Rotational Excitation of Polar Molecules by Electrons

Rotational excitation of polar molecules is calculated in the approximation that the electron transit time is short compared with rotational periods with the result of an $E^{-1\mathrm{}}$ behavior of the cross section. Diffusion cross sections are calculated for $\Delta l=0,1,2\mathrm{}$. Significant corrections to the Born approximation are obtained for large dipole moments. The range (in energy) of applicability of the result is discussed in terms of the energy dependence of the corrections, and a novel energy dependence of these corrections is encountered and explained.