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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 1 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) 2 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 = hmax (whereV0 is the electron accelerating voltage) min 3 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… 4 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. 5 ... - tailieumienphi.vn
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