Alignment of Ar+(2p-12P3/2) ions after electron impact ionisation in the range 1 keV to 268 eV

We have measured the alignment of A20 of Ar+ (2p-1/2 P3/2) ions after electron impact ionization in the range of primary electron energy E0=1000 ... 268 eV (range of excess energy E1=750 ... 19.5 eV) via the anisotropic angular distribution of L3-M2,3 M2,3(1S0) 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 E0≥350 eV agree well with previously existing experimental values of DuBois and Rodbro and with theoretical DWBA results of Berezkho and Kabachnik, but for E0<350 eV they deviate systematically from the DWBA values. For the lowest impact energy E0, which is only 19.5 eV above threshold, we obtained A20=+0.09(16). This value clearly indicates that in the ionisation process near threshold the two low-energy electrons escape not only with a two-electron partial were L=0, according to Wannier's original assumption, but also with partial waves L>0. (orig.)