Electron-Impact Rotational Excitation of a Symmetric-Top Molecule: Application to NH₃

A cross section formula for the rotational transition in a symmetric-top molecule by electron collisions is derived in the Born approximation, with adopting a general form of electron-molecule interaction. Then, the result is applied to the electron-NH 3 collision. The cross section for the transitions Δ K ≠0 ( K being the quantum number representing the angular momentum around the symmetry axis) is calculated for the first time and its magnitude is found to be much less than the dipole excitation cross section (for which Δ K =0). In a large-angle scattering, however, the differential cross section for Δ K ≠0 has a non-negligible effect.