Rotational excitation ofN2by electron impact: 1-4 eV

Abstract

Rotational and rotational-vibrational ($v=0\mathrm{}\to 1$) excitation in ${\mathrm{N}}_2$ have been studied with a crossed-beam electron-impact apparatus. In the energy range 1-4 eV, the elastic and vibrational energy-loss peaks show large rotational broadening compared with the apparatus profile (full width at half-maximum, 18 meV). The branching ratios for rotational transitions with $\Delta j=0, \pm 2, \pm 4$ are obtained with a line-shape analysis applied to the energy-loss profiles. The results for rotational-vibrational excitation at 2.27 eV and scattering angles 30-90° are in good agreement with the calculations using the resonant $d\pi$ waves and the rotational impulse approximation. The corresponding results for pure rotational excitation show that the branches with $\Delta j=\pm 2 \mathrm{and} \pm 4$ are predominantly excited via resonances, while the branch with $\Delta j=0\mathrm{}$ contains a large contribution from direct scattering. The absolute rotational cross sections for $\Delta j=\pm 4$ are measured; they exhibit a large magnitude (${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$) and peak and valley structures in the 1-4 eV range, reminiscent of well-known resonant vibrational excitation. The energy dependence and the absolute magnitude of the rotational cross sections for $\Delta j=\pm 4$ can be understood in terms of a "boomerang" calculation. A comparison of the experiment with the relevant theoretical calculations is made.