Detection of various active species in Ar-diluted NH3 atmospheric-pressure plasma by ultraviolet absorption spectroscopy

Summary form only given. A number of absorption spectra of molecules such as O₃, NH₃ and N₂H₄ exist in the ultraviolet light region. These molecules are chemically useful; O₃ molecules are potent oxidizer; N₂H₄ molecules are used for fuels of rockets. In order to use these molecules effectively, precise diagnostics of those concentrations are inevitable. In this study, we set up a system for measuring absorbance spectra of molecules in the downstream of plasma by ultraviolet absorption spectroscopy. We can calculate an absolute density of molecules from their absorbance. In this paper, we used coaxial dielectric barrier discharge¹ (DBD) for generating plasma at atmospheric pressure. We made an electrode of the coaxial DBD by binding copper tapes to the outside of a glass tube; the copper tapes and the glass tube played roles of high-voltage electrodes and a dielectric barrier in DBD, respectively. We used NH₃ gases which were diluted with Ar gases. The fraction of NH₃ molecules were changed from 0% to 2.5%. The NH₃ and Ar gases were mixed and flown into the glass tube. Ultraviolet light from deuterium lump is through the gases in the downstream of plasma and observed by a spectrometer. From experimental results of the ultraviolet absorption spectroscopy, we got absorption spectra of NH₃, N₂H₄² and NO. When we generated plasma, we confirmed that the absolute density of NH₃ decreased. Also, when we generated no plasma, we couldn´t observe absorption spectra of N₂H₄, but when we generated plasma, we certainly detected it. The reason is that NH₃ molecules decompose and become NH₂ molecules in plasma, and N₂H₄ molecules are generated by three-body collision between NH₂ and Ar molecules. Also, we identified molecular or a- omic species such as N₂ and NH generated in Ar/NH₃ plasma by optical emission spectroscopy. This setup of ultraviolet absorption spectroscopy that we designed can also be utilized for other kinds of gases and plasma. Based on the measured absorption spectra of molecules in plasma, we will study potentials of Ar/NH₃ coaxial DBD for nitriding and reduction process with assistance of other diagnostic methods such as optical emission spectroscopy.