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 in liquids and solutions are performed in a specially designed chamber that sup­ports sample delivery using fast liquid jets with typical thickness of 50-100 µm and flow speeds enabling full sample refresh in the interaction volume at sub-MHz repetition rates. The chamber is constantly flushed with helium gas which, in combination with slits up­stream of the sample position, helps to reduce the background X-ray scatter when performing liquid scattering experiments. FXE specializes in time-re­solved optical pump / X-ray probe measurements, and provides a broad range of available options for optical excitation of samples with femtosecond pulses precisely synchronized in time to the arrival of the X-ray pulses. This allows FXE to routinely meas­ure dynamics in solution samples with 70-100 fs (fwhm) time resolution, depending on the jet thick­ness. The TOPAS system for laser wavelength con­version is capable of producing ~50 fs long (fwhm) NIR pulses near the water O-H stretching and bend­ing overtones, e.g. 1470 nm and 2000 nm. In its standard configuration, the setup at FXE is compati­ble with a very wide range of X-ray photon energies from 5 to 20 keV. The ability to reach high photon energies is particularly beneficial for improving spa­tial 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. In addition to X-ray scattering measurements, the setup is also compati­ble with simultaneous pump-probe hard X-ray non-resonant X-ray emission spectroscopy. This tech­nique can provide local electronic information on transition metal complexes as a complementary probe for solvated species. The capability of FXE to perform femtosecond pump-probe experiments on solution samples in a dedicated sample environment has proven very attractive to the European XFEL user community and is routinely used at the instru­ment for chemistry, biochemistry and material sci­ence research.

 

Key parameters for the FXE instrument:

  • Combined X-ray scattering and spectroscopy in liquids
  • Capability of X-ray photon energies up to 20 keV
  • Various options for optical excitation inclosing NIR fs pulses
  • Low background in forward scattering due to specialized sample environment
  • Temporal resolution in pump-probe experiments down to 70-100 fs depending on the jet thickness
  • Large Pixel Detector is capable of MHz frame rate and dynamic range of 104 photons/pixel/pulse at 12 keV

 

Configuration 1 of the FXE scientific instrument.

 

Configuration 2 of the FXE scientific instrument

 

[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