X-ray laser spectroscopy of highly charged ions at FLASH

Laser spectroscopy, widely applied in physics and chemistry, is extended into the soft x-ray region for the first time. Resonant fluorescence excitation of highly charged ions (HCIs) by soft x-ray free-electron lasers (FELs) shows here the potential for unprecedented precision on photonic transitions hitherto out of reach. The novel experiments combine an electron beam ion trap (EBIT) with the Free-electron LASer at Hamburg (FLASH) to measure resonant fluorescence by trapped HCIs as a function of the laser's wavelength. The present experiments have already reached the performance of conventional soft and hard x-ray spectroscopy. We present the results obtained for three fundamental and theoretically challenging transitions in Li-like ions, namely 1s 2 2s 2 S 1/2 –1s 2 2p 2 P 1/2 in Fe 23+ at 48.6 eV, in Cu 26+ at 55.2 eV and 1s 2 2s 2 S 1/2 –1s 2 2p 2 P 3/2 in Fe 23+ at 65.3 eV. The latter demonstrates laser spectroscopy of multiply or HCIs at more than one order of magnitude higher energies than hitherto reported. Resolving power leading to relative precision up to 6 parts-per-million points to the possibility of providing an atomic absolute wavelength standard in this spectral region, which is still lacking.