Excitation of atomic nitrogen by electron impact

Abstract

Absolute cross sections have been measured for the excitation of the NI(λλ1134 1164, 1168, 1200, 1243, and 1743 Å multiplets by electron impact on atomic nitrogen. The λ1134 and λ1200 Å cross sections are large, reaching ${\text{2.0× 10}}^{\text{−16}}$ and ${\text{2.5× 10}}^{\text{−16}}\hspace{0.17em}{\mathrm{cm}}^2$ at their peaks, respectively. The magnitude of these cross sections is about a factor of two lower than the values reported previously by the authors, due to the discovery of a fault in the apparatus. The presence of vibrationally excited molecular nitrogen in the discharged gas is confirmed and its effect on the measurements is discussed. The ratio of the oscillator strengths of the λ 1200 and $\text{λ\hspace{0.17em}1134\hspace{0.17em}}\mathrm{\AA }$ resonance transitions is measured to be 2.6 ± 0.3. The branching ratio for the $\mathrm{NI}\text{(λ 1131}\mathrm{\AA }\text{/λ 1164}\mathrm{\AA }$ multiplets is measured to be 2.8 ± 0.3. Striking differences in the distribution of intensity between the spectra of atomic nitrogen and molecular nitrogen excited by energetic electrons suggest an optical method for measuring the density of atomic nitrogen in the upper atmosphere.