Cross sections for rotational excitation of CH_4 by 3–20-eV electrons

We report calculated differential, integral, and momentum-transfer cross sections for rotational excitation of CH_4 by electron impact in the 3–20-eV energy range. These cross sections were derived from fixed-nuclei scattering amplitudes obtained using the Schwinger multichannel method. Our results represent the first rotational excitation cross sections obtained for a polyatomic molecule using entirely ab initio procedures. The cross sections agree well with those of earlier model-potential calculations. A comparison of these calculated cross sections with available experimental data is in general encouraging, but some discrepancies remain.