Jyväskylä 2008

MID-IRMatti Hotokka

Department of Physical ChemistryÅbo Akademi University

Jyväskylä 2008

Infrared regions

E [cm-1]Far infrared- FIR- experimentally tricky- difficult to interpret(vibrations, combinationbands, crystalvibrations, rotations, ...

10 400 4000 12 000

Near infrared- NIR- quantitative analysis- overtones of X-Hvibrations- in particular forindustrial processes- requireschemometrics

Mid-infrared- MIR- THE infrared region- molecular vibrations

Jyväskylä 2008

Dispersive

� Based on monochromator

� Old-fashioned, nobody usesFT-IR

� Based on interferometer

� Modern

� Requires a mathematical transformation of theinterferogram to spectrum (FourierTransformation)

Measurement methods

Jyväskylä 2008

FTIR spectrometer

Source

Interferometer

Sample

Detector

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InterferometerMichelson interferometer

M2

M1BSM1 Moving mirrorM2 Stationary mirrorBS Beam splitter

Detector for the HeNe laser

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Michelson interferometerVarious modifications

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Measured: Interferogram P( )

Michelson interferometer

�

P( �)

�

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S P d( ) ( ) cos( )ν ξ πνξ ξ=∞

20

Fourier transformation

InterferogramIR spectrum

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Fourier transformA single frequency (e.g., HeNe laser)

�

IntensityInterferogram

Spectrum

�

FT

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Fourier transformBroad spectral bandCentral burst

Measuringrange

Intensity

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νξN = 1

∆

Fourier transformMeasuring points in optical displacement domain

�

FFT

��

NThe larger mirror movement the higher resolution in spectrum.

Equidistantpoints, �

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Mirror movement Resolution [cm-1]1 mm 101 cm 11 dm 0.11 m 0.012.5 mm 42 cm 0.5

Resolution vs. Displacement

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νξMax = 1

∆

Fourier transformThe more observation points the higher resolution

FFT

�

0 �

NEquidistant points, �

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νξMax = 1

∆

Fourier transformationThe smaller � � the higher frequency

�

FFT

�

0 �

N

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S N M/ ∝

The more scans the better signal-to-noiseratio

Fourier transformation

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Full stop. Show a correlation table

Spectral analysis

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Traditional methods

� Kbr disc

� Liquid samples

� Gas samples

� Polymer films

� Nujol mull technique

� Photoacoustic spectroscopyReflection methodsMicroscopy

Measurement techniques

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Traditional methodsKbr disc technique

4000 3500 3000 2500 2000 1500 1000 5000

100

T %

Wavenumber (1/cm)

-C(O)-NH 2

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Usually a pair of mutually exclusivesolvents such as CCl4 and CS2

Most often interference

Traditional methodsLiquid cell

4000 3500 3000 2500 2000 1500 1000 5000

100

T %

Wavenumber (1/cm)

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Traditional methodsGas cell

14182226 10 6 2

16 20 2412840 28

S D

a)

b)

Pfundt cell

White cell

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Traditional methodsPhotoacoustic spectroscopy

Gas

Mic.

IR light

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Senkrecht and parallel polarizationMethods

� External reflection

� Reflection-absorption spectroscopy

� Grazing-incidence spectroscopy

� Diffuse reflectance

� Internal reflection

Reflectance methods

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Reflection methodsPolarization directions

Plane ofincidence

α α

A pA sR p

R s

z

y

x

0 10 20 30 40 50 60 70 80 900

102030405060708090

100

Angle of incidence

s

p

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Reflection methodsExternal reflection

750100012501500175020000.

0.25

0.5

0.75

k

Wavenumber (1/cm)

750100012501500175020001.2

1.4

1.6

1.8

n

Wavenumber (1/cm)

750100012501500175020000

5

10

15

R %

Wavenumber (1/cm)

The raw reflection spectrum

After Kramers-Kronigtransformation, the refractiveindex component

After Kramers-Kronigtransformation, theabsorbtion index component

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Reflection methodsTransflectance

Metal

Absorbing layer

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Reflection methodsGrazing angle spectroscopy

S polarization P polarization

4006008001000120014000

20

40

60

80

100

k

Wavenumber (1/cm)

1

7

Curve Angle1 15 o

2 30 o

3 45 o

4 60 o

5 70 o

6 80 o

7 85 o

Jyväskylä 2008

Reflection methodsDiffuse reflectance

4001400240034000.

0.25

0.5

0.75

K-M

Wavenumber (1/cm)400140024003400

0.

0.25

0.5

0.75

abs

Wavenumber (1/cm)

Cup

Screen

Mirror Mirror

Spherical reflector

Penetration depth

Raw spectrum After Kubelka-Munk transformation

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Reflection methodsInternal reflection (ATR)

Sample

E

E| |

E⊥

E 0

d p

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dn n np =

−λ

π α2 12

2 12sin ( / )

The penetration depth is larger at smallwavenumbers. The intensity varies!

Reflection methodsHarrick’s formula

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Reflection methodsUsually multiple reflection ATR

α

IRE

Sample

Sample

M 2

M 1

M 3

M 4

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Various objectivesTomographic samplesMappingImaging

Microscopy

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Most normal spectroscopic methods canbe used in micro scale, as well

� KBr microdisc

� Micro-ATR

� Diamond cell

� Grazin angle objective

� Confocal microscopy etc

MicroscopyVarious objectives

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MicroscopyThin layers (multilayer film, paint chips etc)

Resin matrix

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MicroscopyMapping

ScanningMicroscopeWide Field

Microscope

Illuminated area

CCD grid

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MicroscopyImaging

xx

yy

Data cube: the z direction is the spectral frequency axis. The spectrum is stored at every point.