Rotational excitation of symmetric-top molecular ions by electron impact

The theory for the rotational excitation of symmetric-top molecular ions by electron impact is formulated within the Coulomb-Born approximation. Analytical expressions are derived for the rotational transitions induced by dipole and by quadrupole interactions. The cross sections are large and finite at threshold and decrease with increasing electron-impact energy. The validity of the present theory for the description of low-energy collisions is examined. Numerical calculations are reported for the excitation of the hydronium H₃{O}⁺$ ion.