Laser spectroscopy on trapped highly-charged ions using soft x-rays from FLASH

In vast regions of the universe highly charged ions (HCI [1, 2]) are the predominant form of visible matter. Their importance extends to high-temperature terrestrial plasmas, such as those used in fusion research. Yet, accurate prediction of their electronic structure remains a challenge for theory due to the strong electromagnetic field in which the remaining bound electrons dwell. Experimental accuracy has now reached the performance limits of conventional photon spectroscopy in the soft and hard x-ray regions. In this work [3], we report on the resonant laser excitation of the 22S1/2—22P1/2 transition of the Li-like Fe22P1/223+ ion at 48.6 eV, an energy range hitherto unattainable with powerful lasers. The HCI stored in an electron beam ion trap (EBIT [4]) were resonantly excited by ultra-brilliant radiation generated at the Free electron LASer in Hamburg (FLASH [5]). While yielding a relative statistical error of only 2.2·10−5, and extending laser spectroscopy on HCI from the near ultraviolet [6] to the soft X-ray region, this novel experiment demonstrates immediate potential to push the current limits of precision by orders of magnitude. Such experiments allow to verify predictions of quantum electrodynamics (QED) in a strong field environment where perturbation theory [7, 8] fails. Future EBIT experiments at upcoming x-ray free electron lasers (X-FEL) like the Stanford Linear Coherent Light Source (LCLS) or the European X-FEL will pave the way for laser spectroscopy into the hard x-ray region.