Nitrogen vibrational excitation in a non-self-sustained discharge plasma toward efficient nitrogen fixation processes

Inorganic nitrogen fertilizers, such as nitric acid, can be selectively generated with air and water by discharge plasmas, thus potentially become a sustainable method for the nitrogen-fixation with renewable energy resources. Some of suggested reaction pathways in the plasma nitrogen-fixation include vibrationally excited nitrogen molecules as a key reactant, whose efficient generation is important to improve the net efficiency. In a past few years, toward efficient and controlled nitrogen dissociation processes, we have studied on the discharge plasma source for efficient generation of vibrationally exited nitrogen molecule. This work focuses on building a concrete basis to study chemical reactivity of vibrationally excited nitrogen, in a way adoptable to variable energy input with measurements on the vibrational population and the rotational temperature of nitrogen at the ground electronic state. Control of reduced electric field E/N below the self-sustaining discharge voltage is realized by a developed discharge plasma source powered by both a lab-built nanosecond pulse generator and a DC power supply to superimpose the non-self-sustaining voltage for vibrational excitation. Apparent E/N higher than 5 has been achieved up to 0.2 atm with our discharge geometry and the pulse width modulation technique . The superimposed DC voltage is found to populate higher vibrational level more than v = 8 estimated by spontaneous Raman spectroscopy while the gas heating effects were found below the detection limit of the rotational and vibrational Raman spectra fitting. Fluid dynamical and vibrational relaxation modeling in the pulse intervals supports our arguments that the superimposed DC plays a key role in efficient generation of vibrationally excited nitrogen. Further details on the discharge characteristics and measurements on the Raman spectroscopy will be presented.