State-selective influence of the Breit interaction on the angular distribution of emitted photons following dielectronic recombination

We report a measurement of $KLL$ dielectronic recombination in charge states from Kr$^{+34}$ through Kr$^{+28}$, in order to investigate the contribution of Breit interaction for a wide range of resonant states. Highly charged Kr ions were produced in an electron beam ion trap, while the electron-ion collision energy was scanned over a range of dielectronic recombination resonances. The subsequent $K\alpha$ x rays were recorded both along and perpendicular to the electron beam axis, which allowed the observation of the influence of Breit interaction on the angular distribution of the x rays. Experimental results are in good agreement with distorted-wave calculations. We demonstrate, both theoretically and experimentally, that there is a strong state-selective influence of the Breit interaction that can be traced back to the angular and radial properties of the wavefunctions in the dielectronic capture.