Excitation of Several Important Metastable States ofN2by Electron Impact

Abstract

Absolute cross sections for excitation of the $A{}_{}{}^3{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$, $a{}_{}{}^1{}_{}{}^{}\Pi _q^{}{}_{}{}^{}$, and $E{}_{}{}^3{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ metastable states of ${\mathrm{N}}_2$ were deduced from previous time-of-flight studies of metastable nitrogen molecules. The cross section for direct excitation of the $A{}_{}{}^3{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ state was determined from threshold to 40 eV and was found to have a peak value of (${5.3}_{-3.0\mathrm{}}^{+4.0\mathrm{}}$) × ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$ near 11 eV. The total apparent cross section of the $A{}_{}{}^3{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ state was obtained by adding cascade contributions from the $B{}_{}{}^3{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ and $C{}_{}{}^3{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ states and was found to have a value of about 1.2 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at the two peaks near 11.0 and 14.5 eV. The $E{}_{}{}^3{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ state was efficiently excited near threshold and exhibited a resonancelike excitation function with a half-width (full width of half-maximum) at about 0.4 eV and a peak cross section of (7.0 ± 4.0) × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ at 12.2 eV. The excitation function for the $a{}_{}{}^1{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ state was obtained from threshold to about 40 eV and was found to differ markedly from two other measurements. Using the present results, a sample vibrational distribution for noninteracting molecules in the $A{}_{}{}^3{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ state was calculated at selected electron energies near threshold. For most energies, the lowest vibrational levels are most strongly populated in qualitative agreement with atmospheric observations.