Manipulating non-equilibrium by quenching flow in atmospheric-pressure microwave plasmas

Abstract Low-temperature non-equilibrium plasma flows hold great potential for enhancing catalytic reactions, assisting combustion, and synthesizing nanomaterials. In non-equilibrium plasmas, internal energy states of molecules deviate from equilibrium, with vibrationally excited N 2 not following a Boltzmann distribution due to short collision times, long relaxation times, and anharmonic vibrational–vibrational (V–V) pumping effect. To better understand the vibrational–translational and V–V relaxation processes under the quenching effect of cold equilibrium flows, we have developed a non-equilibrium-state-tunable atmospheric-pressure microwave plasma torch and studied it using in-situ laser diagnostics. Specifically, translational ( T g ), rotational ( T r ), and vibrational ( T v ) temperatures have been measured using laser-induced Rayleigh, rotational Raman, and rotational–vibrational Raman scattering, respectively. A direct comparison of these temperatures clearly demonstrates a non-equilibrium-to-equilibrium transition as the quenching flow mixes with the plasma. Non-equilibrium plasma states exhibit different relaxation behaviors under the quenching cold flows. Increasing quenching flow rates can enhance the rotational–translational (R–T) relaxation process, while the population of vibrational states departs from the Boltzmann distribution. This behavior is attributed to the slow relaxation of molecules at high vibrational energy levels compared to the fast relaxation at the ground vibrational level.