The experimental set-up

Contact: christopher.milne@xfel.eu

 

FXE is a versatile instrument for performing femto­second pump-probe experiments on liquids and solids, combining X-ray spectroscopic and scattering techniques. Experiments on solid samples can be performed on goniometers in air and in helium envi­ronments. A vacuum chamber is also available and enables grazing-incidence (GI) as well as transmis­sion-mode wide-angle X-ray scattering (WAXS) ex­periments, optionally under cryogenic conditions at >100 K. Under such cryogenic conditions, amor­phous solid water (ASW) can be grown in-situ and measured by time resolved GI-WAXS (see Figure 2).

 

Layout of the FXE scientific instrument around the sam­ple environment area, showing the locations of the spec­trometers and the Large Pixel Detector (LPD) in the for­ward scattering direction.

 

FXE specializes in time-resolved optical pump / X-ray probe measurements and provides a broad range of options for optical excitation of samples with femto­second pulses precisely synchronized to the X-ray pulses. The almost co-linear configuration of the two beams at a crossing angle of 1° allows FXE to rou­tinely resolve laser-induced processes with 100 fs (fwhm) time resolution. The TOPAS system for laser wavelength conversion is capable of producing ~50 fs long (fwhm) pulses in the wavelengths range from 240 to 2500 nm. In its standard configuration, the setups at FXE are compatible with a very wide range of X-ray photon energies from below 5 to 20 keV. The ability to reach high photon energies is particularly beneficial for improving real-space reso­lution in scattering experiments. Moreover, the pri­mary X-ray detector for forward scattering, the Large Pixel Detector (LPD), allows us to take full advantage of both the MHz pulse repetition rate and the high photon energy of X-rays. The MHz frame rate of the LPD is complemented by the parallel multiple-gain detec­tion technology, which provides a dynamic range of 104 photons/pixel/pulse at 12 keV. Furthermore, a Jungfrau 1M detector is available for diffraction ex­periments. It is mounted on a robot arm above the sample environment and enables the detection of dif­fracted X-rays in most of the upper hemisphere. All scattering setups are also compatible with single-shot techniques that exploit the ~1012 X-ray photons per femtosecond pulse to probe the transient states in processes that cannot be induced repetitively. In addition to X-ray scattering measurements, the set­ups are mostly compatible with simultaneous pump-probe X-ray spectroscopy. This technique can pro­vide site-specific electronic information as a comple­mentary probe.

Key parameters for the FXE instrument:

  • Combined X-ray scattering and spectroscopy
  • Capability of X-ray photon energies from 5 to 20 keV
  • Various options for optical and THz-excitation
  • Temporal resolution in pump-probe experiments >70 fs
  • Large Pixel Detector is capable of MHz frame rate and dynamic range of 10e4 photons/pixel/pulse at 12 keV
  • Various sample environments available for user experiments (liquid jet, goniometer, vacuum chamber, …)

 

The cryo-cooled sample plate in the vacuum chamber, mounted in grazing-incidence geometry, with the water vapour delivery nozzle normal to the plate. The scattering signals shown and labelled as measured on the LPD.

 

[1] A. Galler et al., J. Synch. Rad., 26, 1432 (2019)
[2] D. Khakhulin et al., Appl. Sci., 10(3), 995 (2020)

 

Homepage of the FXE instrument