SXR Overview

SXR Full Name

Soft X-ray Research Instrument for Materials Science

Description of the SXR Instrument

The soft X-ray imaging and pump-probe X-ray spectroscopy program on materials was approved by the LCLS Scientific Advisory Committee (SAC) in 2006, and space was allocated in the LCLS near hall for the accompanying instruments. A consortium was formed in order to fund, design and construct the SXR beamline with members from the Stanford Institute of Material and Energy Sciences (SIMES), the Advanced Light Source (ALS), the University of Hamburg, DESY and the Center for Free Electron Lasers (CFEL) in Hamburg. Operated by the LCLS facility the SXR instrument took first light on May 5, 2010. Initially consortium members and various collaborators brought different endstations and detectors to the SXR instrument providing access to general users via collaborations. Moving forward, the strategic development of end stations by LCLS will provide open access to users in the highest impact scientific areas.

Scientific Program

Pump-Probe Ultrafast Chemistry

The ultimate goal in chemistry and physical chemistry is to understand on a fundamental level how bonds break and reform during chemical reactions. In many cases we arrive at simple pictures of electron motion with respect to electron pair redistributions or electrostatic interactions along a reaction path. For many systems bonding can be understood in terms of molecular orbitals and reactivity in dynamical rearrangements of different molecular states. Such knowledge provides the basis for the understanding of chemical trends and prediction of chemical reactivity for chemical compounds. Since the excitation and probe steps with conventional optical lasers involve valence electrons that are delocalized over many atomic centers it is difficult to study complex systems. Unprecedented insight into chemical reaction dynamics are gained by probing exactly the atomic site involved. X-ray spectroscopies can directly access molecular orbital changes associated with or even during chemical reactions. In particular, accessing core levels in the soft X-ray regime with spectroscopy opens up new prospects to study time-resolved changes in the electronic structure of complex systems containing the essential elements C, O and N or 3D-metal atoms. Detailed insight into surface reactions, catalysis, hydrogen-bonded systems and aqueous solutions can be extracted.

X-ray spectroscopy has the unique ability to provide an atom-specific probe of the electronic structure. In X-ray emission spectroscopy (XES) the atomic or elemental sensitivity arises from the filling of a core hole by valence electrons from the same atomic site. In addition, core-level energy shifts (often denoted chemical shifts) connected with different environments allow for selective probing of chemically non-equivalent atoms (Figure 0-1). The final state of the X-ray emission process is a valence-hole state similar to the final state in valence band photoemission with the unique feature that the valence electronic structure is projected onto a specific atom. Notably, selection rules of XES, and similarly of X-ray photoelectron spectroscopy (XPS), in conjunction with variation of polarization vector of the incident light or angle-resolved detection of electrons allows to access molecular orbital symmetry and associated bond geometry. In addition, XPS can be uniquely tuned to high surface sensitivity, which is particularly desirable when studying interfaces, including the aqueous/vacuum interface. Resonant excitation and Auger electron spectroscopy gives unique access to the electronic structure of atoms and molecules in the gas phase, on surfaces and in liquids and solids.

Strongly Correlated Materials

Soft X-ray scattering experiments on strongly correlated materials probe the charge ordering and elementary charge/magnetic excitations to understand the ground state properties. Recent theoretical developments show that resonance X-ray scattering can provide rich information on many-body wavefunctions. Soft X-rays, being sensitive to valence electrons and in the spectral range of important L edges of transition metals that often are key elements of correlated materials, provide special opportunities for discovery. Using the high pulse intensity and ultra-short X-ray pulse length of LCLS, it is possible to perform optical-pump-and-X-ray-probe experiments to study how the electronic states of strongly-correlated materials relax from an excited state to the ground state. The relaxation process is closely related to the correlation effect among the electrons and the electronic interactions to other degrees of freedom; therefore 'snapshots' obtained from the pump-probe experiments provide important clues to construct a microscopic physics picture of the strongly-correlated systems. In addition, X-ray probe experiment also has some unique advantages, such as element specific information, bulk sensitive signal, and the dynamic structure factors, which are not accessible by the most common ultrafast optical pump-probe experiments in the visible light regime.​

Magnetic Scattering and Imaging

One of the important topics of physics is the study of phase transitions, where structural, electronic, or magnetic properties undergo discontinuous or continuous changes. Powerful symmetry and statistical concepts have been developed to describe such phase transitions, and corresponding scattering and thermodynamic measurements have been used for experimental verifications. Such approaches are quite successful and satisfactory to many. Complementary and equally powerful, the conceptualization of a simple physical picture in the real space has played an important role in the formulation of physical understanding of many phenomena. This method of direct representation, however, has not received equal recognition perhaps due to the lack of corresponding experimental techniques until now. By combining the powerful Fourier Transform Holography (FTH) imaging technique with LCLS' high brightness, short pulse structure, and fully transverse coherence, the dynamics of magnetic fluctuations and magnetization relaxation processes can be visualization at extremely fast time scales and at nm resolutions. These new capabilities not only provide direct experimental proof of the symmetry and statistical concepts in magnetic phase transitions, but also have far-reaching impact beyond magnetism in the studies of critical phenomenon such as other order/disorder transitionsor the demixing of binary alloys.