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X-ray Spectrometry
Lecture Date: February 4th, 2008
Notes
See Chapters 12 and 21 (mostly Chapter 12) of Skoog
This lecture covers both atomic and molecular applications of X-ray spectrometry
X-ray diffraction is only briefly discussed here - it is covered in its own lecture along with its applications to crystallography and solid-state structural analysis
Surface analysis and microscopy is also briefly discussed in advance of its own lecture
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Outline
X-ray absorption/fluorescenceprocesses – Auger electron emission
– Photoelectronemission Excitation of X-rays
– X-ray fluorescence, X-ray emission
X-ray Detection and Spectrometer Design
– Energy-dispersive(ED) spectrometers
– Wavelength-dispersive(WD) spectrometers Methods andApplications
Topics mentioned here but discussed in detail during the SurfaceAnalysis and Microscopy Lecture:
– Scanning electron microscopy – an X-ray emission “microprobe” – Auger electron spectrometry (electron energy)
– X-ray photoelectron spectrometry (again, electron energy)
The Electromagnetic Spectrum
X-rays
(Also gamma rays)
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X-rays
Whatare X-rays? High energy photons.
– Note: gammarays are just high-energy X-rays
Advantages of X-ray spectrometric methods:
– The X-ray spectrum is not very sensitive to molecular effectsor chemicalstate, or excitationconditions
This is because core electrons are usually involved in X-ray transitions – physical and chemical statehave only minuteeffects (I.e. gas vs solid, oxide vs. element).
– Atomizationis not necessary for elemental analysis – Precision and accuracy are good, spectra are simple
– Surface-sensitive(penetration of 100 um at most) Disadvantages of X-ray methods:
– Surface-sensitive,if you want bulk analysis (often not a problem)
– Modest limitsof detection, compared to other elemental methods (e.g.AA, ICP-OES, ICP-MS)
X-ray Production
X-ray are commonly produced by bombarding a target with electrons
The target emitsa spectrum with two components:
– Characteristic radiation
– Continuous radiation (also called white radiation, Bremsstrahlung (braking radiation)
The Duane-Hunt limit explainsthe “cutoff” of the continuous radiation:
eV0 = hc = hmax (whereV0 is the electron accelerating voltage) min
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X-ray Generation: Characteristic Radiation
The characteristics lines in X-ray spectra result from electronic transitionsbetween inner atomic orbitals
The X-ray spectra for most heavy elementsare much simpler than the UV/Visspectra observed in ICP-OES, for example. (Only a few lines!!!)
Big differencebetween X-ray and UV-Vis: The radiation is ionizing, and doesn’t just exciteelectrons to higher levels.
Moseley’s law: Predicts the basic relationship of atom number and the frequency of the characteristic lines
= K(Z − )
where Z is the atomic number, and K and are constants that vary with the spectral series.
X-ray Processes: when an X-ray strikes an atom…
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X-ray Generation: Characteristic Radiation
X-ray transitions: (Here denoted using the Siegbahn notation)
Remember the quantum numbers:
n – principal quantum number
l – angular momentum quantum number
s – spin quantum number (1 and 2 have s = -1/2 and s = +1/2)
j – “inner” quantum number, from coupling of l and s
X-ray Generation: Characteristic Radiation
X-ray transitions, for gold (Z=79), using both optical and X-ray (Siegbahn) notation.
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