Cryo X-ray microscope with flat sample geometry for correlative fluorescence and nanoscale tomographic imaging

a b s t r a c t X-ray imaging offers a new 3-D view into cells. With its ability to penetrate whole hydrated cells it is ideally suited for pairing fluorescence light microscopy and nanoscale X-ray tomography. In this paper, we describe the X-ray optical set-up and the design of the cryo full-field transmission X-ray microscope (TXM) at the electron storage ring BESSY II. Compared to previous TXM set-ups with zone plate condenser monochromator, the new X-ray optical layout employs an undulator source, a spherical grating monochromator and an elliptically shaped glass capillary mirror as condenser. This set-up improves the spectral resolution by an order of magnitude. Furthermore, the partially coherent object illumination improves the contrast transfer of the microscope compared to incoherent conditions. With the new TXM, cells grown on flat support grids can be tilted perpendicular to the optical axis without any geometrical restrictions by the previously required pinhole for the zone plate monochromator close to the sample plane. We also developed an incorporated fluorescence light microscope which permits to record fluorescence, bright field and DIC images of cryogenic cells inside the TXM. For TXM tomography, imaging with multi-keV X-rays is a straightforward approach to increase the depth of focus. Under these conditions phase contrast imaging is necessary. For soft X-rays with shrinking depth of focus towards 10 nm spatial resolution, thin optical sections through a thick specimen might be obtained by deconvolution X-ray microscopy. As alternative 3-D X-ray imaging techniques, the confocal cryo-STXM and the dual beam cryo-FIB/STXM with photoelectron detection are proposed

One-sign order parameter in iron based superconductor

The onset of superconductivity at the transition temperature is marked by the onset of order, which is characterized by an energy gap. Most models of the iron-based superconductors find a sign-changing (s±) order parameter [1–6], with the physical implication that pairing is driven by spin fluctuations. Recent work, however, has indicated that LiFeAs has a simple isotropic order parameter [7–9] and spin fluctuations are not necessary [7,10], contrary to the models [1–6]. The strength of the spin fluctuations has been controversial [11,12], meaning that the mechanism of superconductivity cannot as yet be determined. We report the momentum dependence of the superconducting energy gap, where we find an anisotropy that rules out coupling through spin fluctuations and the sign change. The results instead suggest that orbital fluctuations assisted by phonons [13,14] are the best explanation for superconductivity

Ion beam lithography for Fresnel zone plates in X-ray microscopy

Fresnel Zone Plates (FZP) are to date very successful focusing optics for X-rays. Established methods of fabrication are rather complex and based on electron beam lithography (EBL). Here, we show that ion beam lithography (IBL) may advantageously simplify their preparation. A FZP operable from the extreme UV to the limit of the hard X-ray was prepared and tested from 450 eV to 1500 eV. The trapezoidal profile of the FZP favorably activates its 2nd order focus. The FZP with an outermost zone width of 100 nm allows the visualization of features down to 61, 31 and 21 nm in the 1st, 2nd and 3rd order focus respectively. Measured efficiencies in the 1st and 2nd order of diffraction reach the theoretical predictions

Experience with short-period, small gap undulators at the SwissFEL aramis beamline

The SwissFEL Aramis beamline provides hard X-ray FEL radiation down to 1 Angström with 5.8 GeV and short period, 15mm, in-vacuum undulators (U15). To reach the maximum designed K-value of 1.8 the U15s have to be operated with vacuum gaps down to 3.0 mm. The thirteen-undulator modules are 4m long and each of them is equipped with a pair of permanent magnet quadrupoles at the two ends, aligned magnetically to the undulator axis. Optical systems and dedicated photon diagnostics are used to check the alignment and improve the K-value calibration. In this talk the main steps of the undulator commissioning will be recalled and a systematic comparison between the magnetic results and the electron and photon based measurements will be reported to highlight achievements and open issues.peer-reviewe

a versatile optical pump–soft X-ray probe facility with 100 fs X-ray pulses of variable polarization

Here the major upgrades of the femtoslicing facility at BESSY II (Khan et al., 2006) are reviewed, giving a tutorial on how elliptical-polarized ultrashort soft X-ray pulses from electron storage rings are generated at high repetition rates. Employing a 6 kHz femtosecond-laser system consisting of two amplifiers that are seeded by one Ti:Sa oscillator, the total average flux of photons of 100 fs duration (FWHM) has been increased by a factor of 120 to up to 106 photons s-1 (0.1% bandwidth)-1 on the sample in the range from 250 to 1400 eV. Thanks to a new beamline design, a factor of 20 enhanced flux and improvements of the stability together with the top-up mode of the accelerator have been achieved. The previously unavoidable problem of increased picosecond- background at higher repetition rates, caused by `halo' photons, has also been solved by hopping between different `camshaft' bunches in a dedicated fill pattern (`3+1 camshaft fill') of the storage ring. In addition to an increased X-ray performance at variable (linear and elliptical) polarization, the sample excitation in pump-probe experiments has been considerably extended using an optical parametric amplifier that supports the range from the near-UV to the far-IR regime. Dedicated endstations covering ultrafast magnetism experiments based on time-resolved X-ray circular dichroism have been either upgraded or, in the case of time-resolved resonant soft X-ray diffraction and reflection, newly constructed and adapted to femtoslicing requirements. Experiments at low temperatures down to 6 K and magnetic fields up to 0.5 T are supported. The FemtoSpeX facility is now operated as a 24 h user facility enabling a new class of experiments in ultrafast magnetism and in the field of transient phenomena and phase transitions in solids

Design of soft x-ray gratings for free electron lasers: from specification to characterization

The European XFEL is a large facility under construction in Hamburg, Germany. It will provide a transversally fully coherent X-ray radiation with outstanding characteristics: high repetition rate (up to 2700 pulses with a 0.6 milliseconds long pulse train at 10Hz), short wavelength (down to 0.05 nm), short pulse (in the femtoseconds scale) and high average brilliance (1.6•1025 photons / s / mm2 / mrad2/ 0.1% bandwidth). Due to the very short wavelength and very high pulse energy, mirrors have to present high quality surface, to be very long, and at the same time to implement an effective cooling system. Matching these tight specifications and assessing them with high precision optical measurements is very challenging. One of the three foreseen beamlines operates in the soft X-ray range and it is equipped with a diffractive monochromator. The monochromator is using a variable line spacing grating that covers the wavelength range from 4.6nm to 0.41 nm (energies from 270eV to 3000eV). The grating profile is blazed, and due to the small angle and relatively few lines/mm, it is also very challenging to realize and to be characterized. In this contribution we discuss about the requirements of the optics involved in the soft X-ray monochromator. We describe mirror and grating specifications, and the tests that could be carried out during and after the manufacturing in order to ensure the specifications match

A soft x ray fluorescence spectrometer at BESSY II

A new spectrometer for resonant inelastic x ray scattering experiments has been designed and is currently built at the synchrotron light facility BESSY II. The spectrometer with a total length of 3 m covers the photon energy range from 50 eV to 1000 eV with emphasis on the lower photon energy range. It is designed as plane grating spectrometer with two paraboloidal mirrors for collimation and focusing and a plane grating in between. Two rotational degrees of freedom allow for a variation of the incidence and deflection angles at the grating. The geometrical acceptance of the spectrometer approaches a solid angle of 35 mrad 40 mrad2 at low photon energies with a resolution better than 10 meV at 100 eV. A rotatable delay line detector is used to detect the fluorescence light in the line focu

Methodology for the structural characterisation of VxOy species supported on silica under reaction conditions by means of in situ O K-edge X-ray absorption spectroscopy

A methodology and its application to silica supported vanadia are presented that allows for a detailed analysis of structural peculiarities of the vanadia species on the surface. We introduce in situ oxygen K-edge X-ray absorption spectroscopy (XAS) based on the Auger electron yield (AEY) technique as a characterisation tool to assist vibrational spectroscopy that has been applied extensively in earlier work on this system. The analysis of the O K-near edge X-ray absorption fine structure (NEXAFS) allows a clear distinction between separate vanadia, silica and interface contribution in contrast to vibrational spectroscopy with strongly overlapping contributions due to vibrational coupling. Differently coordinated oxygen can be identified in the O K-NEXAFS spectrum by comparison with theoretical spectra obtained by state of the art density-functional theory (DFT) calculation. A study of catalysts with different V loadings (0 wt% V, 2.7 wt% V, and 10.8 wt% V) shows that the contributions of silica to the NEXAFS appear in an energy region well separated from the spectral signature of oxygen bound to vanadium. Dehydrated catalysts with high (10.8 wt% V) and low (2.7 wt% V) vanadium loading on silica SBA-15 show identical NEXAFS under in situ conditions. This finding indicates that independent of the loading a distribution of vanadia species with a very similar molecular structure, including non-monomeric as well as possibly monomeric configurations and different from crystalline V2O5, is present on this model catalyst