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X-ray free-electron lasers are unique sources of high-brightness coherent radiation. However, existing devices supply only linearly polarized light, precluding studies of chiral dynamics. A device called the Delta undulator has been installed at the Linac Coherent Light Source (LCLS) to provide tunable polarization. With a reverse tapered planar undulator line to pre-microbunch the beam and the novel technique of beam diverting, hundreds of microjoules of circularly polarized X-ray pulses are produced at 500–1,200 eV. These X-ray pulses are tens of femtoseconds long, have a degree of circular polarization of 0.98 –0.04 +0.02 at 707 eV and may be scanned in energy. We also present a new two-colour X-ray pump–X-ray probe operating mode for the LCLS. Energy differences of Δ E / E  = 2.4% are supported, and the second pulse can be adjusted to any elliptical polarization. In this mode, the pointing, timing, intensity and wavelength of the two pulses can be modified. Tunable polarization control and a two-colour X-ray pump–X-ray probe operating mode are demonstrated at the Linac Coherent Light Source (LCLS).

The two single-pass, externally seeded free-electron lasers (FELs) of the FERMI user facility are designed around Apple-II-type undulators that can operate at arbitrary polarization in the vacuum ultraviolet-to-soft x-ray spectral range. Furthermore, within each FEL tuning range, any output wavelength and polarization can be set in less than a minute of routine operations. We report the first demonstration of the full output polarization capabilities of FERMI FEL-1 in a campaign of experiments where the wavelength and nominal polarization are set to a series of representative values, and the polarization of the emitted intense pulses is thoroughly characterized by three independent instruments and methods, expressly developed for the task. The measured radiation polarization is consistently >90% and is not significantly spoiled by the transport optics; differing, relative transport losses for horizontal and vertical polarization become more prominent at longer wavelengths and lead to a non-negligible ellipticity for an originally circularly polarized state. The results from the different polarimeter setups validate each other, allow a cross-calibration of the instruments, and constitute a benchmark for user experiments.

Free electron lasers emit powerful and coherent radiation in a wide wavelength range extending to hard x-rays. This radiation is also characterized by a high degree of polarization that is generally linear and depends on the undulator properties. The possibility of controlling the polarization state of the radiation is an important option for free electron lasers that is critical for a large class of experiments. Such control can be achieved using variable polarization undulators or alternatively via the crossed polarized undulator scheme. We report the results of an extensive study for the characterization of the crossed-polarized undulator scheme in a number of different configurations. A simple model, based on Gaussian mode beam propagation, is presented and used to reproduce the experimental results obtained at the seeded free electron laser FERMI. A good agreement is found between the model and the experiment allowing us to understand the impact of the wavefront properties of the radiation coming from the consecutive undulators on the output radiation. The model is used not only for characterizing the control of the polarization but also for the control of the transverse mode.

The control of polarization state in soft and hard X-ray light is of crucial interest to probe structural and symmetry properties of matter. Thanks to their Apple-II type undulators, the FERMI-Free Electron Lasers are able to provide elliptical, circular or linearly polarized light within the extreme ultraviolet and soft X-ray range. In this paper, we report the characterization of the polarization state of FERMI FEL-2 down to 5 nm. The results show a high degree of polarization of the FEL pulses, typically above 95%. The campaign of measurements was performed at the Low Density Matter beamline using an electron Time-Of-Flight based polarimeter.

NRC publication: Yes

Coherent two-center photoelectron emission has been predicted for all electron subshells. So far, however, is has been only proven for the innermost 1s-subshell only. We present first results for the valence photoionization of N2 and O2 showing oscillatory behavior in both, the partial cross section σ and the angular distribution asymmetry parameter β.

The qualitative and quantitative investigations of historical iron-gall inks by use of μ-XRF spectrometry is a common method for analyzing the differences in their composition. When a fingerprint is established, it is possible to characterize the distinguishable inks used in the production of medieval manuscripts, and, in turn assist in the reconstruction of the manuscript’s history. The characterization of the primary inks in palimpsests, i.e. manuscripts written on re-used parchment, presents a serious problem as the effect of the partial removal on the inks fingerprint is yet known. In this paper we explore our preliminary results concerning the effects of two removal techniques on the fingerprint of iron-gall inks on parchment.

Coherent two-center photoelectron emission has been predicted for all electron subshells. So far, however, is has been only proven for the innermost 1s-subshell only. We present first results for the valence photoionization of N sub(2) and O sub(2) showing oscillatory behavior in both, the partial cross section [sigma] and the angular distribution asymmetry parameter beta .