i r Spectroscopy

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1 Infrared Spectroscopy Infrared Spectroscopy Spectroscopy – Radiation Terminology Wavenumber Wavenumber (ν)the number of waves in a unit of length or distance per cycle - reciprocal of the wavelength ν = 1/ = 1/λ Where: Where: λ in cm in cm Wavenumber is directly proportional to energy – higher number =higher energy Spectroscopy – Radiation Terminology Converting wavelength ( Converting wavelength (μm ) into ) into wavenumber (cm wavenumber (cm -1 ) and reverse ) and reverse cm cm -1 = 10,000/ = 10,000/μm μm = 10,000/cm m = 10,000/cm -1

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Transcript of i r Spectroscopy

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Infrared SpectroscopyInfrared Spectroscopy

Spectroscopy – Radiation Terminology

WavenumberWavenumber ((νν))–– the number of waves in a unit of length or distance per cycle - reciprocal of the wavelength

νν = 1/= 1/λλ Where:Where: λλ in cmin cm

Wavenumber is directly proportional to energy – higher number =higher energy

Spectroscopy – Radiation Terminology

Converting wavelength (Converting wavelength (µµmm ) into ) into wavenumber (cmwavenumber (cm--11) and reverse) and reverse

cmcm--11 = 10,000/= 10,000/µµmm

µµm = 10,000/cmm = 10,000/cm--11

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Infrared SpectroscopyInfrared SpectroscopyRegion Wavelength WavenumberRegion Wavelength Wavenumber

µµm m cmcm--11

NearNear 0.78 0.78 -- 2.512.51 12800 12800 –– 40004000MiddleMiddle 2.5 2.5 –– 5050 4000 4000 –– 200200FarFar 50 50 -- 10001000 200 200 –– 1010

The most useful IR region lies The most useful IR region lies between between 2.5 to 15 2.5 to 15 µµmm oror 4000 4000 –– 670 cm670 cm--11

Below 1500 cmBelow 1500 cm--11 complex spectrum complex spectrum –– Fingerprint for Fingerprint for moleculemolecule

IR IR -- ApplicationsApplications

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Infrared SpectroscopyInfrared Spectroscopy

Low energy Low energy –– vibrational and rotationalvibrational and rotational states; states; change in the dipole moment; change in the dipole moment; Polar molecules have regular fluctuation in Polar molecules have regular fluctuation in dipole momentdipole momentTo interact To interact -- radiation frequency have to match radiation frequency have to match molecule vibrational frequency molecule vibrational frequency When radiation is absorb it changes amplitude When radiation is absorb it changes amplitude of the molecular vibrationof the molecular vibrationSimilarly, the rotation of asymmetric Similarly, the rotation of asymmetric molecules occurmolecules occur

Infrared SpectroscopyInfrared Spectroscopy

Rotational transitionsRotational transitions –– very low energy very low energy required to cause change in rotational level required to cause change in rotational level ––corresponds to 100 cmcorresponds to 100 cm--11 or less (>100 or less (>100 µµm)m)Gases produce in this region, farGases produce in this region, far--infrared, infrared, discrete and well defined lines (absorption discrete and well defined lines (absorption maxima)maxima)Liquid and solids Liquid and solids –– continuum; intramolecular continuum; intramolecular collisions and interactions collisions and interactions

Vibrational Bending IRVibrational Bending IR

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Infrared SpectroscopyInfrared Spectroscopy

Vibrational Vibrational –– Rotational TransitionsRotational Transitions–– energy required is within midenergy required is within mid--infrared infrared

regionregion–– Spectrum of gases consists of a series of Spectrum of gases consists of a series of

closely spaced lines; that are many closely spaced lines; that are many rotational energy states for each rotational energy states for each vibrational statevibrational state

Infrared SpectroscopyInfrared Spectroscopy

RotationRotation is restricted in liquids and is restricted in liquids and solids solids

–– discrete vibrational/rotational discrete vibrational/rotational lines are not present; lines are not present;

-- broadened vibrational peaks broadened vibrational peaks only observedonly observed

Infrared SpectroscopyInfrared Spectroscopy

Molecular VibrationsMolecular Vibrations::Atoms in molecules fluctuate Atoms in molecules fluctuate

continuouslycontinuouslyInteraction in complex moleculesInteraction in complex molecules

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Infrared SpectroscopyInfrared Spectroscopy

Vibrations:Vibrations:StretchingStretching –– continuous change continuous change

in the distance between atoms in the distance between atoms along the bond within along the bond within moleculemolecule

BendingBending –– change in the angle change in the angle between two bonds; types: between two bonds; types: scissoring, rocking, wagging scissoring, rocking, wagging and twistingand twisting

Stretching IRStretching IR

IR Spectroscopy IR Spectroscopy -- AbsorptionAbsorption

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MidMid--range IR Spectrarange IR Spectra

Spectroscope Components Spectroscope Components

Absorption

FluorescencePhosphorescenceScattering

Emission and chemiluminescence

Infrared SpectroscopyInfrared SpectroscopyInfrared Source:Infrared Source:

Usually inert solid material electrically heated to Usually inert solid material electrically heated to 1500 1500 –– 2200K; maximum radiation 5000 2200K; maximum radiation 5000 –– 5900 cm5900 cm--1 1

(2 to 1.7 (2 to 1.7 µµm)m)Nernst GlowerNernst Glower –– rare earth oxides formed in to rare earth oxides formed in to

cylinder cylinder –– electrically heated to 1200K to electrically heated to 1200K to 2200K; 1 2200K; 1 –– 9 9 µµm m

Globar SourceGlobar Source –– silicon carbide rod; heated to silicon carbide rod; heated to 1300K to 1500K; spectrum similar to Nernst 1300K to 1500K; spectrum similar to Nernst with higher energy at the 5 with higher energy at the 5 µµmm

Mercury arcMercury arc –– for spectrum > 50 for spectrum > 50 µµmmTungsten Filament LampTungsten Filament Lamp –– near infrared 2.5 to 0.78 near infrared 2.5 to 0.78

µµmm

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TungstenTungsten--Halogen Lamps for NIRHalogen Lamps for NIR

Spectral Output of IR Radiation SourcesSpectral Output of IR Radiation Sources

IR Radiation Sources

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Spectroscopy Spectroscopy –– Radiation SourceRadiation Source

IR IR –– Nernst Glower Energy Spectrum Nernst Glower Energy Spectrum

Radiation Distribution of IR SourcesRadiation Distribution of IR Sources

Blackbody

Radiation

Characteristics

NIR

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Infrared SpectroscopyInfrared Spectroscopy

Infrared Transducers (Detectors)Infrared Transducers (Detectors)

ThermalThermal –– thermocouples (10thermocouples (10--66K); K); bolometers (thermistors)bolometers (thermistors)

Photoconducting Photoconducting –– semiconductor layer semiconductor layer exposed to IR exposed to IR –– change in resistance is change in resistance is proportional to radiation. Lead sulfite, proportional to radiation. Lead sulfite, cadmium tellurite, indium antimonidecadmium tellurite, indium antimonide

Spectral Response of Various Semiconductor DetectorsSpectral Response of Various Semiconductor Detectors

Operating temperature in K

Infrared SpectroscopyInfrared Spectroscopy

Pyroelectric – temperature dependent capacitors

- Dielectric material, pyroelectricmaterial such as triglycine sulfate between electrodes

- One electrode is expose to IR radiation - Change in temperature change charge

distribution in material and current is measured

Lasers – monochromatic radiation; tunable CO2 laser emits at 1100 – 900 cm-1 –pollution control

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Spectroscopy Spectroscopy –– Radiation SourceRadiation Source

Infrared SpectroscopyInfrared SpectroscopyInstrumentation:Instrumentation:

Classic Classic –– dispersive dispersive –– radiation from the radiation from the source is passed through the sample source is passed through the sample focused on monochromator and dispersed focused on monochromator and dispersed as continuous spectrum on detector; as continuous spectrum on detector; difference in intensity is recorded:difference in intensity is recorded:

T(%) = IT(%) = Iss/I/IRR x 100x 100

peaks of absorbance peaks of absorbance

MonochromatorsMonochromators

Prism and slitPrism and slitPrism and filterPrism and filterGrating and slitGrating and slitGrating and filterGrating and filter

This type of equipment is called This type of equipment is called --Dispersive spectrometer or Dispersive spectrometer or

spectrophotometerspectrophotometer

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SpectrometersSpectrometers

COCO22 and Hand H22O O –– absorption in IR absorption in IR ––sealed optic compartment sealed optic compartment –– filled or filled or flush continuously with dry nitrogen flush continuously with dry nitrogen Double beam systems to compensateDouble beam systems to compensateSingle beam will provide erratic Single beam will provide erratic spectra spectra –– humidity and temperaturehumidity and temperatureMirrors made of polished metalsMirrors made of polished metalsFor spectra moving grating or prismsFor spectra moving grating or prismsOnly about 5% of current instruments Only about 5% of current instruments

are dispersive typeare dispersive type

FT – Fourier Transformed

Mathematical transformation Mathematical transformation converting the signal measured as converting the signal measured as function of time into a function of function of time into a function of reciprocal time reciprocal time –– frequency frequency –– time time seriesseriesFast algorithm available to apply Fast algorithm available to apply this transformationthis transformation

Interferometer

We can measure the radiation intensity at all We can measure the radiation intensity at all wavelength simultaneously and can reconstruct wavelength simultaneously and can reconstruct it in the form of intensity vs. wavelength it in the form of intensity vs. wavelength (spectrum). The wavelength have to be (spectrum). The wavelength have to be encoded in a wellencoded in a well--defined manner as happen in defined manner as happen in interferometer. Wavelength and their interferometer. Wavelength and their corresponding intensities will overlap corresponding intensities will overlap –– the the overlap is used to plot spectrum.overlap is used to plot spectrum.In this case mathematical methods can be In this case mathematical methods can be applied to describe spectrum applied to describe spectrum –– Fourier analysis Fourier analysis -- deconvolutingdeconvolutingAn instrument that does this without a An instrument that does this without a monochromator is called a multiplex monochromator is called a multiplex instrument instrument

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Michelson Interferometer Michelson Interferometer -- FTIRFTIR

ZPDZeroPathlengthDifference

Mirror moves a distanceequal to 1/4λ at constantspeed

Reference

Wave Interaction in InterferometerWave Interaction in Interferometer

In PhaseConstructiveInterference

Out of PhaseDestructiveInterference

Fix and mobile mirrors are at equal distance from splitter or when the path difference is integral multiple of wavelength

Signal at detector

Mobile mirror moves 1/4λand beam will be combined 1800 out of phase

Radiation from splitter

Michelson InterferometerMichelson Interferometer

As the mirror moves, detector sees signal as a As the mirror moves, detector sees signal as a function of path length difference in the form function of path length difference in the form of cosine curveof cosine curve

The frequency of signal will be:The frequency of signal will be:

f = 2f = 2νν//λλ

Where: Where: f f –– frequencyfrequencyνν –– velocity of moving mirrorvelocity of moving mirrorλλ –– wavelength of radiationwavelength of radiation

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InterferometerInterferometerFor monochromatic radiation a plot of the For monochromatic radiation a plot of the signal (intensity) vs. difference in pathlength signal (intensity) vs. difference in pathlength form interferogram and is describe by cosine form interferogram and is describe by cosine functionfunctionThe interferogram it is a record of the The interferogram it is a record of the interference signal at the detector interference signal at the detector –– detector detector response in time response in time –– time domaintime domainIf sample absorbs radiation at a specific If sample absorbs radiation at a specific frequency frequency –– change of amplitude of change of amplitude of frequency will changefrequency will change

Interferometer Detector SignalInterferometer Detector Signal

Sampling the InterferogramSampling the Interferogram

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Interferogram Interferogram --

Infrared SpectroscopyInfrared Spectroscopy

Fourier Transform (FTIR)Fourier Transform (FTIR)Advantage Advantage –– entire spectrum obtained within a few entire spectrum obtained within a few

millisecondsmilliseconds–– hundreds of complete spectra can be hundreds of complete spectra can be

obtained on a sampleobtained on a sample–– spectrometer is recording spectrometer is recording interferogramsinterferograms, ,

which are later which are later deconvoluteddeconvoluted–– Better signal to noise ratios obtained than Better signal to noise ratios obtained than

in dispersive instrumentsin dispersive instruments

Infrared SpectroscopyInfrared Spectroscopy

Sample HandlingSample HandlingGasesGases –– examined without examined without

preparation; water have to be preparation; water have to be eliminated (Pathlength up to 40m eliminated (Pathlength up to 40m used)used)

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Reflectance Cell for IRReflectance Cell for IR

To extent radiation pathlength and attenuate signal

Infrared SpectroscopyInfrared Spectroscopy

Solid samplesSolid samples::–– used as solution in compatible solvent used as solution in compatible solvent

(CCl(CCl44; CS; CS22); variety of cells used made ); variety of cells used made from saltsfrom salts

–– Pressed to pellet form with KBr Pressed to pellet form with KBr ––scattering scattering –– particles; heat and moisture particles; heat and moisture sensitive material; salts selected based on sensitive material; salts selected based on absorption absorption –– Cut off: KBr 400cmCut off: KBr 400cm--11; ; CsICsI200cm200cm--11

–– Mulling Mulling –– dispersing substance in small dispersing substance in small amount of mineral oil (amount of mineral oil (NujolNujol) or ) or fluorinated hydrocarbons (mull). Mixture fluorinated hydrocarbons (mull). Mixture is placed between plates of the cell is placed between plates of the cell

Pellet Sample Die for IRPellet Sample Die for IR

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IR IR –– Sample HolderSample Holder

Liquid sampleLiquid sampleCuvetteCuvetteCardCard

IR IR CuvetteCuvette

Spectral Cards Absorption SpectraSpectral Cards Absorption Spectra

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IR IR –– Solvents LimitationSolvents Limitation

Horizontal Lines Indicates Useful Region

IR Optics Materials IR Optics Materials –– Mid IRMid IR

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Infrared Spectroscopy Infrared Spectroscopy -- PatternsPatterns

Infrared SpectroscopyInfrared Spectroscopy

Coupling of vibrations Coupling of vibrations -- bonds at bonds at singular central atom singular central atom –– change in the change in the characteristics of vibrationscharacteristics of vibrations

MidMid--Range IR SpectraRange IR Spectra

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Michelson InterferometerMichelson Interferometer

Solvents Properties for IRSolvents Properties for IR

Infrared SpectroscopyInfrared Spectroscopy

InstrumentationInstrumentation -- Fourier transform Fourier transform Interference patternInterference pattern –– interferogram interferogram ––complex waveform that contains complex waveform that contains information about all frequencies in the information about all frequencies in the measured infrared region. Using measured infrared region. Using mathematical operation termed a mathematical operation termed a Fourier Fourier transform transform to to deconvolutedeconvolute the interferogram the interferogram to produce a spectrum in frequency domain; to produce a spectrum in frequency domain; the same as in dispersive spectrometer the same as in dispersive spectrometer

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Interferogram FormationInterferogram Formation