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)