TXI‎ > ‎

Science Drivers

The Tender X-ray Imaging (TXI) instrument allows for unique opportunity to utilize both the tender X-ray photon energies and the unique X-ray pump/X-ray probe capacities of LCLS-II

Revealing Biological Function and Dynamics with X-ray Imaging
While crystallography is an extremely powerful tool for elucidating atomic structures, many complex biological machines defy crystallization due to their intrinsic flexibility. One of the original grand visions for X-ray FELs is the ability to image non-crystalline and non-reproducible structures with single femtosecond X-ray pulses. The underlying concept is that the X-ray pulses from FEL sources are so intense that the imaging process outruns the X-ray induced sample damage, while also having enough photons in the incident pulse to produce a measurable and interpretable diffraction pattern in a single shot. Two such imaging methods that can provide alternative paths towards understanding dynamics at low to medium resolution of non-crystalline samples are Fluctuation X-ray Scattering (FXS) and Single Particle Imaging (SPI). 
FXS is a method similar to Small Angle X-ray Scattering (SAXS) and Wide Angle X-ray Scattering (WAXS). The isotropic nature of these SAXS/WAXS diffraction patterns is a result of orientational averaging of the scattering species due to the fact that the X-ray exposure exceeds that of rotational diffusion. The advent of pulsed X-ray sources such as free-electron lasers allows one to reduce the exposure times below that of rotational diffusion such that the non-isotropic intensity fluctuations (or speckle) in the scattering pattern can be resolved. FXS enriches traditional SAXS/WAXS data by providing experimental access to intensity correlations that are directly related to molecular structure. Using iterative or model based phasing routines, one can obtain high-quality 3D representations of the electron density of the model under investigation.
SPI is a coherent diffraction imaging technique, that images isolated biological molecules, complexes, and other objects to near atomic resolution using intense X-ray pulese in a "diffraction-before-distruction" method. TXI has the potential to uniquely benefit single particle imaging for the investigation of biological function. Extrapolating today’s knowledge, there is evidence suggesting 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 
Another important capability of LCLS-II is high-repetition rate pulses in the 2-5 keV range, where absorption edges of many biologically relevant elements exist. For example, phasing close to the sulfur edge could provide a robust technique to phase many novel proteins at low resolution without the need for modifications or the use of other elements.

X-ray Pump & X-ray Probe Techniques
LCLS-II will be capable of producing high-quality pulses with two distinct colors. This will open the door to entirely new fields of nonlinear X-ray science and multi-dimensional X-ray spectroscopy. In general, multi-dimensional X-ray spectroscopy incorporates time-ordered sequences of X-ray pulses to generate a signal that is a function of multiple time delays and/or photon energies. These are nonlinear coherent wave mixing techniques in which X-ray pulses are used as both a pump, to prepare specific near-equilibrium states of matter, and as a probe of these evolving states. Where there are numerous variations of these multidimensional methods, they share in common the attempt to view coherent evolution, which ordinary linear spectroscopy can never see.  One example of such a multi-dimensional method is Stimulated X-ray Raman Spectroscopy (SXRS).  Whereas conventional optical Raman spectroscopy techniques exploit visible or infra-red laser fields to probe lower-frequency vibrational resonances in matter, SXRS uses X-rays to probe valence excitations in matter. One may consider SXRS as a powerful extension (stimulated version) of spontaneous X-ray Raman processes.
Additionally the two-color capability of LCLS-II could also give rise to many new class of experiments - for example fluorescence spectroscopy on shocked or ramp-compressed samples – where the hard X-ray beam would be used to determine lattice parameter, whilst the beam up to 5 keV could be used to generate L or K (for low Z) holes, and the resultant characteristic L or K shell emission lines, which in turn could provide information on the position and shape of the bands within the compressed sample.

Tender X-ray Spectroscopy
The novel access to the tender X-ray region at LCLS would also enable X-ray Absorption Spectroscopy (XAS) of Phosphor, Sulfur, and Calcium , all of them play important roles in many biological systems. The K-edge (at 4 keV) of Ca has a rich pre-edge structure that is very sensitive to the local structure of the binding site. One could follow time resolved changes in the Ca XAS to monitor changes in Ca ligation during an enzymatic reaction. Similarly, sulphur is often involved in metal ligation in active sites and following sulphur XAS (at around 2.47 keV) over a reaction cycle would elucidate the involvement of the sulphur ligands in the reaction mechanism. Here specificity against the large background of sulphur atoms present in the protein could be achieved by difference XANES, as the edge position and shape of sulphur changes over a wide energy range with change in the chemical speciation/oxidation state. Additionally TXI is suitable for studying compounds containing 4d & 5d transition metal elements such as Pd and Pt who's edges in the tender X-ray region.