Hybrid fs/ps CARS for N₂(v) and H₂(v) measurements in non-equilibrium N₂/H­₂ DC discharges

Non-equilibrium plasma creates excited species and radicals that enable new reaction pathways for plasma assisted ammonia catalysis. In recent studies, it is suggested that vibrationally excited N₂(v≥2) can efficiently enhance H₂(v) generation through V-V/V-V’ transfer, populating higher vibrational levels H₂(v≥2), which combines with N atoms to form NH, a key intermediate species for ammonia synthesis. However, quantitative measurements of V-V’ transfer between N₂ and H­­₂ in non-equilibrium plasma remain scarce. In this work, a hybrid fs/ps coherent anti-Stokes Raman scattering diagnostic was developed to measure the vibrational population distribution of both the N₂ and H₂ molecules in DC glow discharges. In addition, the gas temperature was measured through the pure-rotational N₂ Raman transitions to monitor the V-T relaxation process between N₂(v) and H₂. It is observed that an addition of 20% H₂ by volume to a pure N­₂ plasma drastically reduce the N_­2(v) population. However, further addition of H₂ (75% by volume) does not significantly reduce the N₂(v) population. Interestingly, the gas temperature first increases with the 20% H₂ addition, and decreases with 75% H₂ concentration. H₂ CARS spectra will be measured at the same conditions to elucidate the interplay between the V-V’ energy transfer and V-T relaxation processes between N₂ and H₂. These results further provide insights into the kinetics of N₂(v) and H₂(v) in plasma assisted ammonia synthesis.