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 ${}^{28}{\mathrm{N}}_2^{+}$ and ${}^{30}{\mathrm{N}}_2^{+}$ 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 ${\mathrm{A\hspace{0.17em}}}^2{\Pi }_u{\mathrm{\to \; X\hspace{0.17em}}}^2{\Sigma }_g^{+}$ transitions, for A²Π_u vibrational levels with ν′ as high as 30. For an electron energy E_e of 63 eV, about 25% of the ${}^{28}{\mathrm{N}}_2^{+}$ produced is initially in the A²Π_u, 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 ${\mathrm{N}}_2^{+}{\mathrm{\hspace{0.17em}(A\hspace{0.17em}}}^2{\Pi }_u\mathrm{,\hspace{0.17em}v\prime )}$ 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 R̄_e 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 R̄_e and with previously measured lifetimes of the A²Π_u vibrational states. The electron energy dependences measured for light output at selected wavelengths rise from threshold to a maximum around E_e = 32 eV and then decrease at higher E_e. The electron impact ionization process produces rotational populations of ${\mathrm{N}}_2^{+}{\mathrm{\hspace{0.17em}(A\hspace{0.17em}}}^2{\Pi }_u)$ which correspond to an effective temperature higher than the temperature of the target N₂. We suggest that the high A²Π_u vibrational levels, rather than the ${\mathrm{N}}_2^{+}$ quartet states, may be involved in the production of ${\mathrm{N}}_3^{+}$ by electron impact on room‐temperature N₂.