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