Laser Spectroscopy of Highly Charged Ions at Storage Rings

Heavy highly charged ions (HCI) are the simplest few-body systems and the strongest laboratory sources of electromagnetic fields. These ions thus uniquely enable one to probe the effects of quantum electrodynamics and relativity at strong fields, and to verify corresponding theoretical predictions. The impact of these strong fields on the atomic structure can, however, still only be measured by very accurate techniques, such as laser spectroscopy. Unfortunately, the required experimental conditions (e.g. for the HCI to remain in their charge-state) are severe, and the relevant spectroscopic transitions of the ions cannot be directly probed by standard laser systems. In storage rings, such as the experimental storage ring (ESR) of GSI in Darmstadt, the conditions are excellent: a high intensity ion beam, which can also be cooled, traverses the ultra-high vacuum (10−11 mbar) beamline with a revolution frequency of about a MHz. There are thus many ion-photon interactions, and corresponding fluorescence yields are sufficient. The kinetic energy of the ions can be varied over a large range, and the Dopplershift be used to boost the transitions into the laser accessible regime. The future Facility for Antiproton and Ion Research (FAIR) will provide even more possibilities. The Stored Particle Atomic Research Collaboration (SPARC) therefore developed a broad research programme [1]. For example, at the new experimental storage ring (NESR), it is planned to determine QED effects in strong fields via laser spectroscopy of the 1s and 2s hyperfine structure in H- and Li-like ions, respectively [2]. For this purpose, radioactive nuclei or even chains of isotopes can be used to disentangle QED and nuclear effects. In addition, by means of optical pumping with lasers (via the hyperfine structure), spin-polarised ion beams will be produced for e.g. parity non-conservation studies. The high-energy laser PHELIX will generate x-rays which, combined with HCI in the new ESR, enable studies of 2s−2p transitions in high-Z few-electron systems for accurate tests of atomic structure theory in the strong field regime. In the SIS300, laser cooling - the only possible cooling method at these high energies - of Li-like heavy ions is planned [3].
[1] Th. Stöhlker et al., Nucl. Instr. Meth. Phys. Res. B 261 (2007) 234.
[2] D.F.A. Winters et al., Can. J. Phys. 85 (2007) 403.
[3] H. Backe, Hyp. Int. 171 (2007) 93.