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 (T_r) and vibrational (T _v) temperatures are measured by Raman scattering. Maximum values of T _r = 3500 K and T _v = 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.