Excitation mechanisms in a nonequilibrium helium plasma jet emerging in ambient air at 1 atm

Abstract

Species excitation mechanisms were studied, using optical emission spectroscopy, in a helium 200 kHz radio frequency (RF) plasma jet, emerging into the open air at 1 atm. The jet impinged on a dielectric substrate of either MgF2 or quartz. Optical emissions between 115 and 950 nm were recorded through the substrate either along the jet axis or at a steep angle to isolate emissions originating from the region near the substrate surface. Time-resolved emission was observed close to the substrate surface only during a brief period near the positive peak of the applied RF voltage. No emission close to the substrate was observed during the negative voltage with the exception of a weak emission from N2 (C3Πu → B3Πg) just prior to the peak negative voltage. N2+, H, O, OH, and NO emissions along the discharge axis, from impurities in the He feed, or air diffusing into the He jet just downstream of the end of the tube (nozzle), were dominated by Penning ionization of N2 and dissociative excitation of water and O2 by He metastables (He*). Unlike the fully modulated electron-impact excited emission from N2 and He, emissions produced by collisions with He* were weakly modulated during the RF period and were shifted in phase with respect to the peak positive or peak negative voltage. This was attributed to vacuum ultraviolet emissions, in the radiation-trapped environment of atmospheric pressure, that produced He* outside the discharge tube even during the period when the plasma was confined inside the tube. N2 emission revealed a weak ionization wave propagating during the positive voltage period, well before the peak positive voltage. At peak positive voltage, ionization and excitation in the gap between the nozzle and the substrate maximized. The plasma positively charged the substrate, resulting in the brief N2 emission observed during the negative voltage period, as electrons drifted toward the substrate to neutralize the positive charge.