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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
Jyväskylä 2008
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, �
Jyväskylä 2008
νξMax = 1
∆
Fourier transformationThe smaller � � the higher frequency
�
FFT
�
0 �
N
Jyväskylä 2008
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
Jyväskylä 2008
Traditional methodsKbr disc technique
4000 3500 3000 2500 2000 1500 1000 5000
100
T %
Wavenumber (1/cm)
-C(O)-NH 2
Jyväskylä 2008
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
Jyväskylä 2008
Senkrecht and parallel polarizationMethods
� External reflection
� Reflection-absorption spectroscopy
� Grazing-incidence spectroscopy
� Diffuse reflectance
� Internal reflection
Reflectance methods
Jyväskylä 2008
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
Jyväskylä 2008
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
Jyväskylä 2008
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
Jyväskylä 2008
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
Jyväskylä 2008
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.
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