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)