Highly-Charged Ion Precision Spectroscopy Using Free Electron Laser Radiation

Abstract

The characteristics of radiation potentially available from proposed rf-linac free electron lasers1 are considered here for precision spectroscopy of highly-charged ions. Such ions are usually produced in environments not conducive to precision measurements, such as high temperature plasmas or fast ion beams. However, highly-charged recoil ions with eV energies have been produced by impact of fast, stripped ions on atomic targets2, and, using synchrotron radiation, multi-charged ions have been formed at thermal energies by K-shell photoionization followed by Auger emission and electron shake-off3. Using each of these cold ion sources, multi-charged ions have been stored under ultra-high vacuum conditions in Penning4,5 or radio-frequency6 ion traps, while maintaining the low energies at which the ions were generated. Long term storage at low energy in a near perturbation-free environment is consequently a reality for multi-charged ions, and experiments can be designed to exploit these properties for both spectroscopic7 and collision8 measurements. In particular, we discuss precision measurements on hydrogen-like multi-charged ions analagous to measurements on singly-charged alkalai-like ion hyperfine structures made using conventional pulsed laser techniques.9 Precision measurements of interest include, for example, studies of the Z-dependence of the Lamb shift in hydrogen-like and helium-like ions, and studies of the fine structure of helium-like ion n=2 states.