Superelastic scattering of electrons from laser-excited 138Ba (. . . 6s6p 1P1) is used to measure the electron impact coherence parameters for excitation of this state. In particular, the variation with scattering angle of the charge cloud asymmetry parameter, Pl+, and the alignment angle parameter, gamma , have been determined at two incident electron energies. Comparison with recent theoretical results based on the unitarized distorted wave approximation has been made taking into account any possible distortion in the measurements due to the influence of a finite interaction volume. This comparison reveals good quantitative agreement for the gamma parameter over the range of scattering angles studied. Differences between the observed and predicted Pl+ parameters are small for near-forward scattering but become significant at the larger scattering angles studied.
Magnetic sublevel partial differential cross sections for excitation of the Ba(. . .6s6p 6'P, ) level are presented based upon the differential cross section measurements of lensen etaland Wang et a/ in combination with new electron impact coherence parameter measurement8 and electron impact coherence parameter measurements previously reported by Zetner and Li. These partial diflerential cross sections, determined for electron impact energies of Eo= IO, 20, 37, 50 and 80 eV, are compared with available theory.
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