Preferential vibrational excitation in microwave nitrogen plasma assessed by Raman scattering

Vibrational activation of N2 molecules in a flowing microwave plasma is investigated in the context of utilising electrical energy for chemical conversion. Spatial profiles of rotational (Tr) and vibrational (Tv) temperatures are measured by Raman scattering. Maximum values of Tr = 3500 K and Tv = 6000 K were observed in the centre of the plasma at low pressure (50 mbar). A detailed quantification of the local energy content shows how the strong non-equilibrium character of low pressure discharges compares with a closer-to-equilibrium energy distribution at higher pressures. Measurements performed downstream of the plasma display the ability of the microwave flowing reactor to deliver up to 48% of the specific energy input (SEI) into internal degrees of freedom of the gas molecules. Specifically, 23% of the SEI is loaded into the vibrational mode, which is potentially available to enhance chemical reactivity of endothermic reactions.