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1.Fundamentalsofinfraredspectroscopy(FT-IR)
totheblackbodyradiation.TheNerstrodemitsradiationintherangefrom300
to20000nm(fromUVtoNIR),whereastheglobarprovideslightintherangeof
1100-40000nm(NIR,MIRandFIR).Thelightbeamfromthesourcereaches
theinterferometer,theniscollimatedthroughaseriesofmirrorsandfinally
directedtoasample.
103020Interferometer
IntheFourierTransformspectrometers,intensityofabsorptionisnotdirectlyre-
cordedasafunctionoffrequencybutinaformofaninterferogram,whichrepre-
sentsrelationshipbetweenasignalandtime(theopticalpathdiference).Toob-
tainaspectruminthefrequencydomainfromthetimedomain,theinterferogram
istransformedbyamathematicaloperationcalledtheFouriertransformation.Col-
lectiontimeofaspectrumisveryshortduetothefactthattheentirespectralrange
isrecordedsimultaneously.Anumberofspectraregisteredinthetimedomainare
averaged,andthentransformedbytheFouriertransformation.Advantagesofthe
useoftheinterferometerinanyspectrometerareasfollows:
1/Felgett’sadavantage(multiplexadvantage):recordingallwavelengthssimulta-
neouslyresultsinshorteningdatacollectiontimeandimprovementofsignal
(S)tonoiseratio(N)sinceS/N~n1/2(n-numberofspectra).Thisadvantage
isespeciallyimportantfortechniqueswhichgenerateweaksignal(e.g.aRaman
spectrometerwithlaserexcitationinNIRregion),
2/Jacquinot’sadvantage:thelackofslitsdoesnotlimitradiationpathway,
3/Connes’sadvantage:themovementofthemovingmirroriscontrolledoptically
byaHe-Nelaserandthisallowstoachievehighprecisionofthewavenumber
scaleinaspectrum.
4/easychangeofspectralresolution.
Aschematicofatypicalinterferometer(theMichelsoninterferometer)usedin
opticalspectroscopyisillustratedinFigure1.3.Theinterferometerisanelement
splittingtheelectromagneticradiationbeamanditsroleissimilartoagratinginthe
monochromator.Itconsistsoftwomirrors,positionedtoeachotheratanangleof
900,whenoneofthemisstationary,whereasthesecondmirrormovescontrolledby
aHe-Nelaser.Betweenthetwomirrors,abeamsplitterisplacedatanangleof450.
Theradiationfallingonthebeamsplitterisdividedintotwobeamstransferredto
bothmirrors.Thenlightbeamsarereflectedonthemirrorsandnexttheyarecol-
limatedonthebeamsplitter.Lightbeamsinterferetoeachotherdependingonthe
positionofthemovingmirror.Ifthetwomirrorsarelocatedatthesamedistance
fromthebeamsplitter,andhencetheopticalpathdiferenceiszero,thehighest
amplificationofallwavelengthsisobserved.Incaseofanydiferentpositionsof
themovingmirror,bothbeamsarenolongerin-phaseconsistentandonlythose
wavelengthsforwhichtheinterferenceconditionissatisfiedwillbeamplified.