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Molecular photofragmentation has been studied by event imaging on HeH+ ions at 32 nm (38.7 eV) in a fast ion beam crossed with the free-electron laser in Hamburg (FLASH), analyzing neutral He product directions and energies. Fragmentation into He(1snl,n > or = 2)+H+ was observed to yield significant photodissociation at 32 nm with an absolute cross section of (1.4+/-0.7) x 10(-18) cm2, releasing energies of 10-20 eV. A clear dominance of photodissociation perpendicular to the laser polarization was found in contrast to the excitation paths so far emphasized in theoretical studies.

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Abstract. 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 (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.