X-ray Absorption Spectroscopy in the Physical and Biological Sciences · · 2012-08-20X-ray...
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Introduction Experiment Resources
X-ray Absorption Spectroscopy in the Physicaland Biological Sciences
Bruce Ravel
Synchrotron Methods Group, Ceramics DivisionNational Institute of Standards and Technology
Joint InSynC-INCREASE Workshop15 July, 2010
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources
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Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Beamline Physics Periodic table Data
The XAS Experiment
An X-ray Absorption Spectroscopy experiment measures the probabilityas a function of energy that a material will absorb a photon in a givenenergy range.
In its simplest form, an XAS beamline looks like this:
The monochromator uses Bragg diffraction to select the desired energy.The energy is scanned by changing the angle of the mono and the x-raybeam is directed from the mono to the sample.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Beamline Physics Periodic table Data
Model of the atom
All atoms are like little solar systems. Each element on the periodictable has a specific number of protons in the nucleus and electronsorbiting.
The electrons closer to the center are boudn with more energy than thehigher lying electrons.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Beamline Physics Periodic table Data
The basic physical process in XAS and XRF
X-ray in
n=1 K edge
n=2 L edges
n=3 M edgesn=4 N edges
1 An incoming photon interacts with a deep-core electron. Shownhere, a 1s electron is excited for a K-edge spectrum.
2 The deep-core electron is promoted to some unoccupied state abovethe Fermi energy, propagates away, and leaves behind a core-hole.
3 A short time later (1 or 2 femtoseconds), a higher-lying electrondecays into the core-hole and emits a photon.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Beamline Physics Periodic table Data
The basic physical process in XAS and XRF
n=1 K edge
n=2 L edges
n=3 M edgesn=4 N edges
1 An incoming photon interacts with a deep-core electron. Shownhere, a 1s electron is excited for a K-edge spectrum.
2 The deep-core electron is promoted to some unoccupied state abovethe Fermi energy, propagates away, and leaves behind a core-hole.
3 A short time later (1 or 2 femtoseconds), a higher-lying electrondecays into the core-hole and emits a photon.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Beamline Physics Periodic table Data
The basic physical process in XAS and XRF
n=1 K edge
n=2 L edges
n=3 M edgesn=4 N edgesX-ray out
1 An incoming photon interacts with a deep-core electron. Shownhere, a 1s electron is excited for a K-edge spectrum.
2 The deep-core electron is promoted to some unoccupied state abovethe Fermi energy, propagates away, and leaves behind a core-hole.
3 A short time later (1 or 2 femtoseconds), a higher-lying electrondecays into the core-hole and emits a photon.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Beamline Physics Periodic table Data
Characteristic energiesEach element has a characteristic set of excitation and fluorescenceenergies. Two examples:
Iron: Z=26Edge Energy
K 7112L3 706.8L2 719.9L1 844.6
Line Transition Energy StrengthKα1 K-L3 6405.2 0.580Kα2 K-L2 6392.1 0.294Kβ1 K-M3 7059.3 0.082Kβ3 K-M2 7059.3 0.043Kβ5 K-M4,5 7110.0 0.001
Uranium: Z=92Edge Energy
K 115606L3 17166L2 20948L1 21757
Line Transition Energy StrengthLα1 L3-M5 13614.0 0.686Lα2 L3-M4 13438.0 0.077Lβ2 L3-N4,5 16387.7 0.181Lβ5 L3-O4,5 17063.2 0.038Lβ6 L3-N1 15727.0 0.013L` L3-M1 11618.0 0.005
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Beamline Physics Periodic table Data
Data Processing and Atomic Structure
The absorption data show clear differencesfor Mn species of different valence. As thevalence increases (Mn0, Mn2+, Mn3+,Mn4+), the edge position shifts to higherenergy.
The oscillatory portion of the spectrum canbe isolated and ...
... Fourier transformed. This FT functioncan be interpreted to yield a partial pairdistribution functions of atoms about theabsorber. The Mn-O distances aredifferent for the Mn2+, Mn3+, and Mn4+and clearly different from the Mn-Mndistance in Mn metal.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Beamline Physics Periodic table Data
Data Processing and Atomic Structure
The absorption data show clear differencesfor Mn species of different valence. As thevalence increases (Mn0, Mn2+, Mn3+,Mn4+), the edge position shifts to higherenergy.
The oscillatory portion of the spectrum canbe isolated and ...
... Fourier transformed. This FT functioncan be interpreted to yield a partial pairdistribution functions of atoms about theabsorber. The Mn-O distances aredifferent for the Mn2+, Mn3+, and Mn4+and clearly different from the Mn-Mndistance in Mn metal.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Beamline Physics Periodic table Data
Data Processing and Atomic Structure
The absorption data show clear differencesfor Mn species of different valence. As thevalence increases (Mn0, Mn2+, Mn3+,Mn4+), the edge position shifts to higherenergy.
The oscillatory portion of the spectrum canbe isolated and ...
... Fourier transformed. This FT functioncan be interpreted to yield a partial pairdistribution functions of atoms about theabsorber. The Mn-O distances aredifferent for the Mn2+, Mn3+, and Mn4+and clearly different from the Mn-Mndistance in Mn metal.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Beamline Physics Periodic table Data
Information in the XAS Measurement
XAS is used to measure:The valence state of the absorbing atom independently of thechemistry of the rest of the sampleThe local configuration of atoms around the absorbing atom
XAS and Other TechniquesXAS is complementary to other synchrotron and laboratorymeasurements techniques, such as diffraction, NMR, electronmiscroscopy, and many others.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Hutch Applications Example
Standard Hutch Instrumentation
Virtually every beamline provides a basic complement of detectors,optics, and sample positioners.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Hutch Applications Example
Specialized Sample Environments
The fluorescence detector can bereplaced with an energydiscriminating detector whichelectronically isolates the desiredsignal.
This is particulary useful for samplewith many components or with verylow conctrations of the targetelement.
The sample holder can be replacedwith:
electrochemistry cellperistaltic fluid flow apparatusfurnacecryostatmagnet... and so on ...
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Hutch Applications Example
XAS Looks at Matter in All Forms
So ... why do an XAS experiment?XAS can be measured and interpreted with no assumption ofsymmetry or periodictityXAS is non-destructiveX-rays penetrate deeply into the sample containment
XAS is used by researchers in a surprisingly broad array of scientificdisciplines, such as:
Catalysis and energy sciencesEnvironmental sciencesMaterials scienceOrganic and inorganic chemistryLife sciencesand many others
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources Hutch Applications Example
Solving a real-world problem with XAFS
XANES as a fingerprinting techniqueOne of the most powerful uses of XANES data is to simply identifywhat is in front of the beam.
Highly toxic, water soluble CrVI canbe distinguished from the non-toxicCrIII form by simple examination ofthe near-edge spectra.
Here are spectra from coal ashas well as the residue afterwater and acid leachingexperiments.
K2CrO7
Cr2O3
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Quantifying Hazardous Species in Particulate Matter Derived fromFossil-Fuel Combustion, F.E. Huggins, et al., Environ. Sci. Technol.(2004) 38:6, 1836 DOI: 10.1021/es0348748
Introduction Experiment Resources NSLS Web Software
XAS Beamlines at NSLSWe have a comprehensive XAS program at NSLS – some highlights:
X3b Biological applicationsX18b Time resolved XAS
X23a2 NIST + Industrial applicationsX15b and X19a “Tender” x-rays, including S, P, and Cl.
U7a and U4a Soft x-rays, first row elements, transition metal L edges
The beamlines markedin red do XAS or arelated inner shell
spectroscopy.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources NSLS Web Software
Internet Resources
http://xafs.org offers a growingvolume of educational resources.
An earlier version of this talk hasbeen posted on the Tutorials page.
NSLS is developing web-basedsynchrotron education resources,
starting with XAS!
The IFEFFIT mailing list focuses onthe analysis software package Ico-author. It is a great place to askquestions about any aspect of XASmeasurement, theory, orinterpretation.
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences
Introduction Experiment Resources NSLS Web Software
Analysis SoftwareThe IFEFFIT package, written by Matt Newville (GSECARS, APS) andyours truly, is a thorough, high-quality XAS analysis solution. It isopen source, free of cost, available on the web, always underdevelopment, fully supported, cross-platform, and in use by manyhundreds of XAS practitioners worldwide.
ATHENA HEPHAESTUS
Bruce Ravel X-ray Absorption Spectroscopy in the Physical and Biological Sciences