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
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