CHARGE-CLOUD ALIGNMENT IN THE ELECTRON-IMPACT EXCITATION OF BA-138(...6S6P P-1(1))

We have determined charge-cloud alignment parameters P(l)+ and gamma f or electron-impact excitation of the (6s2 S-1(0)) to (6s6p P-1(1)) tra nsition in Ba-138 by measuring the superelastic scattering of electron s from laser-excited Ba-138. Four electron-impact energies were studie d and scattering-angle coverage in each case was sufficient to allow t he observation of pronounced structure in the behavior of these parame ters. Comparison of our measurements with two distorted-wave theories [R. E. H. Clark, J. Abdallah, Jr., G. Csanak, and S. P. Kramer, Phys. Rev. A 40, 2935 (1989); R. Srivastava, T. Zuo, R. P. McEachran, and A. D. Stauffer, J. Phys. B 25, 3709 (1992)] reveals generally excellent agreement except at the lowest impact energy studied where these theor ies fail completely to reproduce the observed behavior of P(l)+. Our l ow-impact-energy data have also been compared with a close-coupling ca lculation [I. I. Fabrikant, Izv. Akad. Nauk Latv. SSR, Ser. Fiz. Tekhn . 6, 11 (1985)] which qualitatively reproduces the observed behavior o f P(l)+ and gamma but fails to give quantitative agreement.