FIU CHM 4130 - CHAPTER 6B_Xiao_Introduction_to_Spectroscopy_2018 (38 pages)

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CHAPTER 6B_Xiao_Introduction_to_Spectroscopy_2018



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Introduction to Spectroscopic Methods Chapter 6 Part 2 Optical Wave Theory 1 2 Light as Particles Eugene Hecht Optics Addison Wesley Reading MA 1998 3 hc E h E Packet Energy h Planck s Constant c Speed of Light meters seconds Wavelength meters Photon Theory 4 5 The Photoelectric Effect Douglas A Skoog et al Principles of Instrumental Analysis Thomson 2011 6 Cut off Observed results Current is propor2onal to the intensity of the radia2on V0 depends on the frequency of the radia2on V0 depends on the chemical composi2on of the coa2ng on the photocathode V0 is independent of the intensity of the incident radia2on Treat electromagnetic radiation as a stream of discrete particles Douglas A Skoog et al Principles of Instrumental Analysis Thomson 2013 7 KEm eV0 h KEm maximum kine2c energy Cut off h Planck s constant frequency work func2on a constant that is characteris2c of surface material E h KEm E h c KEm Douglas A Skoog et al Principles of Instrumental Analysis Thomson 2011 8 Quantum theory 9 Quantum theory Quantum theory by Planck 1900 Two important postulates 1 Atoms ions and molecules can exist in certain discrete states characterized by de nite amounts of energy When a species changes its state it adsorbs or emits an amount of energy exactly equal to the energy di erence between the states 2 When atoms ions or molecules absorb or emit radiaKon in making the transiKon from one energy state to another the frequency or the wavelength of the radiaKon is related to the energy di erence between the states by the equaKon E1 E0 h hc 10 Transi2on Rota2onal transi2on range of absorpKon far IR Vibra2onal transi2on near mid IR Electronic transi2on Electronic transi2on E0 to E1 E0 to E2 near UV VIS far UV 11 Interaction of Radiation and Matter Emission and chemiluminescence Nonradiative excitation processes 12 Interaction of Radiation and Matter Absorption 13 Interaction of Radiation and Matter Photoluminescence 14 Interaction of Radiation and Matter Raman Scattering 15 Emission of Radiation Excitation needs energy Particle or electron bombardment e emit X radiation n Emission n X X h Electrical currents or intense heat source produce ultraviolet visible or infrared radiation Beam of electromagnetic radiation generate fluorescence Exothermic chemical reaction produce chemiluminescence Douglas A Skoog F James Holler and Timothy A Nieman Principles of Instrumental Analysis 2011 16 Emission spectrum Saltwater in a ame Individual atoms Small molecules or radicals Incandescent particles 17 Line Spectra 1 E1 E0 h 1 hc E1 E0 Na as example Individual atoms well separated in a gas phase 18 Band Spectra Vibrational levels Small molecules and radicals 19 ConKnuum Spectra Produced when solid are heated to incandescence Blackbody Radia2on Thermal Radia2on 20 Blackbody RadiaKon A black body is a theoretical object that absorbs 100 of the radiation that hits it Therefore it reflects no radiation and appears perfectly black A blackbody is both a perfect absorber and emitter of radiation Common usage refers to a source of infrared energy as a blackbody when it s emissivity approaches 1 0 usually e 0 99 or better Important sources of infrared visible and long wavelength UV for analytical instruments 21 Blackbody Radiation Wien s Displacement Law 3 max 2 897 10 m K T Stefan Boltzman Law Temperature of body P T4 Surface area of body 5 6697 10 8 Wm 2K 4 Both max and radiation power P are related to TEMPERATURE and current Eugene Hecht Optics Addison Wesley Reading MA 1998 22 AbsorpKon of RadiaKon Is a quanKzed process The energy absorbed is released and transformed to other forms of energy for example to heat Results in excitaKon of a molecule to a higher energy state E E electronic E vibraKonal E rotaKonal 23 AbsorpKon of RadiaKon Douglas A Skoog F James Holler and Timothy A Nieman Principles of Instrumental 24 Analysis Saunders College Publishing Philadelphia 1998 Atomic absorption a few well defined frequencies Molecular absorption absorption band a series of closely spaced absorption lines 25 Atomic Absorption 285 nm 26 Molecular Absorption 27 RelaxaKon RelaxaKon processes permit its return of an atom or molecule excited by absorpKon of radiaKon to the ground state A NonradiaKve relaxaKon excitaKon energy kineKc energy by collision with other molecules B Fluorescence and phosphorescence relaxaKon 28 Non radiaKve relaxaKon Internal Conversion IC radiationless transition to lower state when vibrational energy levels match External Conversion EC radiationless transition to lower state by collisional deactivation Intersystem Crossing ISC transition with spin change e g S to T 29 RadiaKve relaxaKon Fluorescence emission not involving spin change e g S S T T efficient short lived 10 5 s Phosphorescence emission involving spin change T S improbable long lived 10 5 s 30 Fluorescence RelaxaKon Resonance uorescence F A S1 S0 fast 10 9 10 5 s common in atoms Non Resonance uorescence F A S1 S0 fast 10 9 10 5 s common in molecules F A Stokes shia red shiaed emission Non Resonance phosphorescence F A T1 S0 slow 10 5 10 s common in molecules 31 Energy level diagram 32 Spectra 33 Quantitative Aspects of Spectrochemical Measurements Radia2on power P The energy of the a beam of radia2on that reaches a given area per second Radiant energy electrical signal Radiation detector S kP S is a voltage or a current Dark current kd Small constant response of the detector in the absence of radia2on S kP kd 34 Emission Luminescence scabering and absorpKon S k c AdsorpKon Measurements Transmibance T P Po de ni2on Po incident light power P transmibed light power T P Po x 100 Absorbance A log T log P0 P Beer s Law A b c basis of quan2ta2on molar absorpKvity L mol 1 cm 1 b pathlength cm c concentraKon of the absorbing species mol L 1 physical law applicable under certain condi2ons 36 Single beam photometer for absorption measurements in the visible region An overview of electromagnetic radiation absorption 37 Assignment HW Chapter 6 1 2 5 6 10 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