instrumental chemistry chapter 12 atomic x-ray spectroscopic
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Instrumental Chemistry
Chapter 12
Atomic X-Ray Spectroscopic
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Brief Summary
X-ray spectroscopy is a form of optical spectroscopy that utilizes emission, absorption, scattering, fluorescence, and diffraction of X-ray radiation
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About X-Rays
• X-rays are short-wavelength (hence, high frequency, and hence, relatively high energy) electromagnetic radiation. Two ways to produce X-rays:
1) Deceleration of high-energy electrons
2) Electronic transitions involving inner-orbital (e.g. - d or f) electrons
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For analytical purposes, X-rays are generated in three ways:
1) bombardment of metal target with high-energy electron beam
2) exposure of target material to primary X-ray beam to create a secondary beam of X-ray fluorescence
3) use of radioactive materials whose decay patterns include X-ray
emission
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Schematic of an X-ray tube
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Energy-level diagram showing common transitions producing X-rays
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Electron beam sources
In electron beam sources, X-rays are produced by heating a cathode to produce high-energy electrons; these electrons are energetic enough to ionize off the cathode and race towards a metal anode (the target) where, upon collision, X-rays are given off from the target material in response to the colliding electrons.
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The Duane-Hunt law
The maximum photon energy corresponds to total stopping of the electron and is given by:
hvo = (hc)/o = Ve
vo is the maximum frequency
V = accelerating voltage
e = electron charge
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X-ray Fluorescence
Since X-rays are rather energetic, excitation of sample electrons will give rise to fluorescence as the sample electrons are excited and return to their ground states in a series of electronic transitions.
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Bragg equation
sin = (n)/2d
= angle of incidence
= wavelength
d = interplane distance of crystal
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Diffraction of X-rays by a crystal
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X-ray monochromator and detector
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References
http://www.anachem.umu.se/jumpstation.htm http://userwww.service.emory.edu/~kmurray/mslist.html
http://www.chemcenter/org
http://www.sciencemag.org