Density measurement of OH radicals in non-thermal plasmas by laser induced fluorescence and time-resolved absorption spectroscopy

In the present thesis work, we have developed two different experimental setups for the optical detection of the OH radical in discharges at atmospheric pressure. The first one allows us to improve the time-resolved broad-band absorption spectroscopy. The main advances of the new set up are a better collimation of the UV light and a novel gating scheme. They both significantly reduce the interference of the plasma-induced emission on the absorption measurement. The second setup is dedicated to an improved laser induced fluorescence experiment, which takes advantage of a novel multi-transition excitation scheme. This permits the simultaneous measurements of both the OH density and its ground state rotational temperature. In addition, we have developed a new rate-equation model to rationalize LIF spectra, by taking into account the electronic quenching, the vibrational and rotational energy transfers, and the spatial profile of the laser beam. Finally, the electrical power dissipated in the discharge was accurately measured.