26
1 Chapter 3: IR spectroscopy 3.1 Introduction 3.2 Instrumentation 3.3 Technique to improve sensitivity and applicability 3.4 Hyphenated techniques 3.1 Introduction a. Molecular vibrations Electromagnetic field

Chapter 3: IR spectroscopy - nchu.edu.twweb.nchu.edu.tw/~jyisy/courses.files/Advance AC/Chapter 3 - IR... · Chapter 3: IR spectroscopy ... is reduced mass and defined as : =

Embed Size (px)

Citation preview

  • 1

    Chapter3:IRspectroscopy

    3.1Introduction3.2Instrumentation3.3Techniquetoimprovesensitivityandapplicability3.4Hyphenatedtechniques

    3.1Introductiona.Molecularvibrations

    Electromagneticfield

  • 2

    E

    interatomic distancer

    r2Dissociationenergy

    Anharmonic potential(Morsetype)

    harmonicpotential(Hookslaw)

    Harmonicoscillator:Viv =hvi (vi +1/2)wherehisPlancksconstant,vi isthefundamentalfrequencyoftheparticularmodeandvi isthevibrationalquantumnumberoftheith mode.Themoleculeisonlypromotedtotheexcitedstatifitsdipolemoment,m,changesduringthevibration,i.e.providedthat(m/Q) 0.;(Qisdistanceinnormalcoordinate)

    12

    k

    vm=

    Classicalmodelisreducedmassanddefinedas: =(m1.m2)/(m1 +m2)

    12

    k

    vm=

    Classicalmodelisreducedmassanddefinedas: =(m1.m2)/(m1 +m2)

  • 3

    For actual vibrations, anharmonic potential function is used:Viv = h vi (vi +1/2)+ h vi xi (vi +1/2)2where xi is the anharmonicity constant and is dimension-less

    (typically between 0.001 to 0.02)

    Selection rules for vibrational transitions; v = 1 (harmonic oscillator)

    Anharmonic oscillator causes: i. Overtone (v = 2), tripletone (v = 3),

    ii. E becomes smalleriii. Different vibration mode can interact each other to produce

    so called combinational bands or difference bands.

    Fermi resonance: It occurs when an overtone or combination band absorbs at approximately the same frequency of a fundamental mode involving the same atoms.

    The intensity of fundamental bands in IR spectra is proportional to ( / Q)2

    b.VibrationrotationspectroscopyRotationalenergyisusuallyassociatedwithvibrationalenergy.Thetransitionofrotationalenergyisrarelyobservedi.forlargemoleculesinvaporphase(becauserotationalenergytransitionsaretooclosetoberesolved).

    ii.Forcondensedphase(becausecollisionsoccuratagreaterratethantherotationalenergy)

    Therotationalenergylevelsofdiatomicmolecules(rigidrotor)arecharacterizedbyasinglerotationalquantumnumber,J.EJ =BJ(J+1)whereBistherotationalconstantanddefinedasB=h/(82Ic),whereIisthemomentofinertiaofthemoleculeandcisthevelocityoflight.

    E0 = 0E1 = 2B

    E2 = 6B

    E3 = 12B

    E4 = 20B

    J=0

    J=3

    J=1J=2

    J=5

    J=4

    PopulationsforCO(%)

    1.00

    6.25

    2.944.73

    8.29

    7.45

  • 4

    TheselectionrulesforrotationalenergytransitionisJ=1.E=EJ+1 EJ =B(J+1)(J+2) BJ(J+1)=2B(J+1)

    2B2B

    01(2B)

    12(4B)23(6B)

    34(8B)45(10B)

    Rotationalspectra

    Energytransitions

    2B2B2B

    Boltzmanns distributionlawNJ/No =(2J+1)exp(E/KT)2J+1isthenumberofdegeneracyofJth leveld(NJ/No )/dJ =0,whichgivesJmax =(KT/2hB)1/2 1/2

    Diatomicmolecule,XY,haveasinglefundamentalvibrationalmode,ofwavenumber,o,whichisonlyactiveifXY.Simultaneousaccomplishedwithrotationalenergytransitionwith vibrationaltransition,i.e. =1andJ=1.Thus,arigiddiatomicmoleculeconsistsofaseriesofequallyspacedlinesaboveandbelow,correspondingtoJ=+1(Rbranch)andJ=1(Pbranch).BecauseJ 0,noabsorptionlineato.Exceptionrule:moleculesexhibitelectronicangularmomentumin thegroundelectronicstate.J=0transitionisallowedinthistypeofmolecules,i.e.nitricoxide(unpairedelectroningroundstate)exhibitsJ=0transition(Qbranch)

    Rotationalspectra

    Qbranch

    PbranchRbranch

    increaseofenergyDJ=+1

    decreaseofenergyDJ=1

    DJ...2,1,0,1,2...Branch...O,P,Q,R,S

  • 5

    3.2Instrumentationi.DispersiveIRspectrometry

    Withsingleelementdetector

    Witharraydetector

    a.OpticalLayout

    ii. FTIRspectrometryWithsingleelementdetector

  • 6

    i.IRsourceBlockbodyradiations:Nernst Glower:rareearthoxides,12mmi.d.,20mmlong,heatedto1200~2000KGlobar Source:SiliconCarbideRod,5mmi.d.50mmlong,heatedto1300~1500KIncandescentwire:CrwireorRhodiumwireheatedto1000K,weakerintensity

    shorterlifetime

    SynchrotronIRsourceCO2Laser:~10.6m(tunable900~1100cm1,quantitativeanalysisonly)QuantumCascadeIRLaser(FirstdesignedatBellLaboratoriesin1994)

    He/Ne laser632.8nmisusedintheFTIRspectrometerasreference

    b.Components

    QuantumCascadeIRLaser(FirstdesignedatBellLaboratoriesin1994)

    Inquantumcascadestructures,electronsundergointersubband transitionsandphotonsareemitted.Theelectronstunneltothenextperiodofthestructureandtheprocessrepeats.

    Interband transitionsinconventionalsemiconductorlasersemitasinglephoton.

  • 7

    ii.IRDetectorPrinciple:Aftermaterialsabsorbvibrationalenergy,theirtemperaturewillbeincreasedandthe

    propertiesofthematerialswillbealtered.Vibrationalenergyisweakandonlyaroundmwtonw.Toincreasethelargechangesof

    temperatureonthesensingmaterials,theelementshouldbesmallinsizeandthermalnoisefromenvironmentshouldbeavoid.

    Types:Thermocouple,bolometer,pyroelectric detector,photoconductivitydetector Thermocouple

    Fusedtogether

    CuwireConstantanwire

    Afterirradiation,apotentialcanbebuiltandmeasured,whichexhibitslinearrelationshipwithradiantpower(ForIR,bismuth/Antimonypairofwires)

    Potentialdifference

    Potentialdifference

    Ref. Thermopiles:seriesofthermocouples

    BolometerChangeinresistanceasafunctionoftemperatureMaterials:Ni,Pt(bolometer);semiconductor(thermistors)

    Pyroelectric detectorsSensingelementisformedbyplacingpyroelectric materialsbetween

    twoelectrodes.Afterabsorbvibrationalenergy,theinducedpotentialisvariedandrelatedtothetemperatureofthesensingelement.

    Pyroelectric material()

    electrodes

    Pyroelectric material(i.e.triglycinesulfate,TGS[(NH2CH2COOH)3H2SO4]orDeuterated TGS(DTGS)

    +

    +

    + + + + + + +Heatingwhileapplied

    electricfieldtothematerialAfterelectricfieldisremoved,moleculesremainpolarizations

  • 8

    + + + +

    potential1+

    + + +

    +

    potential2+

    potentialwaspartiallycancelled

    Sensingelementshouldbekeptattemperaturelowerthancuriepoint

    topreventlosttheirresidualpolarization(forTGS,curiepointis

    47oC

    Triglycine sulfate(TGS),curiepoint49oC,Deuterated triglycine sulfate(DTGS),curiepoint:60oCDeuterated lanlanine dopedtriglycine sulfate(DLATGS),curiepoint,74oCLithiumtantalate (LiTaO3),lowerpyroelectric coefficientthanTGS,curiepoint,620oC

    PhotoconductivitydetectorPrinciple:conductivityischangedduetoexcitationofelectrons

    byradiationtoitsconductionband

    Conductionband

    Valenceband

    Eg

    Semiconductorsareusedintrinsic:Sx,Se,Sb,Pb,Cd,Ga,Indoped:PbS,CdS,CdSe,InGaAs,PbS,InAs,PtSi,PbSe,InSb,HgCdTe,dopedGe,dopedSi,

    Theenergygapsaresmallthatvibrationalenergycanexcitetheelectrons

    Semiconductor(conductivityincreaseasradiantpowerincreaseOrresistancedecreaseasradiantpowerincrease

  • 9

    ChargetransferdeviceSimilartophotographicfilmtoaccumulatee generatedbyradiationsandcanbetwodimensional

    244

    388 1pixel

    ntypeSisubstrate

    5V 10V + + 5V10V + + 5V

    10V + + 5V10V + +

    v1 v2+++++++

    e

    Step1:Chargeformationandintegration

    Step2:measureV1(blank)

    Step3:measureV2

    Step4:Removecharge

    ++++++ ++++++ ++++++

    Chargeinjectiondevice(CID),usentypeSi (collectcharges)

    Chargecoupleddevice(CCD),useptypeSi (collectelectrons)

    Thermographic Camera:

    Uncooled detectorsaremostlybasedonpyrielectric andferroelectricmaterialsormicrobolometer technology.Thematerialareusedtoformpixelswithhighlytemperaturedependentproperties,whicharethermallyinsulatedfromtheenvironmentandreadelectronically.

  • 10

    WAVELENGTH RANGE FOR IR MATERIALS - MICRONS & CM-1

    AMTIR Barium FluorideBK-7 Borosilicate Crown Glass Cadmium Manganese Mercury TellurideCadmium Manganese Telluride Calcium FluorideCesium Iodide Fused Silica IR GradeFused Silica UV Grade GermaniumPotassium Bromide Potassium ChlorideSapphire SiliconSilver Chloride SSodium ChlorideThallium Bromoiodide Zinc SelenideZinc Sulfide Magnesium Fluoride

    ii.OpticalMaterials

  • 11

    Insolubleinwater.1.5at0.4 4mInfrasil QuartzSiO2Slightlysolubleinacids2.201 14mIrtran2(ZincSulfide)ZnShygroscopic,1.460.18 20mPotassiumChlorideKClHardandbrittle.4.02 11.5mGermaniumGeSensitivetothermalshock.1.37 1.380.11 7.5mMagnesiumFluorideMgF2Insolubleinwater.2.41 18mZincSelenideZnSe

    Insolubleinwater.2.370.5 35mThallium/Bromide/IodideKRS5Insolubleinwater.1.741.5 50mCesiumIodideCsIInsolubleinwater.1.460.2 11.5mBariumFluorideBaF2

    Insolubleinwater.1.400.15 9mCalciumFluorideCaF2Corrosivetometals.2.00.4 23mSilverChlorideAgClHygroscopic.1.530.25 25mPotassiumBromideKBrHygroscopic.1.520.25 15mRockSaltNaCl

    IndexofRefraction

    TransmissionRangeMaterials

    SELECTIONGUIDEFORINFRAREDTRANSMITTINGMATERIALS

    iii.AccessoryGas Cell

  • 12

    LiquidCell Spacer(leadorTeflon)

    KBrDrilledKBr

    Gasket

  • 13

    ReflectionAbsorptionAccessory

    Penetrated radiation

    ReflectiveSample or thin sample

    Diffuse reflectance infrared Fourier Transform Spectrometry(DRIFTs, 1978)

    Diffused radiation

    Ref. KBr powerSample: ~ 5% in KBr

    f(R) = (1- R)/2R= K/S = 2.303 c

    where R = Rs / RKBr

    Function of Kubelka-Munk

    instrumentation

    mirror

    Samplecup

    detectorIR

  • 14

    Wig-L-Bug

    Diffuse-reflectance accessory

    Internalreflection(Attenuatedtotalreflection,ATR)accessory

    dp =

    2n1[sin2 (n2/n1)2] 1/2evanescent wave

    IR in IR out

    Sample

  • 15

    3.3TechniquetoimprovesensitivityandapplicabilityA.SEIRA,B.Emission,C.Polarization

  • 16

    Electromagneticfield

    + + + + + + + +____

    ___ _

    ++++++++________++++++++________

    ++++++++_ _ _ __ _ _ _++++++++________

    +++

    _ __

    ++++++_ _ _ _ _ _

    :Au,Ag,Cu,Pt,In,Hg,Sn,Cd,Zn,polarizability dependent

    OpticalpropertiesSurfaceplasmonic effectEnhancementeffectinRamanandIR

    Link, S et. al., J. Phys. Chem. B 1999, 103, 4212.

    Wavelength (nm)400 600 500 700

    Cex

    t. (n

    orm

    aliz

    ed)

    600 500 700

    1.0

    0.4

    0.2

    0.0

    0.6

    0.8

    400 Wavelength (nm)

    Au NPs

    A.SurfaceenhancedInfraredAbsorptionSpectroscopy

    ElectromagneticField(EF)effect

    ~5nm

    EffectiveDistancefromSurfaceofNanoparticle

    ChargeTransfer(CT)effect

    S

    NH2Electron

  • 17

    + _+ _

    No Net Dipole Moment Produced _ +

    _ +

    + _+ _

    Enlarge Dipole MomentPerpendicular to Surface

    _

    +

    _ +

    SurfaceEnhancedInfraredAbsorption(SEIRA)Spectroscopy

    SurfaceSelectionRuleinSEIRA

    Electricfield

    S

    N+O O Electricfield

    S

    N+O O

    NO2AsymmetricStretchingNO2symmetricStretching

    Enhanced No enhancement

  • 18

    1000

    839

    1500 Wavenumber (cm-1)

    250 ng/cm2

    80 g/cm2

    NO2 Sym. Str., 1339 cm-1

    NO2 Asym. Str., 1503 cm-1

    15771594 1101 854

    Conventional

    Enhanced spectrum

    SH

    N+O O-

    Surface Enhanced Infrared absorption spectroscopy (SEIRA)

    B.EmissionIRspectroscopy

    Principle

    sample

    heater

    emission(a)

    v0 (hingly populated)

    v =1

    v =2 (weakly populated)

    Fundamental overtone

    ~2

    (b)

    v =1

    v =2

    v =0

    ~2(c)

    0

    100

    T

    0

    100

    E

    cm-1 cm-1

    (d)

    absorption

  • 19

    IR source

    Emission accessory

    Extra mirror for emissionbeamsplitter

    interferometer

    detectorsample compartment

    emission sample holder

    IR source

    To interferometer

    Equipment

  • 20

    C.Polarization

  • 21

    3.4HyphenatedtechniquesA.Flowinjectionanalysis FTIRB.Chromatography FTIRC.ThermalGravimetry FTIRD.Electrochemistry FT-IR

  • 22

    A. Flow-injection analysis FT-IR

    B. HPLC-FTIR

  • 23

    C.ThermalGravimetry FTIR

  • 24

    D.Electrochemistry FTIR

  • 25

    Transmission Cells Using Optically Transparent or Perforated Electrodes

    External Reflection-Absorption SEC Cells

  • 26

    Working electrode designs for external reflectance SEC cells. (a) Single element, (b) temperature controlled and (c) multielectrode assembly