Hybrid fs/ps CARS Characterization of Rotational and Vibrational Excitation of N₂ in RF Non-Equilibrium Plasma

This manuscript presents spatially resolved measurements of vibrational and rotational excitation in intermediate pressure (20 Torr) N₂ plasmas sustained in a radio-frequency (RF) dielectric barrier discharge (DBD) reactor, using a three-beam hybrid femtosecond/picosecond (fs/ps) coherent anti-Stokes Raman scattering (CARS) diagnostic technique. The measurements span RF input powers from 20 to 55 W and include vertical spatial scans across the 4 mm interelectrode gap with a 0.5 mm step size. Notably, we report the experimental observation of vibrational levels up to v = 6 in a pure N₂ RF-DBD plasma at 20 Torr. With increasing power, the vibrational temperature rises sharply and then plateaus at higher power inputs, indicating a transition to a regime dominated by enhanced V–T relaxation and dissociation from highly excited vibrational states. Spatially, the vibrational temperature profiles peak near the center of the reactor and decrease toward both electrodes, with a mild asymmetry likely arising from geometric differences between the electrodes and the corresponding sheath dynamics. These results confirm that vibrational excitation in RF-DBD plasmas is structured and non-uniform, despite the discharge’s visually diffuse appearance in optical emission.