Electron - Impact Excitation of Interstellar Molecules

Electron-molecule collisions play a crucial role in astrophysical environments where the electron fraction is higher than about 10−5, e.g. in the diffuse ISM, PDRs or comets. We present here a brief review of our recent R-matrix calculations of rate coefficients for electron-impact rotational excitation of interstellar molecules. Our major result is the prediction of emission lines from higher rotational states than previously assumed. Rate coefficients for transitions with ∆j > 1 are indeed found to be significant and in some cases larger than those with ∆j = 1, the only ones considered previously. This reflects the importance of short-range interactions which are ignored in the standard theories. Illustrative results are presented for CH, H3 and H2O.