretical collision-induced Raman spectra for two rare gas mixtures, Ne-Ar and Kr-Xe. Such heteronuclar systems are crucially relevant in order to explain the phenomenon of opacity observed in the atmospheres of planets.

Postdoctoral Fellowship