A laboratory study of the dissociative excitation of the N + ( 5 S) state

The prominent auroral emission feature near λ2145 Å has been tentatively identified as the N + ( 5 S) doublet lines [λ2139.68 Å and λ2143.55 Å]. Dissociative excitation of N 2 by electron impact with a predicted cross section value ≥2 × 10 −18 cm² has been proposed as the source of this metastable species. We have observed the N + ( 5 S) emission lines for the first time in a laboratory electron‐impact experiment and found that the cross section has a value of ∼ 3 × 10 −21 cm² at 100 eV. This result eliminates dissociative excitation as a significant N + ( 5 S) source in auroras. Observation of the spatial distribution of the doublet radiation around the electron beam has provided some insights into the radiative lifetime of the 5 S state, and we show that τ r ∼ 5 µsec if the fragment ions are produced with a kinetic energy of ∼ 3 eV but ∼ 100 µsec if the N + ions are formed with thermal energies.