Rotational Excitation of the Interstellar CN Molecule

Rotational excitation of the CN molecule by thermal electron impact is studied in a close-coupling method to test the Born approximation previously used. It is found that the Born approximation gives a fairly accurate cross section even for the molecular dipole moment as large as 2.5 debye. The excitation by slow protons is also studied in a semi-classical method. Here again the higher-order processes are taken into account to see the deviation from the first-order perturbation theory. The results of these calculations are applied to the interstellar CN molecules. It is concluded that the observed population in the rotationally excited level cannot be explained in terms of the collisional excitation, unless the molecules are in the H II region of relatively high density.