Resonant structure in electron impact excitation of CO near threshold

Electron impact excitation functions of numerous states in CO have been measured at 45° scattering angle with resolutions of 16–23 meV FWHM. The decay peak of the 10.04 eV resonance can be seen in the results for the a 3Π, a′ 3Σ+, and A 1Π vibrational levels. There was no evidence of resonant excitation of the a′ 3Σ+ state near 8 eV as suggested by Newton and Thomas. Excitation functions of the b 3Σ+, B 1Σ+, C 1Σ+, c 3Π, and E 1Π states, and a previously unobserbed state at 11.26 eV show numerous sharp resonances in the first few eV above threshold. Energy loss spectra in the 8–14 eV loss region show peaks corresponding to known states as well as to unidentified states. No sign of the metastable state at about 9.5 eV seen by Wells, Borst, and Zipf could be detected in direct excitation, but an indirect excitation process involving the A 1Π state is consistent with the data.