Experimental station Bernina

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Femtosecond X-Ray Pump-Probe Diffraction and Scattering Electronic and Magnetic Ordered Crystalline Materials


Experimental Station B (ESB) is designed for femtosecond pump-probe experiments in condensed matter and material science, employing photon-in photon-out techniques in the energy range 4.5 – 12.4 keV [1,2]. It includes a femtosecond optical laser system to generate a variety of pump beams (from the UV up to the THz range), a X-ray optical scheme to tailor the X-ray probe beam, shot-to-shot diagnostics to monitor the X-ray intensity and arrival time, and two endstations operated at a single focus position with spot size of 2-200 μm:


  1. ) The XPP-XRD station (see Fig 1.) is dedicated to X-ray pump-probe (XPP) resonant and non-resonant X-ray diffraction (XRD) experiments [3-5]. It consists of a heavy load six-circle Kappa-diffractometer with dual-detector arm carrying a 4M 2D pixel detector (Jungfrau) [6] and a polarization analyzer stage with point detector, respectively. Experiments for samples under a variety of environmental conditions are done by replacing the in-air Kappa-goniometer by custom built sample environmental modules.
  2. ) The XPP-GPS station (see Fig 1.) is a general purpose station for XPP experiments in non-scanning mode. It consists of a heavy load sample goniometer and a robot detector arm carrying a 16M 2D pixel detector (Jungfrau) [7]. By swapping stages, a variety of sample environmental modules can be accommodated. The robot detector arm is mounted to the ceiling and can be retracted up to 3 m downstream (3.7 m from the focal position) to enable coherent diffraction and SAXS experiments.

Until endstation ESC becomes operational, fixed target protein crystallography will be offered at XPP-GPS [8]. A module is being developed for serial femtosecond crystallography at 100 Hz of 3-D micro-crystals of size < 5 μm in a cryo/in-air or He-environment.


Pump options

For pump-probe experiments several pumping options (UV/NIR/FIR) are available. Special emphasis is given to provide intense pulses in the midIR/THz range a very important wavelength range since it permits the selective control of material properties by addressing low energy excitations such as electro-magnons [4], spin waves [9] or phonons [10]. An optical parametric amplifier (OPA) with subsequent difference frequency generation covers the spectral range from 1100 to >15000 nm. The same output is used to generate intense THz pulses in organic crystals with field strengths exceeding 1 MV/cm [11] and pulse energy up to 10 μJ in the frequency range 1-10 THz. Very short pulses (< 10 fs) are available at 800 nm by pulse compression in a hollow core fibre [12]. To compensate for drifts in the amplifier system, a laser arrival time monitor (LAM) will be installed directly after the compressor. A variety of laser diagnostics will provide all relevant laser parameters for the user.


X-ray timing diagnostic

The X-ray timing diagnostic setup is installed in the ESB hutch upstream of the diamond/Si solid state attenuator (SSA). Two different techniques are used to measure the X-ray arrival time, the spectral encoding based on optical gating [13] and the photon arrival and length monitor (PALM) based on THz/IR streaking [14]. For the latter the accuracy is 0.5-5 fs rms depending on the X-ray wavelength and laser jitter. The intense THz radiation is generated by the tilted pulse front method in LiNbO3 [15]. The third method with less timing accuracy employs the X-ray arrival time derived from the electron beam arrival monitor (BAM) installed at the end of the ARAMIS undulator.


X-ray Optics

The layout of the X-ray optics [16] is shown in the figure below. For pink beam a pair of bendable plane elliptical mirrors (offset mirrors) installed in the optics hutch shift the beam vertically by vertically by 20 mm. At working distance 2.5 m upstream of the endstation, a pair of bendable deflecting (8 & 12 mrad) KB-mirrors (coating Mo/B4C) provide achromatic focusing with horizontal and vertical spot size of <2 - 200 μm FWHM.
For monochromatic beam the offset mirrors are retracted. In this case the double crystal monochromator (DCM) is the first optical element in the beam. A motorized horizontal translation allows to switch between Si(111), Si(311) and Si(511) crystal pairs. The DCM has a variable offset (20 - 32 mm) to optionally operate high harmonic rejection mirrors (HHRM) with variable deflection angles in a 4-bounce scheme. The beam is kept constant at height 1420 mm as for the pink beam. A HHR of <10−4 is achieved with attenuation 100 of the fundamental. There is the option to install diamond X-ray phase retarders (XPR) in the OH hutch downstream of DCM-HHRM. Operated in Bragg/Laue transmission geometry such XPR can provide flexible linear and circular X-rays with high degree of polarization [17,18].


Experimental Hutch

Experimental Station B (ESB) is designed as a pump-probe experimental station. It combines time-resolved laser spectroscopy methods and X-ray scattering techniques to study the dynamics of cooperative interactions in crystalline materials that exhibit long-range electronic and magnetic order. The experiments will be carried out in a pump-probe mode, with an atomic resolution on the timescale of a millionth part of a billionth of a second.

X-ray pump-probe (XPP) experiment

An X-ray pump-probe (XPP) scattering experiment works as depicted in the figure below. A laser- or THz-pump-pulse excites a crystalline material, for example a CMR manganite, which exhibits long-range lattice-, charge-, orbital- and spin-order. The response of the material is measured by photon-in/photon-out scattering of a X-ray probe-pulse focussed onto the same spot. Before reaching the sample, the pump pulse bounces of a movable mirror pair (not shown) that provides a time delay between the excitation of the pump and the arrival of the probe. A 2D pixel detector records the transient, pump-induced fractional change in scattered intensity I0 (normalized to the incoming X-ray intensity I0)as a function of the time delay from the moment of pump excitation. The data are read out from the detector shot-by-shot and are written onto disk.

References

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