Electron-impact rotational excitation of water

Rotational excitation of H2O, HDO and D2O by thermal electron impact is studied using the molecular R-matrix method. Rate coefficients are obtained up to electron temperatures of 8000 K. De-excitation rates and critical electron densities are also given. It is shown that the dominant transitions are those for which J = 0, ±1, as predicted by the dipolar Born approximation. However, a pure Born treatment is found to overestimate the cross-sections close to threshold energies and to neglect important (dipole forbidden) transitions, owing to the importance of short-range and threshold effects. In the context of cometary water, the contribution of electron collisions might explain the need for large H2O–H2O collisional excitation rates in population models that neglect electrons.