Electronic Excitation of Helium and Krypton by Electron Impact.

A crossed-beam electron impact spectrometer is modified to enable the study of near-threshold electronic excitation of He and Kr by electron impact. Absolute differential cross sections for excitation of the individual n = 2 states of He and 4p('5)5s states of Kr are measured within the first four electron volts of threshold. In He pronounced angle-dependent structures are apparent in the threshold excitation of the 2('3)P state and in the near-threshold excitation of the 2('3)S state. These experimental observations, when viewed in light of recent theoretical considerations, suggest the existence of a new ('2)P resonance near 20.8 eV. The results on Kr demonstrate that the angular momentum of the Kr('+) ion core (4p('5)) has a strong influence on the excitation cross sections, both on the relative magnitudes of the cross sections and on the relative strengths of resonant structures in the excitation functions. The angular momentum of the entire electron cloud of an excited state has less of an effect on the cross sections. It manifests itself primarily in the relative magnitudes of the two metasable cross sections. These observations emphasize the weakness of the coupling between the core and the outer electrons in the electron-Kr system.