Secondary-electron-production cross sections for electron-impact ionization of molecular nitrogen.

Abstract

Measurements of the double-differential cross section (DDCS), as a function of the ejected energy, angle, and primary energy for electron-impact ionization of molecular nitrogen are reported at incident energies of 200, 500, 1000, and 2000 eV. The ejection angle was varied from 30\ifmmode^\circ\else\textdegree\fi{} to 150\ifmmode^\circ\else\textdegree\fi{} in steps of 15\ifmmode^\circ\else\textdegree\fi{}. The cross sections were obtained by use of a crossed-beam apparatus with an effusive gas source and a pulsed electron beam. Scattered and ejected electrons were energy analyzed by time-of-flight analysis from below 2 eV to the primary energy. The relative measurements were placed on an absolute scale by matching the experimental elastic differential cross sections to absolute measurements at each primary energy. Comparisons of the DDCS with previous reported values revealed significant differences. The DDCS were fitted to a Legendre polynomial expansion as a function of the ejection angle. Platzman plot analysis was carried out on the energy distributions determined from the fit coefficients. The total ionization cross sections at these primary energies were deduced from this plot. An autoionization feature at 2.3 eV was observed for the first time in measurements of this nature and has been assigned as due to a Rydberg state converging to the B ${\mathrm{}}^{2}$${\ensuremath{\Sigma}}_{u}^{+}$ ionic state which decays to the X ${\mathrm{}}^{2}$${\ensuremath{\Sigma}}_{g}^{+}$ ground state of ${\mathrm{N}}_{2}$${\mathrm{}}^{+}$. An analysis of the autoionization lines observed in the present work in the range 0.4--2.5 eV is also presented.