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FIU CHM 4130 - CHAPTER 8_Xiao_Introduction_to_Optical_Atomic_Spectrometry_2018 (1)

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Optical Atomic Spectroscopy The elements present in a sample are atomized by atomization process. • Optical Spectrometry – Absorption – Emission – Fluorescence • Mass Spectrometry • X-Ray SpectrometryAn iron atom and its four unpaired electronsOrbitals and ElectronsElectrons exist at distinct orbitals and have spinMotion of the electronsIntroduction to Atomic Optical Spectroscopy (Chapter 8)Sample is atomized (atoms/ions)absorption or emission measuredEnergy Level DiagramsEvery elements has unique set of atomic orbitalsp,d,f... levels split by spin-orbit couplingSpin (s) and orbital (l) motion create magnetic fields that perturbeach other (couple)if fields parallel - slightly higher energyif fields antiparallel - slightly lower energyNSNSvs.NSSNDefine SO coupling by J (total angular momentum)J=L+S (L = l S = s∑∑) (positive values only)CEM 333 page 7.1Na atom and Na ion Valence ElectronEnergy level diagrams: single external electronEnergy level diagram for atomic magnesiumSpin orientations Spins are paired No split Spins are unpaired Energy splittingFig 8-1• Similar pattern between atoms but different spacing• Spectrum of ion different to atom• Separations measured in electronvolts (eV)1eV = 1.602x10−19C ×1 V J / C( )= 1.602x10−19J= 96. 484 kJ ⋅mol−1• As # of electrons increases, # of levels increasesEmission spectra become more complexLi 30 lines, Cs 645 lines, Cr 2277 linesCEM 333 page 7.3Atomic spectroscopy • Absorption • Emission • FluorescenceAtomic Absorption SpectraEnergy level diagrams Absorption 3s 3p 3s 4pAtomic Emission SpectraEnergy level diagrams Emission 3p 3s 4d 3p3s 3s 3p 3p 4p Mg Na Atomic Fluorescence Spectra λ = 285.2 nm λ = 330.3 nm λ = 589.0 nm and 589.6 nmAtomic Fluorescence Spectra Resonance fluorescence Nonresonance fluorescenceDesire narrow lines for accurate identificationBroadened by(i) uncertainty principle(ii) pressure broadening(iii) Doppler effect(iv) (electric and magnetic fields)Atomic line widths:(i) Uncertainty Principle:Quantum mechanical idea states must measure for some minimumtime to tell two frequencies apart Δt⋅ΔE ≥ hΔtminimumtime formeasurement{⋅ Δνminimumdetectabledifference infrequencies{≥ 1Shows up in lifetime of excited state• if lifetime infinitely long, ΔE infinitely narrow• if lifetime short, ΔE is broadenedCEM 333 page 7.4Line Broadening1. Uncertainty Effects – Heisenberg uncertainty principle: The nature of the matter places limits on the precision with which certain pairs of physical measurements can be made. One of the important forms Heisenberg uncertainty principle: ΔtΔν ≥ 1 p156 Δt : minimum time for measurement; Δν : minimum detectable difference in frequencies; To determine Δν with negligibly small uncertainty, a huge measurement time is required. Desire narrow lines for accurate identificationBroadened by(i) uncertainty principle(ii) pressure broadening(iii) Doppler effect(iv) (electric and magnetic fields)Atomic line widths:(i) Uncertainty Principle:Quantum mechanical idea states must measure for some minimumtime to tell two frequencies apart Δt⋅ΔE ≥ hΔtminimumtime formeasurement{⋅ Δνminimumdetectabledifference infrequencies{≥ 1Shows up in lifetime of excited state• if lifetime infinitely long, ΔE infinitely narrow• if lifetime short, ΔE is broadenedCEM 333 page 7.42. Pressure broadening (ii) Pressure broadening:Collisions with atoms/molecules transfers small quantities ofvibrational energy (heat) - ill-defined ground state energyEffect worse at high pressures• For low pressure hollow cathode lamps (1-10 torr) 10-1-10-2 Å• For high pressure Xe lamps (>10,000 torr) 100-1000 Å (turnslines into continua!)(iii) Doppler broadening:Change in frequency produced by motion relative to detectorIn gas, broadens line symmetricallyDoppler broadening increases with T• At room T ~10-2-10-3 ÅTotal linewidth typically 0.01-0.1 ÅCEM 333 page 7.63. Doppler broadening Change in frequency produced by motion relative to detector (ii) Pressure broadening:Collisions with atoms/molecules transfers small quantities ofvibrational energy (heat) - ill-defined ground state energyEffect worse at high pressures• For low pressure hollow cathode lamps (1-10 torr) 10-1-10-2 Å• For high pressure Xe lamps (>10,000 torr) 100-1000 Å (turnslines into continua!)(iii) Doppler broadening:Change in frequency produced by motion relative to detectorIn gas, broadens line symmetricallyDoppler broadening increases with T• At room T ~10-2-10-3 ÅTotal linewidth typically 0.01-0.1 ÅCEM 333 page 7.63. Doppler shift: The wavelength of radiation emitted or absorbed by a rapidly moving atom decreases if the motion is toward a transducer, and increases if the motion is receding from the transducer. In flame, Doppler broadening is much larger than natural line width Δλλ=νcTemperature Effect on Atomic Spectra T changes # of atoms in ground and excite states. • Bolzmann equation • Effects on AAS, AFS, and AES )exp(00kTEggNNjjj−=)exp(00kTEggNNjjj−=• Nj/No%&=%1.72%x%10&4%(172%excited%atoms%for%each%106%atoms%in%ground%state)%!!!!!Sugges'ng!a!very!high!popula'on!of!the!ground!state!even!at!high!temperatures.!Temperature Effect on Atomic Absorption and EmissionSignal of atomic emission is dependent on the number of atoms in the excited state. Other Effects of T on Atomic Spectrometry:T changes # of atoms in ground and excited statesBoltzmann equationN1N0=P1P0exp −ΔEkT# $ % & # atoms in level transition energy E1-E0# levels at each energy Boltzmann constant 1.38x10-23 J·K-1Important in emission measurements relying on thermal excitationNa atoms at 2500 K, only 0.02 % atoms in first excited state!Less important in absorption measurements - 99.98 % atoms inground state!CEM 333 page 7.7Other Effects of T on Atomic Spectrometry:T changes # of atoms in ground and excited statesBoltzmann equationN1N0=P1P0exp −ΔEkT# $ % & # atoms in level transition energy E1-E0# levels at each energy Boltzmann constant 1.38x10-23 J·K-1Important in emission measurements relying on thermal excitationNa atoms at 2500 K, only 0.02 % atoms in first excited state!Less important in absorption measurements - 99.98 % atoms inground state!CEM 333 page 7.7Signal of atomic absorption and fluorescence is dependent on the number


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