Studies of N4+ and N3+ Ion Formation in Nitrogen Using High-Pressure Mass Spectrometry

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Abstract
The formation of the complex ions of nitrogen N4+ and N3+ has been studied at pressures up to 0.1 torr using a differentially pumped mass spectrometer and monoenergetic electron-beam techniques. The N4+ ion has an appearance potential of 15.0±0.1 eV, and exhibits a quadratic pressure dependence in the threshold region, consistent with the secondary reaction N2*+N2→N4++e. At electron energies greater than 17.5 eV, a cubic pressure dependence is observed, consistent with the secondary reaction N2++2N2→N4++N2. The reverse of this reaction has been shown to lead to an early onset of N2+ when a sufficiently large extraction field is used to impart the necessary kinetic energy to the N4+ ion. The N3+ ion has an appearance potential of 21.1±0.1 eV and exhibits a quadratic dependence at low pressures, consistent with the secondary reaction N2+(4Σu+)+N2→N3++N, proposed previously by Cermak and Herman. At higher pressures the N3+ pressure dependence deviates from quadratic due to the depopulation of the N2+(4Σu+) state. An analysis of this deviation in the N3+ pressure dependence provides an approximate estimate of the radiative lifetime of the N2+(4Σu+) of τ=8×10−6 sec and of the collisional destruction rate coefficient 〈QDv〉Av=2×10−9 cm3/sec.