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Stanford Synchrotron Radiation Lightsource Sources and Optics for XAS Apurva Mehta
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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
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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
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Apurva Mehta Two ways of collecting data Monochromatic “Scanning” Measurement sample detector
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Apurva Mehta Two ways of collecting data Monochromatic “Scanning” Measurement sample detector
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Apurva Mehta Two ways of collecting data Monochromatic “Scanning” Measurement sample detector
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Apurva Mehta Two ways of collecting data Monochromatic “Scanning” Measurement sample detector
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Apurva Mehta Two ways of collecting data Monochromatic “Scanning” Measurement sample detector Slow but doable
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Apurva Mehta Outline Sources Slits Monochromator Mirrors XAS BL Layout Wiggler vs. Undulator Energy Resolution “Glitches” Harmonic rejection High Flux Density
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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
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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
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Apurva Mehta How to Use an Undulator Changing the Undulator K – Scanning the Gap
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Apurva Mehta Double Crystal Monochromator Bragg’s Law: 2dsin( ) = h/E Not Right!
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Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution Use Higher Order Reflection
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Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution Use Narrower Io Slits
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Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution Use Narrower Mono Slits
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Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution Use Collimated Beam
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Apurva Mehta Double Crystal Monochromator Harmonics 220 8keV 440 16keV Rejection
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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
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Apurva Mehta Double Crystal Monochromator Harmonics 220 8keV 440 16keV detuning Rejection
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Apurva Mehta Double Crystal Monochromator Collimating mirror for Harmonic Rejection
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Apurva Mehta Harmonic Rejection Mirror Ni kedge Mn kedge
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Apurva Mehta Focusing Mirror A Cylindrical Mirror Bent into a Torous Focuses Vertically and Horizonally
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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
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Apurva Mehta
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Monochromator “Glitches” 220
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Apurva Mehta Scattering from a Single crystal 22 Bragg’s Law: 2dsin( ) = h/E
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Apurva Mehta Real Space X-ray Diffraction Momentum Transfer Space Scattering from a Single crystal Real Space LatticeReciprocal Lattice
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Apurva Mehta Q1Q1 Q0Q0 QDQD Scattering from a Single crystal 29 Ewald’s Sphere
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Apurva Mehta Multiple Reflections 30 Ewald’s Sphere Resonance between the two reflections Energy Transfer between the two
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Apurva Mehta Multiple Reflections – Phi Rotation 31 Ewald’s Sphere rotation eliminates the secondary reflection eliminates the resonance/glitch Primary reflection
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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.
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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
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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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Stanford Synchrotron Radiation Lightsource Thanks Questions?
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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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