Resonance-induced deviations of β from 2.0 for rare gas s-subshell photoionization

We investigate the case of resonance-induced deviations of the angular distribution anisotropy parameter, ?, from its nominal value of 2.0 for the Kr 4s-subshell at the 3d ? mp(m ? 5) excitations and the Xe 5s-subshell in the region of the 4d ? mp(m ? 6) excitations. We observe small but unmistakable variations of ? from 2.0 at these resonances. In the case of Kr the largest deviation from 2.0 is a mere -0.030(6), ? 4s ? 1.970(6), while in Xe it is definitively larger at -0.080(6), ? 5s ? 1.920(6). To understand these results in more detail, we have carried out relativistic random phase and relativistic multi-channel quantum defect calculations. Our theoretical results clearly indicate deviations of ? from 2.0 for these photolines in these resonance regions, although the degree of variation is predicted to be larger than what is observed. � 2006 IOP Publishing Ltd.