Electron-impact rotational excitation of linear molecular ions

Molecular R-matrix calculations are performed to give rotational excitation rates for electron collisions with linear molecular ions. Results are presented for CO, HCO, NO and H2 up to electron temperatures of 10 000 K. De-excitation rates and critical electron densities are also given. It is shown that the widely used Coulomb–Born approximation is valid for Dj 1⁄4 1 transitions when the molecular ion has a dipole greater than about 2D, but otherwise is not reliable for studying electron-impact rotational excitation. In particular, transitions with Dj . 1 are found to have appreciable rates and are found to be entirely dominated by short-