Selective generation of excited species in ns pulse/RF hybrid plasmas for plasma chemistry applications

Abstract

Hybrid plasmas, sustained by a repetitive ns pulse discharge and a sub-breakdown RF waveform in N2 and its mixtures with H2, CO, and CO2, are studied using laser diagnostics and kinetic modeling. Plasma emission images show that adding the RF waveform to the ns pulse train does not result in a discharge instability development, since the RF field does not produce additional ionization. Unlike a ns pulse/DC discharge, the ns pulse/RF plasma is sustained using a single pair of electrodes external to the discharge cell. Measurements of electronically excited molecules, N2(A3Σu +), and vibrationally excited molecules in the ground electronic state, N2(X1Σg +, v), demonstrate that these species are generated selectively. N2(A3Σu +) molecules are produced predominantly by the ns pulse discharge waveform, while vibrational excitation of the ground electronic state N2 is mainly due to the RF waveform. Strong vibrational nonequilibrium is maintained at a low translational–rotational temperature. The ns pulse/RF discharge data demonstrate that the quenching of N2(A3Σu +) is not affected by N2 vibrational excitation. Kinetic modeling shows that the rate of N2(A3Σu +) quenching in a ns pulse discharge in nitrogen is underpredicted, and the modeling predictions agree with the data only if the rate of N atom generation by electron impact dissociation of N2 is increased by approximately an order of magnitude. This suggests a significant effect of excited electronic states on the net dissociation rate. Infrared emission spectra of ns pulse/RF hybrid plasmas in CO–N2 and CO2–N2 mixtures show that the present approach also generates strong vibrational excitation of CO and CO2, with the CO yield in the CO2–N2 mixture approximately a factor of two higher compared to that in a ns pulse discharge alone. This indicates a significant contribution of the vibrationally enhanced CO2 dissociation in the hybrid plasma. The present results demonstrate that sustaining the hybrid plasma in reacting molecular gas mixtures may isolate the plasma chemical reaction pathways dominated by vibrationally excited molecules from those of electronically excited molecules and atomic species.