X-ray Pump/Probe and X-ray Wave-mixing
This new instrumentation will offer a unique capability to combine X-rays from two independent FELs. This will open entirely new fields of nonlinear X-ray science and two-color X-ray pump and X-ray probe methods. For example, Fourier-transform inelastic X-ray scattering (FT-IXS) has been demonstrated at LCLS as a powerful method for probing collective mode dynamics (e.g. phonon coupling) in the excited state.
However, present experiments are limited to modes that are accessible by optical excitation (at q=0). First experiments in this area will employ element-specific soft X-ray excitation (e.g. O K-edge, or Cu L-edge) for example to probe charge collective modes of cuprate superconductors.
Coherent X-ray Imaging
Single particle imaging experiments at LCLS have produced single-shot coherent diffraction images of viruses, bacteriophages, organelles, and cyanobacteria to name a few. The latest results yield scattering information (significantly above background) out to 3.5 Å, and push the state-of-the-art for 3D image reconstruction to below 10 nm.
The optimum conditions for single shot particle imaging are the subject of much active research, and evidence suggests that the optimum region for single particle imaging is in the tender X-ray range (between 2 keV and 6 keV) which may represent the best compromise between scattering cross-section and resolution.
Sample heterogeneity combined with the low number of snapshots (at present low repetition rates) further complicates the assembly of complete data sets. Furthermore, this heterogeneity of objects (e.g.conformational heterogeneity in bio-molecules, or structural heterogeneity in nanocatalysis) is central to understanding how they function.
New instrumentation at LCLS-II will support a wide range of science in the tender X-ray range including: coherent X-ray diffractive imaging, small angle X-ray scattering (SAXS), fluctuation SAXS, and time-resolved SAXS. The high repetition rate will open new opportunities for characterizing heterogeneous ensembles of particles in operating (or near-native) environments.