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Soft X-ray Research (SXR) Instrument for Materials Science
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.
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
Figure 3: Experimental Setup: A scanning electron microscopy image of a 15-reference gold holography mask, showing the aperture and the references. The sample aperture diameter is 1.45 micrometers. (b) A CCD camera located 490 mm downstream records the spectrohologram in the far field. (c) Reconstruction of the initial magnetic domain state from a low-fluence accumulated spectrohologram with 58% circularly polarized x-ray pulses (< 2 mJ/cm2). The dark and light regions are 100–150 nm wide domains with opposite out- of-plane magnetization directions. (d) Single shot reconstruction of the sample after illumination at 30 mJ/cm2. From: T. Wang, et al., PRL, 108, 267403 (2012)
Near Experimental Hall, Hutch 2 » complete instrument map
SLAC National Accelerator Laboratory, Menlo Park, CA
Operated by Stanford University for the U.S. Dept. of Energy