Electron impact ionizationndash;excitation of cold, supersonic CO beams has been investigated in order to understand the rotational selection rules when the excited ionic state has2Pgr; symmetry. Variable electron energy (150ndash;900 eV) was used, and the recorded emission spectra had the rotational structure almost completely resolved. Assuming the electron dipole selection rule to be valid, a simple model of rotational transitions within ionizationndash;excitation to a2Pgr; state was developed. Synthetic spectra, calculated on the basis of this model, were in excellent agreement with the experimental spectra at 900 eV. The possibility that there might be deviations from the electric dipole selection rule at lower electron impact energies was considered. However, because of the complicated relationship between ground and excited state populations, we did not expect to be able to draw any definitive conclusions about such effects. Experimentally, definite evidence was obtained for such deviations at electron impact energies below 200 eV.
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