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 intensityx= the thickness of the sampleμ= 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

• Hydrocarbon Analysis• Environmental toxicology• Protein Biochemistry• Protein Biochemistry

ProsCons

• Any state can be used,

• Gives electron structure of analyte,

• Wide range of applications,

• Structural information of analyte

•Synchrotron needed,•Carefully planned experiment,•Data interpretation sometimes difficult.•$$$

Reference• 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/X-ray_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).