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 Te≅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 ∼3x10-5 Torr, a nearly electron-free plasma is formed with a residual electron-to-ion density ratio ne/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-/ne for different pressures in both C7F14 and SF6 indicate that for thermal electrons at 2300 K larger values of n-/ne are obtained in C7F14 at a lower pressure than in SF6