Time-resolved X-ray diffraction (trXRD)
Since the discovery made by Max von Laue in 1912 of the diffraction of X-rays by crystals, the X-ray diffraction technique has played a strategic role in direct probing and in revealing the time-averaged static structure of matter. Because of these diffraction techniques, three-dimensional structures are now solvable with the atomic scale resolution in systems ranging from small molecules to crystals and from DNA and proteins to viruses and particles . Diffraction provides a way of easily understanding why the properties of diamond are so different from those of graphite in spite of the fact that both diamond and graphite consist of the same chemical element of carbon. This is because diamond and graphite possess different special distributions of elementary carbon atoms as shown in the figure above.
However, a full description of phase transformation, chemical reactions, or biological functions requires a real-time visualization of the actual steps characterizing a given process. The time scales for our areas of interest can range from femtoseconds for molecular and phase transformations through picoseconds to the millisecond scale for visco-elastic flow and dissipation in liquids and gasses. One of the great dreams in science is to be able to create an “atomic/molecular movie” that permits the recording of motions as the causal events of interest. The aim is to observe the whole process of transition from an initial state/configuration of matter through intermediate states to a final state/configuration during a chemical reaction, a phase transformation, or any other type of dynamical process. Therefore, trXRD, which is often called ultrafast X-ray diffraction (UXRD) in the literature, has become more and more prevalent in various scientific disciplines that involve the direct observation of atomic motion in real time.
Typical trXRD experiments are carried out in a pump-probe mode. This method, suitable for studying quantitatively reproducible dynamics, relies on creating a time-delayed sequence of two short pulses: a “pump” pulse excites the matter from the balance and a “probe” pulse of X-rays is used to create or take ‘snapshots’ of the matter structure at a particular time during a dynamical response of the matter. It is then possible to trace out the structural response of a system as a function of time by repeatedly applying these two pulses to the matter with different relative delays.
The ELI Beamlines facility will develop different types of secondary X-ray sources. The X-ray diffraction beam line will rely on an X-ray plasma source driven by the laser system L1 with high average pulse power up to 100 mJ, a 1 kHz repetition pulse rate, and pulse duration of about 100 fs. The X-ray energy of spectra lines is expected to be from 5 to 10 keV. The part of the X-rays that was originally illuminated into a whole 4π space will be collected using suitable optics. The X-rays that are collected will be focused on the sample situated in the middle of a goniometer. The expected intensity on the sample should be about 109 photons/s with a spot size of a few tens to a few hundreds µm2. Diffracted intensity will be detected by a two-dimensional detector working in a single photon counting mode. One great advantage of the fact that trXRD uses a laser-driven X-ray source will be near-perfect synchronization of the temporal overlap of the pump laser pulses and X-ray probe pulses (low-jitter).
Time-resolved X-ray spectroscopy
The ultrafast pump-probe X-ray Absorption and Fluorescence Spectroscopy will be used to capture the photo-initiated chemical dynamics. Using either the Plasma X-ray Source (PXS) or Betatron source, K- and L-edge spectroscopy will be realized by imaging into the sample X-rays with photon energies in the range of ten keV and hundreds of eV, respectively. The X-ray sources will emit, depending on the type and driving laser conditions, suitable broad bandwidth X-radiation from the soft to hard X-ray regime. The polychromatic X-ray emission will be captured and focused into the optically pumped interaction region of the sample using a fiber-optic X-ray lens. After transmission through the sample solution, the X-ray radiation will be spectrally dispersed and detected by an X-ray CCD camera or Dectris Eiger X pixel array detector. Owing to the ultrashort X-ray pulse length, measurements of structural dynamics will be possible with a temporal resolution of 300 fs. The liquid samples will be continuously pumped through a fused silica capillary with about 500 μm inner diameter, or through a microfluidic nozzle creating a flat sheet jet.
 A.H. Zewail. “The new age of structural dynamics,” Acta Crystallographica A 66, 135-136 (2010).
Štefan Michalik (now at Diamond Light Source)