Quantitative femtosecond two-photon absorption laser induced fluorescence measurements of hydrogen and nitrogen atoms in an AC dielectric barrier discharge

Plasma represents one of the most attractive ways to synthesize ammonia (NH₃), since it uses renewable electricity to produce NH₃ at low temperatures. For plasma assisted NH₃ synthesis, the underlying chemical kinetics involving gas phase chemistry is not well understood, especially for atomic species like nitrogen (N) and hydrogen (H) atoms. Therefore, this work reports quantitative measurements of atomic N and H in a N₂/H₂ AC plasma using femtosecond two-photon absorption laser induced fluorescence (code-named fs-TALIF). Effects of key AC plasma properties including frequency and applied voltage on the N and H number densities were studied. The results show two major observations. First, the N number density is approximately one order of magnitude lower than the H number density, indicating that producing N atom is critical in plasma assisted NH₃ synthesis. Second, due to the competing factors including N₂/H₂ dissociations by excited species and electrons and N/H consumptions via radical quenching and propagation, the number density ratio of N to H (a key indicator to NH₃ production) is nonlinearly dependent on frequency. The number density ratio of N to H first decreases as the frequency increases from 5 kHz to 15 kHz, and then rises back at 20 kHz.