Mass Spectrometry of Free Radicals and Vibronically Excited Molecules Produced by Pulsed Electrical Discharges

Free radicals and metastable molecules produced by short-duration pulsed electrical discharges have been studied with a mass spectrometer employing a collision-free molecular-beam sampling system. Nitrogen atoms generated by extremely short pulses (0.035-μsec half-width) have been used to study the dynamics of the gas-sampling system. NH free radicals produced by electrical discharges in ammonia were detected in the X 3Σ− ground state and in the a 1Δ electronically excited state. It is estimated that approximately 22% of the NH radicals were in the a 1Δ state. Direct measurement of the ionization potential of NH in the ground state gives I (NH) = 13.1 eV. Vibronically excited nitrogen molecules from a discharge in a N2–He mixture have been observed with excitation energies up to about 9 eV, indicating that either or both of the metastable electronic states, a 1Πg and a′ 1Σu−, are substantially populated by the discharge. Evidence for excitation of N2 X 1Σg+ ground-state molecules to high vibrational quantum numbers has been obtained from the appearance-potential curve of the N2+ ion.