Excitation of metastable N2(A 3Σu+) vibrational levels by electron impact

Absolute cross sections for the excitation of individual vibrational levels of the metastable ${\mathrm{A\hspace{0.17em}}}^3{\Sigma }_u^{+}$ state of nitrogen by electron impact were calculated using recent experimental data for the direct excitation of the ${\mathrm{A\hspace{0.17em}}}^3{\Sigma }_u^{+}$ state. The calculations include direct excitation as well as fast cascade contributions from the B ³Π_g and C ³Π_u states of N₂. The data input to the calculations consists of cross sections for direct excitation of the entire A, B, and C states, Franck‐Condon factors for the transition from the ${\mathrm{N}}_2{\mathrm{(X\hspace{0.17em}}}^1{\Sigma }_g^{+}\text{,\hspace{0.17em} v = 0)}$ ground state to the vibrational levels of the A, B, and C states, and transition probabilities for the transitions C → B and B → A. The results obtained consist of graphs displaying the direct, cascade, and total cross sections of the A state both as a function of electron energy as well as vibrational quantum number. Electron energies range from threshold near 6 eV to 45 eV and vibrational quantum numbers range from ν = 0 to ν = 22. The results show that the population rates for the lowest vibrational levels of the A state are dominated by cascade, whereas the higher vibrational levels are directly excited from the N₂ ground state. In general, the lowest vibrational levels of the A state are most efficiently populated by monoenergetic electrons and the nonmonoenergetic electron fluxes which are encountered in the aurora. Applications of the present results to processes in the upper atmosphere are briefly discussed.