Electron attachment to C7F14 and SF6 in a thermally ionized potassium plasma

Electron attachment to perfluoromethylcyclohexane (C7F14) and sulfur hexafluoride (SF6) is studied in a Q machine which produces a thermally ionized potassium plasma at an electron temperature T-e similar or equal to 0.2 eV (2300 K). Negative ion formation is observed by Langmuir probe measurements of the reduction in electron density as electrons attach to C7F14 to form C7F14- or to SF6 to produce SF6-. In C7F14 at a pressure similar to 3x10(-5) Torr, a nearly electron-free plasma is formed with a residual electron-to-ion density ratio n(e)/n(+)< 10(-4). Formation of the C7F14- negative ion was confirmed by the presence of the C7F14- electrostatic ion cyclotron wave mode in the power spectrum of current-driven plasma oscillations. Measurements of the negative ion-to-electron density ratio, n(-)/n(e) for different pressures in both C7F14 and SF6 indicate that for thermal electrons at 2300 K larger values of n(-)/n(e) are obtained in C7F14 at a lower pressure than in SF6.