High precision measurements of forbidden transitions in highly charged ions at the Heidelberg EBIT

Few-electron ions, which can be produced and studied at rest in electron beam ion traps (EBITs) are very well suited for the study of nuclear size effects and QED in strong fields, but the various contributions are usually entangled. Therefore, combinations of experiments with ions in different charge states are required to separate those contributions. In order to achieve this, several spectroscopic techniques have been recently implemented at the Heidelberg EBIT, aiming at high resolution and accuracy. In the optical region the most accurate wavelengths ever reported for highly charged ions [Draganić et al., Phys. Rev. Lett. 91 (2003) 183001] have been obtained, the results being sensitive to isotopic shifts [Tupitsyn et al., Phys. Rev. A 68 (2003) 022511] at the 0.01 meV level. The forbidden transitions of B-like ArXIV and Be-like ArXV ions studied here are especially interesting, since the QED contributions are as large as 0.2%. Improved atomic structure calculations allow to determine their values with growing accuracy, although the theoretical accuracy still lags three to four orders of magnitude behind the experimental one. In a different experiment, the lifetime of the corresponding metastable level has also been measured with an uncertainty of less than 0.2% thus becoming sensitive to the influence of the bound electron anomalous magnetic moment, an almost experimentally unexplored QED effect so far. A new laser spectroscopic setup aims at facilitating future studies of the hyperfine structure of heavy hydrogenic ions. Through the study of the dielectronic recombination, information on rare processes, such as two-electron-one-photon transitions in Ar16+ [Zou et al., Phys. Rev. A 67 (2003) 42703] at energies of around 2 keV, or the interference effects between dielectronic and radiative recombination in Hg77+ at 50 keV, and accurate values for the excitation energies of very heavy HCI have been obtained. A novel X-ray crystal spectrometer allowing absolute X-ray wavelength measurements in the range up to 15 keV with very high precision and reproducibility is currently used to study the Lyman series of H-like ions of medium-Z ions and the 2s-2p transitions of very heavy Li-like ions.