Screened Radiative Corrections from Hyperfine-Split Dielectronic Resonances in Lithiumlike Scandium

Term energies for dielectronic-recombination Rydberg resonances below 0.07 eV are determined for ${\mathrm{Sc}}^{18+}$ with absolute accuracies below 0.0002 eV by electron collision spectroscopy in an ion storage ring, using the twin-electron-beam technique and a cryogenic photocathode. The lithiumlike $2s_{1/2}\mathrm{\text{−}}2p_{3/2}$ transition energy for $Z=21$ is determined to 4.6 ppm, less than 1% of the few-body effects on radiative corrections. Features from the hyperfine structure of the $2s$ state could be resolved in the dielectronic-recombination spectrum.

Figure 1

Electron collision spectrum of the lowest ${\mathrm{Sc}}^{18+}$ DR resonances assigned to three doubly excited Rydberg terms of different $J$, whose hyperfine-unsplit energies are indicated in the bottom together with the energy shifts caused by the splitting between two $F$-levels of the $1s^{2}2s_{1/2}$ ground state. Labels ($J$, $F$) are applied near the resulting hyperfine-split contributions in the experimental spectrum. The further splitting of each ($J$, $F$) contribution through the HFS in $1s^{2}2p_{3/2}$ and the relative intensities of the HFS subcomponents are marked by stick diagrams. The fitted rate coefficient and broad minor features included in the fit are indicated by thick and thin curves, respectively.

Figure 2

Relative contributions to the $2s_{1/2}\mathrm{\text{−}}2p_{3/2}$ transition energy in lithiumlike ions from the screening of the electron-nucleus QED [16, 18] and from two-loop QED [11, 19, 34] compared to experimental errors for electron-beam ion traps [3, 4, 5] (circles), optical spectroscopy (6 and references therein, squares), and previous [22] and present electron collision spectroscopy (filled squares) on the $2s_{1/2}\mathrm{\text{−}}2p_{3/2}$ splitting.