Monochromatic “Scanning” Measurement
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Sources and Optics for XAS Apurva Mehta Stanford Synchrotron Radiation Lightsource X-ray absorption Spectroscopy Apurva Mehta 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 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 Two ways of collecting data detector Monochromatic “Scanning” Measurement sample Apurva Mehta Two ways of collecting data detector Monochromatic “Scanning” Measurement sample Apurva Mehta Two ways of collecting data detector Monochromatic “Scanning” Measurement sample Apurva Mehta Two ways of collecting data detector Monochromatic “Scanning” Measurement sample Apurva Mehta Two ways of collecting data detector Monochromatic “Scanning” Measurement sample Apurva Mehta Slow but doable Outline Apurva Mehta Sources Wiggler vs. Undulator Slits Monochromator Energy Resolution “Glitches” Harmonic rejection Mirrors High Flux Density XAS BL Layout Transparent Detector - Io Xanes detector Measurement Requirements sample Apurva Mehta Det/Io = signal EXAFS Very Robust Normalization Energy Resolution Homogeneous Beam Very High Signal to Noise Storage ring with straight sections Apurva Mehta Sources insertion device Bend magnets and Wigglers bending magnet - a “sweeping searchlight” 3.60E+013 Intensity 3.20E+013 2.80E+013 Good for EXAFS 2.40E+013 wiggler - incoherent superposition 2.00E+013 2000 4000 6000 8000 10000 Energy eV Undulators 6.00E+012 Bad for EXAFS 5.00E+012 Intensity 4.00E+012 undulator - coherent interference 3.00E+012 2.00E+012 1.00E+012 0.00E+000 2000 3000 4000 5000 6000 Energy eV 7000 8000 9000 10000 Apurva Mehta How to Use an Undulator Changing the Undulator K – Scanning the Gap 6.00E+012 3.00E+012 0.00E+000 5000 5200 5400 5600 Energy eV 5800 6000 Apurva Mehta Double Crystal Monochromator 1.0 0.8 0.6 0.4 0.2 0.0 -50 Bragg’s Law: 2dsin(q) = h/E 0 angle/energy 50 100 Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution 1.0 0.8 0.6 0.4 0.2 0.0 -50 0 50 100 50 100 angle/energy 1.0 0.8 0.6 0.4 Use Higher Order Reflection 0.2 0.0 -50 0 angle/energy Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution Use Narrower Io Slits Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution Use Narrower Mono Slits Apurva Mehta Double Crystal Monochromator Increasing Energy Resolution Use Collimated Beam Apurva Mehta Double Crystal Monochromator Harmonics Rejection 8keV 16keV Transparent Detector - Io detector Why Harmonics are a Problem sample Apurva Mehta Required Measurement = DetF IoF Actual Measurement = DetF + DetH IoF + IoH Det/Io = signal Apurva Mehta Double Crystal Monochromator Harmonics Rejection 8keV 220 @ 8 keV 440 @ 16keV 1.0 16keV 0.8 0.6 0.4 0.2 0.0 -50 0 angle (microrad) detuning 50 100 Double Crystal Monochromator Apurva Mehta Collimating mirror for Harmonic Rejection Harmonic Rejection Mirror 1.0 0.8 Reflectivity Apurva Mehta 0.6 0.4 0.2 0.0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 Energy eV Ni kedge Mn kedge Apurva Mehta Focusing Mirror A Cylindrical Mirror Bent into a Torous Focuses Vertically and Horizonally XAS BL Optics Layout Apurva Mehta Bend Magnet or Focusing Wiggler Preferred. Mirror Undulator – should be scanned. Double Crystal Monochromator M1 Slits Storage ring with straight sections Mo Slits insertion device Io Slits Expt. Hutch “Detune” Mono Slits 1. Energy Resolution 2. Harmonic Rejection Collimating/ Harmonic Rejection Mirror 3. High Flux Density Apurva Mehta Apurva Mehta Monochromator “Glitches” Apurva Mehta Scattering from a Single crystal 2q w Bragg’s Law: 2dsin(q) = h/E Apurva Mehta Scattering from a Single crystal Real Space X-ray Diffraction Momentum Transfer Space Real Space Lattice Reciprocal Lattice Scattering from a Single crystal Apurva Mehta QD Q1 Q0 Ewald’s Sphere 29 Multiple Reflections Apurva Mehta Ewald’s Sphere Resonance between the two reflections Energy Transfer between the two 30 Multiple Reflections – Phi Rotation Apurva Mehta Primary reflection Ewald’s Sphere F rotation eliminates the secondary reflection eliminates the resonance/glitch 31 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. 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 XAS BL Optics Layout Apurva Mehta Bend Magnet or Focusing Wiggler Preferred. Mirror Undulator – should be scanned. Double Crystal Monochromator M1 Slits Storage ring with straight sections Mo Slits insertion device Io Slits “Detune” Mono Slits Collimating/ Harmonic Rejection Mirror Expt. Hutch Thanks Questions? Stanford Synchrotron Radiation Lightsource XAS BL Optics Layout Apurva Mehta Bend Magnet or Focusing Wiggler Preferred. Mirror Undulator – should be scanned. Double Crystal Monochromator M1 Slits Storage ring with straight sections Mo Slits insertion device Io Slits “Detune” Mono Slits Collimating/ Harmonic Rejection Mirror Expt. Hutch
