Absolute differential cross sections for electron-impact excitation of CO near threshold: I. The valence states of CO

Absolute differential cross sections for the electronic excitation of valence states in CO have been measured in the energy range from threshold to 3.7 eV. In particular, a newly developed experimental technique to control the analyser's efficiency allowed us to measure absolute angular dependences of those states for scattering angles between and . The transitions from the ground state to the and states are forbidden by spin selection rules. It is found that the shape resonance located at 1.8 eV partly decays into these valence states, which are located several eV above the resonance. The decay occurs by ejection of a core electron whose spin is antiparallel to that of the incident electron causing a sharp rise of the cross sections just at threshold. Three new core-excited shape resonances were found, one for each of , and symmetry. By comparison with recently calculated cross sections their electron configurations have been classified. Resonant excitation via these three resonances or via the low-lying resonance dominates the cross sections of the two spin forbidden transitions in the detected energy range. The excitation of the optically allowed transition, however, is dominated by non-resonant processes.