Studies of N₄⁺ and N₃⁺ ion formation in nitrogen using high pressure mass spectrometry

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⁺(⁴∑_u⁺)+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⁺(⁴∑_u⁺) state. An analysis of this deviation in the N₃⁺ pressure dependence provides an approximate estimate of the radiative lifetime of the N₂⁺(⁴∑_u⁺) of τ=8×10^-6 sec and of the collisional destruction rate coefficient Q_Dv>_Av=2×10^-9 cm³/sec.