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Sources and Optics for XAS

Sources and Optics for XAS. Apurva Mehta. X-ray absorption Spectroscopy. Basic Experiment :. =XANES ( X-ay Absorption Near Edge Structure ) =NEXAFS ( Near Edge X ray Absorption Fine Structure ). (EXAFS = Extended X ray Absorption Fine Structure ). E b. EXAFS. Xanes.

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Sources and Optics for XAS

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  1. Sources and Optics for XAS Apurva Mehta Stanford Synchrotron Radiation Lightsource

  2. X-ray absorption Spectroscopy Basic Experiment : =XANES (X-ay Absorption Near Edge Structure) =NEXAFS (Near Edge X ray Absorption Fine Structure) (EXAFS = Extended X ray Absorption Fine Structure) Eb EXAFS Xanes Core electron binding energy, Eb

  3. Two ways of collecting data White Beam Energy Dispersive Spectrum in Single Shot Optics and Detector were not Available But bent crystal optics is making some of this possible XES – Uwe

  4. Two ways of collecting data Monochromatic “Scanning” Measurement detector sample

  5. Two ways of collecting data Monochromatic “Scanning” Measurement detector sample

  6. Two ways of collecting data Monochromatic “Scanning” Measurement detector sample

  7. Two ways of collecting data Monochromatic “Scanning” Measurement detector sample

  8. detector sample Two ways of collecting data Monochromatic “Scanning” Measurement Slow but doable

  9. Sources Slits Monochromator Mirrors XAS BL Layout Wiggler vs. Undulator Energy Resolution “Glitches” Harmonic rejection High Flux Density Outline

  10. Transparent Detector - Io detector sample Measurement Requirements Det/Io =signal EXAFS Xanes Very Robust Normalization Homogeneous Beam Energy Resolution Very High Signal to Noise

  11. bending magnet - a “sweeping searchlight” wiggler - incoherent superposition Storage ring with straight sections insertion device Sources Bend magnets and Wigglers Good for EXAFS Undulators Bad for EXAFS undulator - coherent interference

  12. How to Use an Undulator Changing the Undulator K – Scanning the Gap

  13. Double Crystal Monochromator Bragg’s Law: 2dsin(q) = h/E Not Right!

  14. Double Crystal MonochromatorIncreasing Energy Resolution Use Higher Order Reflection

  15. Double Crystal MonochromatorIncreasing Energy Resolution Use Narrower Io Slits

  16. Double Crystal MonochromatorIncreasing Energy Resolution Use Narrower Mono Slits

  17. Use Collimated Beam Double Crystal MonochromatorIncreasing Energy Resolution

  18. 440 16keV 440 Double Crystal MonochromatorHarmonics Rejection 220 8keV 220

  19. Transparent Detector - Io detector sample Why Harmonics are a Problem Det/Io =signal DetF Required Measurement = IoF DetF + DetH Actual Measurement = IoF + IoH

  20. 440 16keV 440 detuning Double Crystal MonochromatorHarmonics Rejection 220 8keV 220

  21. Double Crystal MonochromatorCollimating mirror for Harmonic Rejection

  22. Harmonic Rejection Mirror Ni kedge Mn kedge

  23. Focusing Mirror A Cylindrical Mirror Bent into a Torous Focuses Vertically and Horizonally

  24. Io Slits M1 Slits Storage ring with straight sections Expt. Hutch “Detune” Mo Slits Mono Slits insertion device XAS BL Optics Layout Focusing Mirror Bend Magnet or Wiggler Preferred. Undulator – should be scanned. Double Crystal Monochromator • Energy Resolution • Harmonic Rejection • High Flux Density Collimating/ Harmonic Rejection Mirror

  25. Monochromator “Glitches” 220 220

  26. Scattering from a Single crystal 2q w Bragg’s Law: 2dsin(q) = h/E

  27. Scattering from a Single crystal Real Space X-ray Diffraction Momentum Transfer Space Real Space Lattice Reciprocal Lattice

  28. Scattering from a Single crystal 29 Q1 QD Q0 Ewald’s Sphere

  29. Multiple Reflections 30 Ewald’s Sphere Resonance between the two reflections Energy Transfer between the two

  30. Multiple Reflections – Phi Rotation 31 Primary reflection Ewald’s Sphere F rotation eliminates the secondary reflection  eliminates the resonance/glitch

  31. Monochromator “Glitches” • Location Depends on Phi Orientation • Severity depends on precise crystal orientation, difficult to predict • Can not be eliminated, but sometimes can be made sufficiently narrow by slits adjustment that EXAFS are not affected.

  32. Preparation for an XAS Experiment • Absorbing Element • K, L, M edge • BL with the appropriate energy range • Energy Resolution • Monochromator crystal order (e.g., 111, 220) • Crystal Orientation – phi cut – “Glitch” spectrum. • Collimation of the beam prior to the Mono • Narrow slits • Harmonic Rejection Strategy • Mo angle adjustment for appropriate cut-off • Detune the monochromator • Flux Density on the sample • Select an insertion device source if available • Adjust the M1 focus • Open Slits

  33. Io Slits M1 Slits Storage ring with straight sections Expt. Hutch “Detune” Mo Slits Mono Slits insertion device XAS BL Optics Layout Focusing Mirror Bend Magnet or Wiggler Preferred. Undulator – should be scanned. Double Crystal Monochromator Collimating/ Harmonic Rejection Mirror

  34. Thanks Questions? Stanford Synchrotron Radiation Lightsource

  35. Io Slits M1 Slits Storage ring with straight sections Expt. Hutch “Detune” Mo Slits Mono Slits insertion device XAS BL Optics Layout Focusing Mirror Bend Magnet or Wiggler Preferred. Undulator – should be scanned. Double Crystal Monochromator Collimating/ Harmonic Rejection Mirror

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