X-ray laser spectroscopy of highly charged ions at FLASH

Abstract

Abstract\nLaser 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 (HCI) by soft x-ray free-electron lasers 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 HCI as a function of the wavelength. The present experiments reach already 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 27+ 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 highly charged ions 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 standards in this spectral region, which is still lacking.