Measurements of spatially resolved electron number densities and modes temperatures using optical emission spectroscopy of atmospheric pressure microplasma jet

The electrical gas discharge parameters of direct-current non-thermal microplasma jet in Ar-2%H₂ flow at open atmospheric air was investigated by using spatially resolved optical emission spectroscopy (OES). The jet was confined from microhollow of tungsten-carbide (～500 μm inner diameter) to a molybdenum foil. Despite its small volume, the atmospheric pressure microplasma jet provides a range of power densities, from low to ～10¹² W m^?3 generated either in rare gases or in molecular gases. A high resolution spectrometer (Jobin-Yvon, Czerny-Turner model THR1000, resolution of 0.001 nm, with focal length of 1.0 m and numerical aperture of 0.13 ? f/7.5) was used to allow registration of OH (A ²Σ⁺, ν = 0 → X ²Π, ν′ = 0) rotational bands at 306.357 nm, Ar I 603.213 nm line and N₂ (C ³Π _u , ν = 0 → B ³Π _g , ν′ = 0) second positive system with the band head at 337.13 nm in order to estimate the rotational temperature from the cathode sheath of the plasma jet to the anode. For currents ranging from 20 to 100 mA and for a particular excited levels, the excitation temperature was measured in the negative glow region either from a Boltzmann plot of Ar I 4p–4s and 5p–4s transitions of excited argon or using the Mo I (from 440 to 450 nm) two-lines method of excited Mo atoms sputtered from the cathode surface, giving 24 000 K (100 mA at 100 μm) and 7000 K (20 mA at 500 μm from the cathode). From the N₂ (C ³Π _u , ν = 0 → B ³Π _g , ν′ = 0) rotational transition the rotational temperature along the positive column was estimated. The vibrational temperature of the bulk plasma (1400 to 4500 K) was estimated for a current varying from 20 to 120 mA using the N₂ second positive system with Δν = ?2. Using the broadening of H _β Balmer line it was possible to estimate the electron number density of the negative glow (10¹⁴ to 10¹⁵ cm^?3) as a function of the current.