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

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Presentation on theme: "Stanford Synchrotron Radiation Lightsource Sources and Optics for XAS Apurva Mehta."— Presentation transcript:

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

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

3 Apurva Mehta 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 Apurva Mehta Two ways of collecting data Monochromatic “Scanning” Measurement sample detector

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

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

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

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

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

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

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

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

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

14 Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution Use Higher Order Reflection

15 Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution Use Narrower Io Slits

16 Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution Use Narrower Mono Slits

17 Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution Use Collimated Beam

18 Apurva Mehta Double Crystal Monochromator Harmonics 220 8keV keV Rejection

19 Apurva Mehta Why Harmonics are a Problem sample detector Transparent Detector - Io Det/Io = signal Required Measurement = Det F Io F Actual Measurement = Det F + Det H Io F + Io H

20 Apurva Mehta Double Crystal Monochromator Harmonics 220 8keV keV detuning Rejection

21 Apurva Mehta Double Crystal Monochromator Collimating mirror for Harmonic Rejection

22 Apurva Mehta Harmonic Rejection Mirror Ni kedge Mn kedge

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

24 Apurva Mehta XAS BL Optics Layout insertion device Storage ring with straight sections Bend Magnet or Wiggler Preferred. Undulator – should be scanned. Collimating/ Harmonic Rejection Mirror Focusing Mirror Double Crystal Monochromator Expt. Hutch “Detune” Mono Slits Io Slits M1 Slits Mo Slits 1.Energy Resolution 2.Harmonic Rejection 3.High Flux Density

25 Apurva Mehta

26 Monochromator “Glitches” 220

27 Apurva Mehta Scattering from a Single crystal 22  Bragg’s Law: 2dsin(  ) = h/E

28 Apurva Mehta Real Space X-ray Diffraction Momentum Transfer Space Scattering from a Single crystal Real Space LatticeReciprocal Lattice

29 Apurva Mehta Q1Q1 Q0Q0 QDQD Scattering from a Single crystal 29 Ewald’s Sphere

30 Apurva Mehta Multiple Reflections 30 Ewald’s Sphere Resonance between the two reflections Energy Transfer between the two

31 Apurva Mehta Multiple Reflections – Phi Rotation 31 Ewald’s Sphere  rotation eliminates the secondary reflection  eliminates the resonance/glitch Primary reflection

32 Apurva Mehta 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.

33 Apurva Mehta 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

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

35 Stanford Synchrotron Radiation Lightsource Thanks Questions?

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


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