1 Introduction to Spectroscopic Methods Chapter 6 - Part 2 Optical Wave Theory23 Light as Particles Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.4 Photon Theory E = Packet Energy h = Planck’s Constant c = Speed of Light (meters/seconds) = Wavelength (meters) λλ=ν=c Ehh5The Photoelectric Effect 6 Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2011Observed(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(7 Cut-off ν Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2013 Treat electromagnetic radiation as a stream of discrete particles.(((((((((((((((KEm=(eV0(=(hν(@(ω(((((KEm((@(maximum(kine2c(energy((((((((h(–(Planck’s(constant;((((((((ν"@(frequency;(((((((((ω(@(work(func2on(–(a(constant(that(is(characteris2c(of(surface(material;(((((((((((((E(=(hν"="KEm+(ω(((((((((E(=(h"(c/λ)"="KEm+(ω(((8 Cut-off ν Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2011Quantum theory 9Quantum theory Quantum'theory'by'Planck'(1900)'7 Two'important'postulates:''''''1.'Atoms,'ions,'and'molecules'can'exist'in'certain'discrete'states,'characterized'by'definite'amounts'of'energy.'When'a'species'changes'its'state,'it'adsorbs'or'emits'an'amount'of'energy'exactly'equal'to'the'energy'difference'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'ν'or'the'wavelength'λ of'the'radiaKon'is'related'to'the'energy'difference'between'the'states'by'the'equaKon:''E17E0'='hν'='hc/λ1011 Transi2on( Rota2onal(transi2on(Vibra2onal(transi2on(Electronic(transi2on(Electronic(transi2on(E0'to'E1'E0'to'E2'range'of'absorpKon' far7IR' near7'&'mid7'IR' near7UV/VIS' far7UV'Interaction of Radiation and Matter -Emission and chemiluminescence 12 Nonradiative excitation processes13 Interaction of Radiation and Matter -Absorption14 Interaction of Radiation and Matter -Photoluminescence15 Interaction of Radiation and Matter -Raman ScatteringEmission of Radiation 16 Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Instrumental Analysis, 2011. n Emission n X* → X + hν Excitation needs energy! • Particle or electron bombardment (e-) - emit X-radiation • Electrical currents or intense heat source - produce ultraviolet, visible, or infrared radiation • Beam of electromagnetic radiation - generate fluorescence • Exothermic chemical reaction - produce chemiluminescenceEmission'spectrum:'Saltwater'in'a'flame'17 Individual atoms Small molecules or radicals Incandescent particles18 Line'Spectra'Individual atoms, well separated, in a gas phase ν1= (E1− E0) / hλ1= hc / (E1− E0) Na as example19 Band'Spectra'Small molecules and radicals Vibrational levelsConKnuum'Spectra'• Produced(when(solid(are(heated(to(incandescence.((• Blackbody(Radia2on((Thermal(Radia2on)((20Blackbody'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. 2122 Wien’s Displacement Law λmax = 2.897 × 103 m KTEugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998. Stefan-Boltzman Law Temperature of body P = σΑT4 Surface area of body σ = 5.6697 × 10-8 Wm-2K-4 Blackbody Radiation Both λmax and radiation power (P) are related to TEMPERATURE and current!AbsorpKo n'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'23AbsorpKon'of'RadiaKon'24 Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.25 Atomic absorption - a few well-defined frequencies Molecular absorption - absorption band (a series of closely spaced absorption linesAtomic Absorption 26 285 nm27 Molecular AbsorptionRelaxaKon''• 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''2829 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) Non7radiaKve'relaxaKon'30 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 RadiaKve'relaxaKon'Fluorescence'RelaxaKon''• Resonance(fluorescence(– λF'='λA''(S1''''S0,'fast'1079'–'1075's,'common'in'atoms')'• Non@Resonance(fluorescence(– λF'≠'λA'(S1''''S0,'fast'1079'–'1075's,'common'in'molecules')'– λF'>'λA''Stokes'shia'(red'shiaed'emission)'• Non@Resonance(phosphorescence(((((((@@(λF'≠'λA'(T1''''S0,'slow'1075'–'10's,'common'in'molecules')'(((3132 Energy-level diagram33 SpectraQuantitative 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(– S(=(kP(( (S(is(a(voltage(or(a(current(• Dark(current((kd)(– Small(constant(response(of(the(detec tor (in(the(absence(of(radia2on(– S(=kP(+(kd(34 Radiation detectorEmission,'Luminescence,'scabering'and'absorpKon'S = k’cAdsorpKon'Measurements'• Transmibance'T'='P/Po'(defini2on)(• Po'7'incident'light'power'• P'7'transmibed'light'power'%T'='P/Po'x'100'%''•