Hybrid fs/ps CARS for N₂ vibrational population measurements in non-equilibrium N₂/H­₂ DC plasmas

Vibrationally excited molecules generated in non-equilibrium low temperature plasmas create new reaction pathways and promote reaction efficiency in plasma chemical synthesis. Recently, it is shown that vibrationally excited H₂(v), populated via V-V (H₂(v) – H₂(v)) and V-V’ (N₂(v) – H₂(v)) energy transfer, significantly enhances the H and NH production and directly promotes ammonia production. Therefore, quantitative measurements of vibrationally excited N_­2(v=1-5) and H₂(v=1-4) in non-equilibrium plasma provide insights into key V-V/V-V’ energy transfer and their impact on plasma-assisted ammonia synthesis. Further, these measurements help optimize the plasma discharge properties, reduce the energy cost, and improve yield in ammonia synthesis. In this work, a hybrid fs/ps coherent anti-Stokes Raman scattering system is developed for rotational and vibrational temperature measurements in low pressure, non-equilibrium N₂/H­₂ DC plasmas. A 3-beam configuration is adapted to excite both the pure-rotational and rovibrational Raman transitions of either the N₂ or the H_­2 molecule by tuning the output frequency of the optical parametric amplifier. Time-resolved vibrational population distribution of N₂(X) is calculated based on the measured vibrational temperatures. In addition, N₂ CARS spectra will be measured in H₂/N₂ mixtures with different H₂ mole fractions. These measurements help illustrate the effect of H₂ concentration on the population distribution of vibrationally excited N₂ in plasma-assisted ammonia synthesis.