N2 vibrational excitation in atmospheric pressure ns pulse and RF plasma jets

Time-resolved N-2 vibrational temperature and translational-rotational temperature in quasi-two-dimensional atmospheric pressure plasma jets sustained by ns pulse and RF discharges in nitrogen/noble gas mixtures are measured by the broadband vibrational Coherent Anti-Stokes Raman Scattering (CARS) . The results indicate a much stronger vibrational excitation in the RF plasma jet, due to the lower reduced electric field and higher discharge power. In a ns pulse discharge in N-2/He, N-2 vibrational temperature is significantly lower compared to that in N-2/Ar, due to the more rapid vibration-translation (V-T) relaxation of nitrogen by helium atoms. In the RF plasma jets in N-2/Ne and N-2/Ar, the vibrational excitation increases considerably as the nitrogen fraction in the mixture is reduced. The experimental data in the RF plasma jet in N-2/Ar jet are compared with the kinetic modeling predictions. The results indicate that nitrogen vibrational excitation in N-2/Ar plasma jets with a small N-2 fraction in the mixture (several percent) is controlled primarily by electron impact, anharmonic vibration-vibration (V-V) pumping, and V-T relaxation by N atoms. In comparison, V-V energy transfer from the vibrationally excited molecules in the first excited electronic state, N-2(A(3)sigma(u) (+), v), which are generated primarily by the energy transfer from the metastable Ar atoms, has a minor effect on the vibrational populations of the ground electronic state, N-2(X-1 sigma(g) (+), v). Although the discharge energy fraction going to electronic excitation is significant, the predicted quasi-steady-state N-2(A(3)sigma(u) (+)) number density, controlled by the energy pooling and quenching by N atoms, remains relatively low. Because of this, the net rate of N-2(X-1 sigma(g) (+)) vibrational excitation by the V-V energy transfer from N-2(A(3)sigma(u) (+)) is much lower compared to that by the direct electron impact. The results show that atmospheric pressure RF plasma jets can be used as sources of highly vibrationally excited N-2 molecules and N atoms.