Absolute cross section for the formation of N-2(+) (X (2)Sigma(+)(g)) ionsproduced by electron impact on N-2

A combination of electron scattering and laser-induced fluorescence (LIF) t echniques has been employed in an absolute experimental determination of th e N-2(+) (X(2)Sigma(g)(+)) ionization cross section as a function of electr on energy from threshold to 200 eV. N-2(+): ground-state ions, resulting fr om electron impact on Na, are detected by laser-induced fluorescence of the N-2(+); X(2)Sigma(g)(+) --> B(2)Sigma(u)(+) transition at 391 nm. The rela tive ionization cross section obtained from LIF spectra taken at different electron energies is put on an absolute scale by calibration relative to th e well known electron-impact cross section for the formation of helium atom s in the 2(3)S state from the 1(1)S ground state. The He 2(3)S atoms are pr obed via LIF of the 2(3)S --> 3(3)P transition at 389 nm in the same appara tus using the same detection equipment and laser-induced fluorescence techn ique at essentially the same wavelength that is used in the measurement of the N-2(+)(X) cross section. The cross section peaks at 60 eV with a maximu m value of 80 x 10(-18) cm(2). At 100 eV, our cross section value is (74.9 +/- 7.5) x 10(-18) cm(2), which is in good agreement with the value of (86. 9 +/- 7.0) x 10(-18) cm(2) reported by Doering and Yang (1996 J. Geophys. R es. 102 9683) who used a different experimental technique.