Breit interaction in dielectronic recombination of hydrogenlike uranium

Absolute rate coefficients for dielectronic recombination (DR) of H-like ${\mathrm{U}}^{91+}$ ions have been measured. The electron-ion merged-beam technique at a heavy-ion storage ring was employed using a stochastically cooled ion beam. Thereby, the previously accessible electron-ion collision energies could be greatly extended to the range 63–90 keV. High-resolution DR spectra were measured covering all $\mathit{KLL}$ and $\mathit{KLM}$ resonances. For the resonance strengths, excellent agreement between relativistic theory and experiment is found only if the Breit contribution to the electron-electron interaction is included in the calculations. For the $\mathit{KL}$${}_{1/2}$$L$${}_{1/2}$ and $\mathit{KL}$${}_{1/2}$$M$${}_{1/2}$ groups the Breit contribution amounts to 44% of their total resonance strengths.

Figure 1

Measured ${\mathrm{U}}^{91+}$ merged-beams DR rate coefficient. The data were obtained in several individual runs. For display, effects from the drift tube used for the energy scan and the beam merging have been deconvolved.

Figure 2

Theoretical DR rate coefficients including GBI (red line) and without (blue line). The line shape is natural line widths only. The individual resonances that form the respective group are indicated by black dashed lines (GBI included).

Figure 3

Measured (symbols) and simulated (lines) ${\mathrm{U}}^{91+}$ DR rate coefficient for the $\mathit{KLL}$ resonance groups. The simulations comprise the theory values from Table  including GBI (red) and without (blue). The beam parameters are mimicked by an ion beam momentum spread of $\delta p/p=7.5\times {10}^{-4}$, electron beam temperatures of ${\mathit{kT}}_{\parallel }=0.2$ meV, and ${\mathit{kT}}_{\perp }=120$ meV as well as the effects from drift tube, merging and demerging. It is noted that experiment and simulation are on independently absolute scales.