[en] The observed deficit of the solar neutrino flux is now well established. This puzzling problem of today's particle physics could be resolved soon. The most likely explanation would be the vacuum neutrino oscillation phenomenon, indirectly proving the non-zero mass of these fleeting particles. Following the proposition of Raghavan of using ¹⁷⁶Yb as a target for low-energy solar neutrino spectroscopy, an intense R and D work has started a few years ago to define a suitable scintillator incorporating a large amount of ytterbium. Recently, the observation of UV scintillation in mixed yttrium/ytterbium aluminium garnets opened the field of investigation to a new class of scintillating crystals with interesting luminescence properties, very attractive not only for neutrino physics but also for radiation detection, in general. Their luminescence properties present some peculiarities that make them interesting by themselves

[en] Luminescence of Yb³⁺ from the charge-transfer state with broad emission bands and short radiative lifetimes (few to tens of nanoseconds depending on the host lattice and the temperature) is attractive for the development of fast scintillators capable of discriminating very short events. The most important currently considered application is that in solar neutrino (ν_e) real-time spectroscopy, since the ν_e capture by ¹⁷⁶Yb is followed by a specific emission signature which can accordingly excite the Yb³⁺ fluorescence. Studies on scintillation and luminescence in aluminium garnets containing Yb³⁺ have shown that these materials meet some of the required properties for such scintillators. In defining our priorities, the best compromise between host crystal, Yb³⁺ concentration, production method, post-growth treatment and performance is to be considered based on the studies of charge-transfer luminescence and quenching mechanisms. The experiments have been extended to a large number of compounds: YAG:Yb-YbAG, YGG:Yb-YbGG, YAP:Yb-YbAP, LaYbO₃ in the form of single crystals and/or powders. In garnets, the temperature-dependent fluorescence intensity and decay time under X-ray and VUV excitations decrease at low temperatures (T<100 K) and demonstrate the important role played by the traps. The thermoluminescence peaks show a strong dependence on the crystal history, composition and impurities introduced intentionally. The fluorescence intensity and decay time are also dependent on Yb³⁺ concentration and the presence of Yb²⁺. The results trace the major directions to optimised scintillators in terms of their efficiency and lifetime

[en] We report on a resonant magnetic scattering experiment using soft x-ray pulses generated from a free-electron laser (FEL). The free-electron laser was operated at a fundamental wavelength of 7.97 nm and radiation at the fifth harmonic originating from self-amplified stimulated emission at 1.59 nm with an average energy of 4 nJ per pulse was detected. We demonstrate the feasibility of resonant magnetic scattering at FEL sources by using a Co/Pd multilayer as prototype sample that was illuminated with 20-fs-long soft x-ray pulses tuned to the Co L₃ absorption edge at 778.1 eV (1.59 nm).

[en] FLASH, the Free-electron LASer in Hamburg, is a worldwide unique source for extremely bright ultra-short laser-like pulses tunable in a wide spectral range in the extreme ultraviolet and soft x-ray region (Ackermann et al 2007 Nat. Photonics 1 336-42). To fully exploit the features of this new generation of light sources, a user facility with efficient radiation transport to the experimental area and novel online photon diagnostics capable of characterizing the unique parameters of the FLASH radiation has been built. It serves a broad user community active in many scientific fields ranging from atomic and molecular physics to plasma and solid state physics as well as chemistry and biology. A special focus is placed on the exploitation of the ultra-short FLASH pulses using pump-probe techniques. Thus, the facility is equipped with optical and THz sources synchronized to FLASH. This paper gives a detailed overview of the FLASH user facility.

[en] Coherent diffractive imaging for the reconstruction of a two-dimensional (2D) finite crystal structure with a single pulse train of free-electron laser radiation at 7.97 nm wavelength is demonstrated. This measurement shows an advance on traditional coherent imaging techniques by applying it to a periodic structure. It is also significant that this approach paves the way for the imaging of the class of specimens which readily form 2D, but not three-dimensional crystals. We show that the structure is reconstructed to the detected resolution, given an adequate signal-to-noise ratio

[en] The dissociation pathways of HeH⁺ have been investigated below the first ionization continuum by photoabsorption at 32 nm, using fragment momentum imaging in a crossed-beams experiment at the free-electron laser in Hamburg (FLASH). Investigations were done both for ions with several vibrational levels excited in the ion source and for ions vibrationally cooled in an electrostatic ion trap prior to the irradiation. The product channels He⁺(1s)+H(nl) and He(1snl)+H⁺ were separated and the He(1snl)+H⁺ channel was particularly studied by coincidence detection of the He and H⁺ fragments on two separate fragment detectors. At 32 nm excitation, the branching ratio between the product channels was found to be σ_He⁺+H/σ_He+H⁺=0.96±0.11 for vibrationally hot and 1.70±0.48 for vibrationally cold ions. The spectra of kinetic energy releases for both channels revealed that photodissociation at 32 nm leads to high Rydberg states (n > or approx. 3-4) of the emerging atomic fragments irrespective of the initial vibrational excitation of HeH⁺. The fragment angular distributions showed that dissociation into the He+H⁺ channel mostly (∼70%) proceeds through ¹Π states, while for the He⁺+H channel ¹Σ and ¹Π states are of about equal importance.

[en] 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 1s²2s ²S_1/2-1s²2p ²P_1/2 in Fe²³⁺ at 48.6 eV, in Cu²⁶⁺ at 55.2 eV and 1s²2s ²S_1/2-1s²2p ²P_3/2 in Fe²³⁺ 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.

[en] The fragmentation of the hydronium cation H₃O⁺ after photoabsorption at 13.5 nm has been investigated with a crossed-photon-and-ion-beams experiment making pulsing and trapping techniques available for fragment momentum imaging at the intense Free-electron LASer in Hamburg. The observed photofragmentation patterns demonstrate that the photolysis of H₃O⁺ proceeds by valence ionization into H₃O²⁺ which subsequently fragments to mainly OH+2H⁺ and H₂O⁺+H⁺ with a branching ratio of up to 0.6:1 and with different degrees of excitation of the molecular fragment. The cross section for fragmentation into OH+2H⁺ is found to be (0.37±0.18)x10^-18 cm², while the total photoabsorption cross section is estimated to be greater than 0.95x10^-18 cm². The data suggest that ionization mainly occurs from the 3a₁ and 1e valence orbitals and that initial ionization from 3a₁ mainly leads to fragmentation into H₂O⁺(A ²A₁)+H⁺ while initial ionization from the 1e orbital predominantly populates the H₂O⁺(B ²B₂)+H⁺ and OH(X²Π)+2H⁺ channels. The results are of significance for astrophysical models of gas clouds in the vicinity of hot radiating objects and for models of the chemistry of planetary and lunar ionospheres under solar irradiation.

[en] Ytterbium doped borate crystals are promising laser media, e.g. in LaSc₃(BO₃)₄ (LSB) matrices large distance between ytterbium ions results in reduced concentration quenching of the ytterbium f-f luminescence [Petermann, K., Fagundes-Peters, D., Johansen, O., Mond, M., Peters, V., Romero, J.J., Kutovoi, S., Speiser, J., Giesen, A., 2005. Highly Yb-doped oxides for thin-disc lasers. J. Crystal Growth 275, 135-140]. Yb³⁺ ions in complex oxides in addition to the 4f → 4f transitions often manifest fast charge transfer luminescence (CTL) in the UV-visible range. In some borates it was not observed at all, like in orthoborates of Sc, Y and La [Van Pieterson, L., Heeroma, M., de Heer, E., Meijerink, A., 2000. Charge transfer luminescence of Yb³⁺. J. Lumin. 91, 177-193]; in haloborates Sr₂B₅O₉X, where X = Cl, Br, the UV/visible luminescence was attributed to ytterbium CTL though it looked substantially different from other matrices [Dotsenko, V.P., Berezovskaya, I.V., Pyrogenko, P.V., Efryushina, N.P., Rodniy, P.A., Eijk van, C.W.E., Sidorenko, A.V., 2002. Valence states and luminescence properties of ytterbium ions in strontium haloborates. J. Solid State Chem. 166, 271-276]; while in oxyborate Li₂Lu₅O₄(BO₃)₃ 'classical' CTL was observed [Jubera, V., Garcia, A., Chaminade, J.P., Guillen, F., Sablayrolles, Jean, Fouassier, C., 2007. Yb³⁺ and Yb³⁺-Eu³⁺ luminescent properties of the Li₂Lu₅O₄(BO₃)₃ phase. J. Lumin. 124(1), 10-14]. In this work the luminescence properties of another borate, namely LSB doped by Yb are presented

[en] Coherent x-ray imaging represents a new window to imaging non-crystalline, biological specimens at unprecedented resolutions. The advent of free-electron lasers (FEL) allows extremely high flux densities to be delivered to a specimen resulting in stronger scattered signal from these samples to be measured. In the best case scenario, the diffraction pattern is measured before the sample is destroyed by these intense pulses, as the processes involved in radiation damage may be substantially slower than the pulse duration. In this case, the scattered signal can be interpreted and reconstructed to yield a faithful image of the sample at a resolution beyond the conventional radiation damage limit. We employ coherent x-ray diffraction imaging (CXDI) using the free-electron LASer in Hamburg (FLASH) in a non-destructive regime to compare images of a biological sample reconstructed using different, single, femtosecond pulses of FEL radiation. Furthermore, for the first time, we demonstrate CXDI, in-line holography and Fourier transform holography (FTH) of the same unicellular marine organism using an FEL and present diffraction data collected using the third harmonic of FLASH, reaching into the water window. We provide quantitative results for the resolution of the CXDI images as a function of pulse intensity, and compare this with the resolutions achieved with in-line holography and FTH.