The time-resolved hard X-ray diffraction endstation KMC-3 XPP at BESSY†II.

The time-resolved hard X-ray diffraction endstation KMC-3 XPP for optical pump/X-ray probe experiments at the electron storage ring BESSY II is dedicated to investigating the structural response of thin film samples and heterostructures after their excitation with ultrashort laser pulses and/or electric field pulses. It enables experiments with access to symmetric and asymmetric Bragg reflections via a four-circle diffractometer and it is possible to keep the sample in high vacuum and vary the sample temperature between ∼15 K and 350 K. The femtosecond laser system permanently installed at the beamline allows for optical excitation of the sample at 1028 nm. A non-linear optical setup enables the sample excitation also at 514 nm and 343 nm. A time-resolution of 17 ps is achieved with the `low-α' operation mode of the storage ring and an electronic variation of the delay between optical pump and hard X-ray probe pulse conveniently accesses picosecond to microsecond timescales. Direct time-resolved detection of the diffracted hard X-ray synchrotron pulses use a gated area pixel detector or a fast point detector in single photon counting mode. The range of experiments that are reliably conducted at the endstation and that detect structural dynamics of samples excited by laser pulses or electric fields are presented.

Single crystals of ferroelectric lithium niobate-tantalate LiNb1-xTaxO3 solid solutions for high-temperature sensor and actuator applications.

Ferroelectric LiNb1-xTaxO3 solid solutions with various Nb/Ta ratio were grown from the melt by the Czochralski method. The exact composition of the grown crystals was determined by inductively coupled plasma atomic mass spectrometry. The dependence of the crystal composition on the composition of the initial melt was obtained and explained by a wide separation between the phase boundaries of the liquid and solid phases on the LiNbO3-LiTaO3 phase diagram. Using high-resolution X-ray diffraction, the parameters a and c of a crystal unit cell were determined (LiNb0.88Ta0.12O3: a = 5.1574 Šand c = 13.8498 Å). Further, the Curie temperature TC of the crystals was measured using the differential scanning calorimetry technique. TC was found to depend on the composition of the crystals that allowed conditions for the monodomainization of the grown crystals to be defined (LiNb0.88Ta0.12O3: TC = 1102°C; LiNb0.33Ta0.67O3: TC = 794°C). Finally, the velocity of surface acoustic waves was determined by scanning electron microscopy and X-ray diffraction techniques (YZ-cut of a LiNb0.88Ta0.12O3 crystal: V = 3440 m s-1).

Non-uniform nanosecond gate-delay of hybrid pixel detectors.

A simple experiment to characterize the gating properties of X-ray area detectors using pulsed X-ray sources is presented. For a number of time-resolved experiments the gating uniformity of area detectors is important. Relative gating delays between individual modules and readout chips of PILATUS2 series area X-ray detectors have been observed. For three modules of a PILATUS 300K-W unit the maximum gating offset between the modules is found to be as large as 30 ns. On average, the first photosensor module is found to be triggered 15 ns and 30 ns later than the second and the third modules, respectively.

Observation of sagittal X-ray diffraction by surface acoustic waves in Bragg geometry.

X-ray Bragg diffraction in sagittal geometry on a Y-cut langasite crystal (La3Ga5SiO14) modulated by Λ = 3 µm Rayleigh surface acoustic waves was studied at the BESSY II synchrotron radiation facility. Owing to the crystal lattice modulation by the surface acoustic wave diffraction, satellites appear. Their intensity and angular separation depend on the amplitude and wavelength of the ultrasonic superlattice. Experimental results are compared with the corresponding theoretical model that exploits the kinematical diffraction theory. This experiment shows that the propagation of the surface acoustic waves creates a dynamical diffraction grating on the crystal surface, and this can be used for space-time modulation of an X-ray beam.

Optimized spatial overlap in optical pump-X-ray probe experiments with high repetition rate using laser-induced surface distortions.

Ultrafast X-ray diffraction experiments require careful adjustment of the spatial overlap between the optical excitation and the X-ray probe pulse. This is especially challenging at high laser repetition rates. Sample distortions caused by the large heat load on the sample and the relatively low optical energy per pulse lead to only tiny signal changes. In consequence, this results in small footprints of the optical excitation on the sample, which turns the adjustment of the overlap difficult. Here a method for reliable overlap adjustment based on reciprocal space mapping of a laser excited thin film is presented.

Ultrafast switching of hard X-rays.

A new concept for shortening hard X-ray pulses emitted from a third-generation synchrotron source down to few picoseconds is presented. The device, called the PicoSwitch, exploits the dynamics of coherent acoustic phonons in a photo-excited thin film. A characterization of the structure demonstrates switching times of ≤ 5 ps and a peak reflectivity of ∼10(-3). The device is tested in a real synchrotron-based pump-probe experiment and reveals features of coherent phonon propagation in a second thin film sample, thus demonstrating the potential to significantly improve the temporal resolution at existing synchrotron facilities.

Ultrafast lattice response of photoexcited thin films studied by X-ray diffraction.

Using ultrafast X-ray diffraction, we study the coherent picosecond lattice dynamics of photoexcited thin films in the two limiting cases, where the photoinduced stress profile decays on a length scale larger and smaller than the film thickness. We solve a unifying analytical model of the strain propagation for acoustic impedance-matched opaque films on a semi-infinite transparent substrate, showing that the lattice dynamics essentially depend on two parameters: One for the spatial profile and one for the amplitude of the strain. We illustrate the results by comparison with high-quality ultrafast X-ray diffraction data of SrRuO3 films on SrTiO3 substrates.

In situ x-ray reflectivity and grazing incidence x-ray diffraction study of L 1(0) ordering in (57)Fe/Pt multilayers.

In situ high temperature x-ray reflectivity and grazing incidence x-ray diffraction measurements in the energy dispersive mode are used to study the ordered face-centered tetragonal (fct) L 1(0) phase formation in [Fe(19 Å)/Pt(25 Å)]( × 10) multilayers prepared by ion beam sputtering. With the in situ x-ray measurements it is observed that (i) the multilayer structure first transforms to a disordered FePt and subsequently to an ordered fct L 1(0) phase, (ii) the ordered fct L 1(0) FePt peaks start to appear at 320 °C annealing, (iii) the activation energy of the interdiffusion is 0.8 eV and (iv) ordered fct FePt grains have preferential out-of-plane texture. The magneto-optical Kerr effect and conversion electron Mössbauer spectroscopies are used to study the magnetic properties of the as-deposited and 400 °C annealed multilayers. The magnetic data for the 400 °C annealed sample indicate that the magnetization is at an angle of ∼50° from the plane of the film.

Application of a pnCCD in X-ray diffraction: a three-dimensional X-ray detector.

The first application of a pnCCD detector for X-ray scattering experiments using white synchrotron radiation at BESSY II is presented. A Cd arachidate multilayer was investigated in reflection geometry within the energy range 7 keV < E < 35 keV. At fixed angle of incidence the two-dimensional diffraction pattern containing several multilayer Bragg peaks and respective diffuse-resonant Bragg sheets were observed. Since every pixel of the detector is able to determine the energy of every incoming photon with a resolution DeltaE/E approximately 10(-2), a three-dimensional dataset is finally obtained. In order to achieve this energy resolution the detector was operated in the so-called single-photon-counting mode. A full dataset was evaluated taking into account all photons recorded within 10(5) detector frames at a readout rate of 200 Hz. By representing the data in reciprocal-space coordinates, it becomes obvious that this experiment with the pnCCD detector provides the same information as that obtained by combining a large number of monochromatic scattering experiments using conventional area detectors.

A monolithic Fresnel bimirror for hard X-rays and its application for coherence measurements.

Experiments using a simple X-ray interferometer to measure the degree of spatial coherence of hard X-rays are reported. A monolithic Fresnel bimirror is used at small incidence angles to investigate synchrotron radiation in the energy interval 5-50 keV with monochromatic and white beam. The experimental set-up was equivalent to a Young's double-slit experiment for hard X-rays with slit dimensions in the micrometre range. From the high-contrast interference pattern the degree of coherence was determined.

Pinhole interferometry with coherent hard X-rays.

This paper discusses the experimental realisation of two types of X-ray interferometer based on pinhole diffraction. In both interferometers the beam splitter was a thin metal foil containing micrometer pinholes to divide the incident X-ray wave into two coherent waves. The interference pattern was studied using an energy-dispersive detector to simultaneously investigate in a large spectral range the diffraction properties of the white synchrotron radiation. For a highly absorbing pinhole mask the interference fringes from the classical Young's double-pinhole experiment were recorded and the degree of coherence of X-rays could be determined. In the case of low absorption of the metal foil at higher X-ray energies (>15 keV) the interference pattern of a point diffraction interferometer was observed using the same set-up. The spectral refraction index of the metal foil was determined.