Electrical characterization of the flowing afterglow of N2 and N2/O2 microwave plasmas at reduced pressure

A cylindrical Langmuir probe was used to analyze the spatial distribution of the number density of positive ions and electrons as well as the electron energy distribution function (EEDF) in the flowing afterglow of a 6 Torr N-2 and N-2/O-2 plasma sustained by a propagating electromagnetic surface wave in the microwave regime. In pure N2 discharges, ion densities were in the mid 10(14) m(-3) in the pink afterglow and in the mid 10(12) m(-3) early in the late afterglow. In both pink and late afterglows, the ion population was much higher than the electron population, indicating non-macroscopically neutral media. The EEDF was close to a Maxwellian with an electron temperature of 0.5 +/- 60.1 eV, except in the pink afterglow where the temperature rose to 1.1 +/- 0.2 eV. This latter behavior is ascribed to N2 vibration-vibration pumping in the pink afterglow that increases the concentration of high N2 vibrational states and thus rises the electron temperature by vibration-electron collisions. After addition of small amounts of O-2 in the nominally pure N-2 discharge, the charged particles densities and average electron energy first strongly increased and then decreased with increasing O-2 concentration. Based on these data and the evolution of the N-2(+)(B) band emission intensities, it is concluded that a significant change in the positive ion composition of the flowing afterglow occurs, going from N-2(+) in nominally pure N-2 discharges to NO+ after addition of trace amounts of O-2 in N-2. (C) 2014 AIP Publishing LLC.