X-Ray Spectroscopy
Andrew Hornberger
What is X-ray Spectroscopy
• A technique used to determine the elements
that are present and there abundance in the
sample. Also give bond length, angle and
coordination numbers/geometry.
• It consists of several different techniques:
Fluorescence, Diffraction, Emission,
Absorption.
X-Ray Absorption
Cont.. (Lamberts Beers law)
• ln (I0 /I) = μx
I0= initial intensity
I= transmitted intensity
x= the thickness of the sample
μ= the linear absorption coefficient (different for
each atom)
When μ increases drastically, it is known as the
absorption edge
How it Happens
• A x-ray photon is shot from a synchrotron into the
analyte,
• The analyte’s inner core electron is excited by the
photon to a photoelectron and is ejected out of the
atom to an unoccupied electron state or further.
• It leaves as a spherical wave and effects the
surrounding atoms
• This happens at the absorption edge and corresponds
to the binding energy.
• Each edge is different due to the nucleus charge being
different
Basic Instrumentation
Trends
• As the atomic number increase, the energy
spikes at the edges increase as well, mainly
caused by the oxidation state or, the amount
of valence electrons in the outer shells
Regions of X-ray absorption
X-ray Absorption Near Edge structure (XANES)
• The transfer of the low core energy state to the higher
unoccupied state.
• Provides information on the oxidation state of the
absorbing atom and symmetry.
Near Edge X-ray Absorption Fine Structure (NEXAFS)
• The core electron is excited past the unoccupied shell and
the photoelectron is backscattered by all of the neighboring
atoms.
• It gives good data on what is the neighboring atoms, how
many and the orientation around the atom that is ejecting
the electron.
• Term more popular in soft x-ray
Regions of X-ray absorption Continue…
Extended X-ray Absorption Fine Structure (EXAFS)
•
When it is well above the ionization stateits
•
Its de Broglie wavelength becomes comparable to the distance to neighboring
atoms
•
It gives good data on what is the neighboring atoms, how many and the distance
between the two atoms.
•
~50‐ 1000eV after the absorption edge
•
You can model EXAFS with the equation above :Where Fj(k) = backscattering
from each neighboring atom, Nj = number of neighboring atoms of type j, σj =
Debye‐Waller factor, rj = distance of jth atom from absorbing atom, φj = total
phase shift of photoelectron, k is the wave number corresponding to an energy,
and X(k) is related to the absorption coefficient.
Data of X-Ray Absorption Spec.
Data of X-Ray Absorption Spec.
Data of X-Ray Absorption Spec.
Applications
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Hydrocarbon Analysis
Environmental toxicology
Protein Biochemistry
Protein Biochemistry
Pros
• Any state can be
used,
• Gives electron
structure of analyte,
• Wide range of
applications,
• Structural
information of
analyte
Cons
•Synchrotron needed,
•Carefully planned
experiment,
•Data interpretation
sometimes difficult.
•$$$
Reference
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The Nilsson Research Group at Stanford University.
http://ssrl.slac.stanford.edu/nilssongroup/corelevel.html (accessed Dec. 9,2011).
Farideh Jalilehvand’s X-Ray Absorption Spectroscopy at the University of Calgary.
http://www.chem.ucalgary.ca/research/groups/faridehj/xas.pdf (accessed Dec. 9,2011).
X-ray Absorption Spectroscopy at Lunds University.
http://www.chemphys.lu.se/research/techniques/xrayxas/ (accessed Dec. 9,2011).
ZeNan Chang, Changyi Li’s lecture at Cal Tech. http://tone.caltech.edu/W8.pdf (accessed Dec.
9,2011).
Chemwiki of XAS Applications at the University of California, Davis.
http://chemwiki.ucdavis.edu/Wikitexts/UCD_Chem_205:_Larsen/ChemWiki_Module_Topics/XAS:_
Analytical (accessed Dec. 9,2011).
Jagdeep Singh and Jeroen A. van Bokhoven’s X-Ray Absorption Spectroscopy
http://www.vanbokhoven.ethz.ch/education/Bok_1 (accessed Dec. 9,2011). #*
Chemwiki of EXAFS Applications at the University of California, Davis.
http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Xray_Spectroscopy/EXAFS:_Theory (accessed Dec. 9,2011).
Rigaku’s X-ray Absorption Spectroscopy. http://www.rigaku.com/xas/ (accessed Dec. 9,2011).
Dr. Tony VanBuuren’s X-Ray Absorption Spectroscopy - XAFS, NEXAFS, XANES, EXAFS lecture at the
University of Berkeley. http://www.youtube.com/watch?v=70RHRCe1eRc&feature=relmfu
(accessed Dec. 9,2011).