Development of instrumentation for simultaneous THz and IR spectroscopy : application to HONO equilibrium ; Développement d'instrumentation pour une spectroscopie simultanée THz et IR : application à l'équilibre de HONO

Measuring trace gases having a notable impact on human health, climate and the stability of the ozone layer constitutes an extremely important challenge. In the coming years, new, higher sensitivity satellite instruments will improve atmospheric sounding only if the necessary spectral parameters are available. For some species of atmospheric interest such as nitrous acid (HONO) and hypobromous acid (HOBr), spectroscopic data are incomplete or almost non-existent. The challenge in this thesis is to get quantitative spectroscopic parameters for these species. To achieve this objective, an innovative instrumental set-up involving the simultaneous use of two instruments, an IR/THz dual beam experiment, has been built: a thermostatically controlled cell (200-350K) made of inert materials, glass and Teflon, capable of accommodating a 0.1-1.1 THz spectrometer and coupled to a high resolution (HR) Fourier Transform spectrometer (FTS) in the IR range. The THz spectrometer allows to select rotational lines of the target molecule for partial pressure determination, and the FTS enables simultaneous measurement of the rovibrational spectrum in an IR atmospheric window for quantification of the species of interest. The adopted strategy relies on the fact that the intensities of the rotational spectra simply depend on the permanent dipole moment of the molecule of interest, determined with high accuracy from Stark effect measurements. In the case of HONO, which only exists in laboratory conditions in the form of an equilibrium mixture with other species like H2O, NO and NO2, the lack of knowledge of the partial pressure in the gas mixture leads to serious difficulties for quantitative measurements. In addition, HONO exists in two conformer forms, and the height of the conformer barrier between cis- and trans-HONO DeltaCis-Trans is still poorly known, which affects lines positions and especially lines intensities. To improve the determination of the energy difference between the ground vibrational state of the cis- and ...