Visible and near ultraviolet light produced by the radiative decay of long-lived states in a nitrogen ion beam

Optical emissions from the decay of long-lived, excited states of N2+28 and N2+30 in ion beams have been studied. Spectra are obtained for wavelengths between 3200 and 6000 Å. Most (∼ 80%−90%) of the emission is consistent with light emitted in A 2Πu → X 2Σg+ transitions, for A2Πu vibrational levels with ν′ as high as 30. For an electron energy Ee of 63 eV, about 25% of the N2+28 produced is initially in the A2Πu, ν′ = 2–9 levels; 0.7% is in the ν′ = 10–19 levels; and 0.3% is in the ν′ = 20–30 levels. Energies of N2+ (A 2Πu, v′) states for ν′≤ 30 are found to be close to the energies calculated from formulas that are known to be valid when ν′ ≤ 9. The electronic transition moment R̄e is found to vary rather slowly for internuclear distances between 0.92 and 1.1 Å. Apparent lifetimes of 6–7 μsec have been measured for emission in selected spectral ranges; these apparent lifetimes are consistent with the observed R̄e and with previously measured lifetimes of the A2Πu vibrational states. The electron energy dependences measured for light output at selected wavelengths rise from threshold to a maximum around Ee = 32 eV and then decrease at higher Ee. The electron impact ionization process produces rotational populations of N2+ (A 2Πu) which correspond to an effective temperature higher than the temperature of the target N2. We suggest that the high A2Πu vibrational levels, rather than the N2+ quartet states, may be involved in the production of N3+ by electron impact on room-temperature N2.