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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
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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