Kinetics of N2 vibrational excitation and NO formation in nanosecond-pulsed air discharge: can vibrational-translational nonequilibrium be exploited for efficient N2 fixation?

Plasma-assisted nitrogen fixation (NF) offers a sustainable pathway by selectively populating vibrationally excited nitrogen molecules (N2(v)), yet its energy efficiency remains constrained by poorly understood vibration-to-chemistry coupling mechanisms. This study integrates coherent anti-Stokes Raman scattering, laser-induced fluorescence, and vibrational state-resolved kinetic modeling to unravel the dynamics of N2(v) under vibrational-translational (VT) non-equilibrium (Tv ≈ 7000 K, Tv /Tr ≈ 5) in a 10 kPa nanosecond-pulsed air discharge. Our experiments however reveal that over 99% of the vibrational energy is dissipated via either N2–O VT relaxation (46.97%) and diffusion (52.8%), with less than 1% contributing to NO production through the Zeldovich reactions. In contrast, our analysis identifies the electronically excited states as the dominant precursors for NO formation. We can conclude that, in medium-to-high-pressure (>10 kPa) air discharges for NF, the strategy of selectively enhancing N2 vibrational excitation to improve the energy efficiency is intrinsically limited by the rapid VT relaxation. This work emphasizes the need of combining multiscale diagnostics and kinetic models to gain sufficient foundational insights for achieving energy-efficient plasma-based NF.