Time-of-flight appearance potential mass spectrometry of a radio frequency discharge supersonic jet nitrogen source

Appearance potential mass spectrometry (APMS), time-of-flight (TOF) methods, and optical emission spectroscopy (OES) were employed to characterize the active nitrogen species emitted from a 13.56-MHz radio frequency discharge (RFD) supersonic jet (SSJ) source. OES of the RFD-SSJ plume evidenced only N2 first positive (1+) series bands arising from N2(B3Πg)→N2(A3Σu+) transitions in addition to He* and Ar* emission lines. APMS yields the relative concentrations of various active nitrogen species (N2+ ions, N2 metastables, and ground-state N atoms) by varying the electron impact ionization energy. TOF sampling was used to discriminate between background and beam signals, and TOF velocity analysis was used to measure the kinetic energies and energy spread of the beam components. By TOF-APMS, we demonstrate that the RFD-SSJ nitrogen source produces primarily ground-state N atoms (4S) with very small concentrations of metastable molecular nitrogen (A3Σu+) and molecular ions. N2 dissociation fractions of 10%–25% were inferred. Plasma neutral temperatures ranged from 1200 to 1650 K, and energy analysis of TOF velocity data indicated N atom kinetic energies of 0.45–0.49 eV.