DOC PREVIEW
FIU CHM 4130 - CHAPTER 12_Xiao_X_Ray_Spectrometry_2018

This preview shows page 1-2-3-4-5-39-40-41-42-43-44-78-79-80-81-82 out of 82 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 82 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

Atomic X-Ray Spectroscopy Chapter 12 X-ray range à 10-5Å to 100 Å Usedà 0.1Å to 25 Å8-1Atomic X-Ray Spectrometry$ Emission, absorption, scattering, fluorescence and diffraction$ Fundamentals$ Instruments$ X-ray Fluorescence$ X-ray Absorption$ X-ray diffraction8-1Atomic X-Ray Spectrometry$ Emission, absorption, scattering, fluorescence and diffraction$ Fundamentals$ Instruments$ X-ray Fluorescence$ X-ray Absorption$ X-ray diffractionMaking X-rays • 1. By bombardment of a metal target with a beam of high-energy electrons • 2. By expose of a substance to a primary beam of X-rays to generate a secondary beam of X-ray fluorescence • 3. By use of a radioactive source whose decay process results in X-ray emission • 4. From a synchroton radiation sourceFormation of X-Rays (emission) • Bombardment of a metal target with a beam of high-energy electronsFormation of X-Rays (fluorescence) • Exposure of a substance to x-ray radiation à absorption and then à secondary fluorescenceFormation of X-Rays (decay, synchroton) • Radioactive decay à X-ray emission (common in medicine) • Synchrotron source radiation (accelerated particles) very few of these available!X-Rays are just like any kind of electromagnetic radiation Two different atomic processes to produce X-ray photons: • Bremsstrahlung • K-shell emission !Bremsstrahlung (braking radiation) !• X-rays are generated by interactions encountered by free electrons • Tungsten is the best element – a high melting point and a good heat conducting. • The same bremsstrahlung pattern for most of heavy elements. • A range of photons emitted !X-rayA simplified diagram of a water cooled X-ray tubeX-ray tube emission λ0 λ0 = 12,398/V Duane-Hunt Law • Independent of material • Related to acceleration voltage à E Continuum Spectra: Results from Collisions between the electrons and the atoms of target materials Ee = E’e + hν At λo, E’e = 0 hν0 = hc/λo = Ve V: accelerating voltage e: charge on electronK-shell emission!• X-rays are generated by electrons changing energy levels within an atom • K-shell knock-out on Innermost electron • K-shell spectrum depends on the target element.Line spectra is possible! Line Spectrum of a Molybdenum target λ0 • Atomic number >23 • 2 line series K and L • E K> EL • Atomic number < 23 • K only L From electron transitions involving inner shells A minimum acceleration voltage required for each element increases with atomic numberLine spectra λ0Bohr’s!atomic!model,!shell!model!X-ray line labelingElectron Transitions è X-Rays 1. Transitions that involve the innermost atomic orbitals. 2. Energy difference between the L and K levels > that between the M and L levels. 3. Energy difference between the transitions labeled α1 and α2 as well as those between β1 and β2 are so small – single lines. 4. Energy difference between the levels increase regularly with atomic number. 5. Energy of characteristic X-ray lines are independent of the physical and chemical state of the element.Relationship between X-ray emission frequency and atomic number for Kα1 and Lα1• Line spectra from fluorescent sources • Line or continuum spectra from radioactive sources Fe(26)-55 Mn(25)-55 + hνInterac3on!of!X6Ray!with!Ma;er!1. X-ray absorptionTransitions resulting from absorption of X-rayX-ray absorption spectra Ln P0/P = μX --- (1) μ is the linear absorption coefficient is characteristic of the Element and # of atoms in the path of the beam. X is sample thickness Ln P0/P = μMηX --- (2) η is density of the sample μM is mass absorption coefficient Absorption edgesFrom: X-ray spectroscopy: Transferring electrons to water. Anders Nilsson Nature Chemistry 2, 800–802 (2010) 2. X-Ray FluorescenceThe!Basic!Process!X6Ray!Fluorescence!6!Intensity!Mul3ple!Transi3ons!What!is!X6ray!Diffrac3on?!3. X-Ray DiffractionBragg’s Law of Diffraction light scattering by lattice of atoms! dddPCAPPCAP2nsinsin2nsinnλθθλθλ=====+Constructive interference only at angles proportional to λ and d! If λ is known and θ can be measured then you can calculate d! If d is known and θ can be measured then you can calculate λ!Instrumenta3on!of!Atomic!X6ray!Spectrometry!Instrument components 1. Sources - Tubes - Radioisotopes - Secondary fluorescence sourceX-Ray Tube (electron beam sources) 100KV! Controlling the intensity of the emitted X-Ray Determining the energy of the X-RayRadioisotopes – line spectra or continuumSecondary Fluorescent source – line spectra As a source for absorption or fluorescence studiesInstrumenta3on!of!Atomic!X6ray!Spectrometry!Instrument components 1. Monochromators - Filters - Monochromators1. FiltersTarget-filter combination2. X-ray MonochromatorsFlat Crystal Design d2nsinλθ=How a collimator filters a stream of raysSimplicity Decreased radiation Increased scatteringFlat crystal with Soller CollimatorsRowland circle Rowland circleCurved crystal with slitsHigher intensities Higher resolution Lower background Bent Crystal DesignAt least two interchangeable crystals dθdλ=n2d cosθDouble Crystal DesignInstrumenta3on!of!Atomic!X6ray!Spectrometry!Transducers – photon counters - Gas filled counters Ionization due to photo interaction with gas Three types: Ionization chambers Proportional counters Geiger counters - Scintillation counters - Semiconductor transducers Lithium-drifted silicon detectors Lithium-drifted germanum detectorsGas-Filled Transducers e- e- Ar+ ArGas!amplifica3on!for!various!types!of!gas6filled!detectors!Geiger counterScheme of scintillation counters• Crystal of pure silicon, with lithium diffused in to compensate for any residual carriers • Much greater depletion depths - about 3mm thick and 3-6 mm diameter • Electrodes plated on front and back • Front electrode is thin to allow X-rays to enter • Biased by a voltage of 3-500V • Cooled to Liq. N2 Si(Li) crystalSemiconductorsSi(Li) semiconductorSi(Li) semiconductor • Energy of an x-ray generates electron-hole pairs • These are swept from the crystal by the bias voltage, and are detected in the external circuitry as a pulse of charge • Since the average energy required to create


View Full Document
Download CHAPTER 12_Xiao_X_Ray_Spectrometry_2018
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view CHAPTER 12_Xiao_X_Ray_Spectrometry_2018 and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view CHAPTER 12_Xiao_X_Ray_Spectrometry_2018 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?