Vibrational Excitation by Electron Impact near Threshold in H2, D2, N2, and CO

Abstract

The trapped-electron method is applied to the measurement of the slopes of the vibrational cross sections near their respective thresholds. Absolute values are obtained by normalization to positive-ion cross sections. The values obtained in ${\mathrm{H}}_2$ are 43 for $v=1, 0.72$, for $v=2, 0.04$, for $v=3, \mathrm{and} 0.003$, for $v=4\mathrm{}$, in units of ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$/eV. The values for ${\mathrm{N}}_2$ are 2.5 for $v=1\mathrm{}$ and about 0.02 for $v=2\mathrm{}$, whereas, the values for CO are much larger, i.e., 120 for $v=1\mathrm{} \mathrm{and} 2.4$ for $v=2\mathrm{}$. Various corrections that are necessary for determining absolute cross sections from trapped-electron data are discussed. The validity of the corrections is studied for the case of the $2{}_{}{}^3{}_{}{}^{}S$ excitation function in He.