Spectroscopic characterization of rovibrational temperatures in atmospheric pressure He/CH₄ plasmas

Atmospheric pressure of helium (He) and methane (C _H4) mixture discharge characteristics are investigated using emission spectroscopic methods. Plasmas are produced in a radio frequency capacitively coupled device at atmospheric pressure in the ambient air. Without the C _H4 gas introduced in the plasma, the emission spectrum exhibits typical helium discharge characteristics showing helium atomic lines with nitrogen molecular bands and oxygen atomic lines resulting from air impurities. Addition of a small amount (<1%) of C _H4 to the supplied He results in the emission of CN (^{B2 +} - ^{X2 +}: violet system) and CH (^A2 Δ- ^X2: 430 nm system) molecular bands. Analyzing the CN and CH diatomic molecular emission spectra, the vibrational temperature (_Tvib) and rotational temperature (_Trot) are simultaneously obtained. As input power levels are raised from 20 W to 200 W, _Tvib and _Trot are increased from 4230 K to 6310 K and from 340 K to 500 K, respectively. On the contrary, increasing the C _H4 amount brings about the decrease of both temperatures because C _H4 is harder to ionize than He. The emission intensities of CN and CH radicals, which are important in plasma processing, are also changed along with the temperature variation. From the results, the atmospheric pressure plasma shows strong nonequilibrium discharge properties, which may be effectively utilized for thermal damage free material treatments.