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The low density matter end-station at the new seeded free electron laser FERMI@Elettra is a versatile instrument for the study of atoms, molecules and clusters by means of electron and ion spectroscopies. Beams of atoms, molecules and helium droplets as well as clusters of atoms, molecules and metals can be produced by three different pulsed valves. The atomic and molecular beams may be seeded, and the clusters and droplets may be pure, or doped with other atoms and molecules. The electrons and ions produced by the ionization and fragmentation of the samples by the intense light of FERMI can be analysed by the available spectrometers, to give mass spectra and energy as well as angular distributions of charged particles. The design of the detector allows simultaneous detection of electrons and ions using velocity map imaging and time-of-flight techniques respectively. The instruments have a high energy/mass resolution and large solid-angle collection efficiency. We describe the current status of the apparatus and illustrate the potential for future experiments.

Present performance and future development of the free-electron lasers (FELs) in Hamburg—FLASH for the extreme ultraviolet and the European XFEL for the soft and hard x-ray regimes—are presented. As an illustration of the unprecedented characteristics of these sources a few recent examples of experiments performed in the area of atomic, molecular and optical (AMO) physics are described. The results highlight in particular the available high photon intensities, the short pulse durations and the coherence of the FEL beam. Nonlinear processes involving for the first time inner-shell electrons, time-resolved experiments on the few femtosecond timescales, and imaging experiments on small particles have been the focus of these studies, demonstrating the unique potential of short-wavelength FELs and pointing to numerous exciting future opportunities.

The ionization dynamics of He nanodroplets irradiated with intense femtosecond extreme ultraviolet pulses of up to 10 13 W=cm 2 power density have been investigated by photoelectron spectroscopy. Helium droplets were resonantly excited to atomiclike 2p states with a photon energy of 21.4 eV, below the ionization potential (I p ), and directly into the ionization continuum with 42.8 eV photons. While electron emission following direct ionization above I p is well explained within a model based on a sequence of direct electron emission events, the resonant excitation provides evidence of a new, collective ionization mechanism involving many excited atomiclike 2p states. With increasing power density the direct photoline due to an interatomic Coulombic decay disappears. It indicates that ionization occurs due to energy exchange between at least three excited atoms proceeding on a femtosecond time scale. In agreement with recent theoretical work the novel ionization process is very efficient and it is expected to be important for many other systems. With the advent of short-wavelength free-electron lasers (FELs) the interaction between intense, high-energy light pulses and matter has become a very active field of research [1][2][3] and one of the most exciting topics in atomic and molecular science. Key questions are related to ionization dynamics on an atomic level, answers to which will help to develop an understanding of processes in more complex systems. In pioneering experiments and theoretical studies, various new phenomena such as absorption enhancement [1,4], bleaching [3,5,6], as well as modification [7] and suppression [8] of electron emission were discovered.At high power densities a nanoscale sample, such as a large molecule or cluster, can absorb a large number of photons and the system undergoes a transition to a highly excited, nonequilibrium state. Ionization in this case is strongly interlinked with correlated electron dynamics, either due to multielectron collisions with energy exchange [7] or by novel types of autoionization processes related to interatomic Coulombic decay (ICD), as predicted recently [9]. According to this work, clusters resonantly irradiated by intense light pulses with photon energies insufficient to ionize the atoms by single photon absorption are efficiently autoionized due to the energy exchange between two excited electrons [ Fig. 1(a)]. As a result, an unusual form of a collectively excited, plasmalike state may be formed which is expected to autoionize on a fs-ps time scale [9]. Initial evidence for such an ionization process in Ne clusters has been reported recently [10,11].In this Letter we report a study of electron emission from He clusters irradiated by intense pulses from the new seeded-FEL FERMI [12] at power densities where such collective autoionization (CAI) processes are expected to occur [13]. He droplets were either resonantly excited to the 2p atomiclike state [14], which is well below the ionization potential (I p ), or excited into the continuum. The elect...