We have measured the alignment A20 of K*(3p54s2 2P3/2) and the ratio of cross sections R0 = sigma (3/2)/ sigma (1/2) of fine structure states K*(3p54s2 2P3/2, 2P1/2) for electron impact excitation in the range of incident energy E0 = 31.4-500 eV. We have also calculated these quantities in the distorted wave Born approximation (DWBA) and the plane wave Born approximation (PWBA). There is good agreement between experiment and the present DWBA and PWBA values for the alignment A20 whereas earlier DWBA values by Pangantiwar and Srivastava (1987) deviate systematically from experiment for E0 < 100 eV. For the ratio of cross sections R0 the present DWBA values agree only qualitatively with the experiment for E0 < 100 eV.

Abstract.We have measured the alignment sJ20 of Ar + (2p-~ 2P3/z) ions after electron impact ionization in the range of primary electron energy E0 = 1000... 268 eV (range of excess energy E I = 750... 19.5 eV) via the anisotropic angular distribution of 3 M2,3 (ISo) Auger electrons. On decomposing the Auger spectra into their components special care was taken by including the effect of the postcollision interaction on the shape of Auger lines. The present alignment values for E o > 350 eV agree well with previously existing experimental values of DuBois and Rodbro and with theoretical DWBA results of Berezkho and Kabachnik, but for Eo < 350 eV they deviate systematically from the DWBA values. For the lowest impact energy Eo, which is only 19.5 eV above threshold, we obtained ~2o= +0.09(i6). This value clearly indicates that in the ionisation process near threshold the two low-energy electrons escape not only with a two-electron partial wave L=0, according to Wannier's original assumption, but also with partial waves L > 0.

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