The rotational excitation of polar molecules by electrons

Cross sections and rates have been calculated for the mod Delta j mod =1 rotational transitions of linear polar molecules due to electron impact. First-order time-dependent perturbation theory, modified to ensure unitarity and approximate angular momentum conservation, has been used and simple analytic forms for the cross sections and rates are obtained. Comparisons with more sophisticated theories suggest that the author's cross sections are accurate to within 15% over a wide range of energies and dipole moments. Their results agree well with recent beam experiments on CsF, KI and CsCl at higher electron energies, but confirm other calculations which lie significantly above the measured values at the lower electron energies for all three molecules.