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The emission spectrum (200 – 1000 nm) and discharge characteristics of an atmospheric pressure Ar/CH4 capacitively coupled plasma, 275 W, 27.12 MHz, 0.7 L min-1 Ar and 0-7.5 mL min-1 CH4 as collision/reaction gas were studied. The emission spectrum of pure Ar plasma is dominated by molecular bands of OH and N2 second positive system in UV as well as Ar I (4p→4s) lines in VIS range. The addition of CH₄ results in the appearance of the C₂ Swan band and quenching of OH, N₂ and Ar emission. Quenching is the consequence of the second-kind collisions of CH₄ molecule with Ar, N₂ excited species and chemical reaction with OH radical, respectively. Processes were confirmed by the decrease of the excitation temperature of Ar and electron number density when adding CH₄ in Ar plasma. The Ar/CH₄ plasma is of great interest for applications in atomic spectroscopy due to the possibility to remove spectral interferences by the collision/reaction gas method and the use of low resolution microspectrometers.

Keywords: capacitively coupled plasma, molecular emission, collision/reaction gas, plasma diagnosis