Emission Cross Sections of N2 in the Vacuum Ultraviolet by Electron Impact

The electron beam excitation spectrum of N₂ at low pressure (0.12–0.18 μ) in the wavelength range 1200–2000 Å was recorded for monoenergetic electron impact energies from 10 to 208 eV. The spectral features observed were the Lyman–Birge–Hopfield vibrational transitions ${\mathrm{(a}}^1{\Pi }_g{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{\Sigma }_g$ and several atomic nitrogen multiplets produced by dissociative excitation of N₂. The emission cross sections of the Lyman–Birge–Hopfield vibrational transitions were measured and summed to give the cross section for the entire electronic transition. The energy dependence of all vibronic transitions was the same. The atomic nitrogen transitions studied were the multiplets at 1200 Å ${(}^4{\mathrm{P\hspace{0.17em}\to \hspace{0.17em}}}^4{S}^0)$ , 1493 Å ${(}^2{\mathrm{P\hspace{0.17em}\to \hspace{0.17em}}}^2{D}^0)$ and 1743 Å ${(}^2{\mathrm{P\hspace{0.17em}\to \hspace{0.17em}}}^2{P}^0)$ . These emissions originated from dissociative excitation of molecular nitrogen. The emission cross sections of these transitions typically have two thresholds, corresponding to dissociative excitation of N₂ into two nitrogen atoms and to dissociative ionization excitation into an atom and an ion.