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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
Jyväskylä 2008
InterferometerMichelson interferometer
M2
M1BSM1 Moving mirrorM2 Stationary mirrorBS Beam splitter
Detector for the HeNe laser
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Michelson interferometerVarious modifications
Jyväskylä 2008
Measured: Interferogram P( )
Michelson interferometer
�
P( �)
�
Jyväskylä 2008
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
Jyväskylä 2008
νξN = 1
∆
Fourier transformMeasuring points in optical displacement domain
�
FFT
��
NThe larger mirror movement the higher resolution in spectrum.
Equidistantpoints, �
Jyväskylä 2008
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
Jyväskylä 2008
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
Jyväskylä 2008
Full stop. Show a correlation table
Spectral analysis
Jyväskylä 2008
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)
Jyväskylä 2008
Traditional methodsGas cell
14182226 10 6 2
16 20 2412840 28
S D
a)
b)
Pfundt cell
White cell
Jyväskylä 2008
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
Jyväskylä 2008
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
Jyväskylä 2008
Reflection methodsInternal reflection (ATR)
Sample
E
E| |
E⊥
E 0
d p
Jyväskylä 2008
dn n np =
−λ
π α2 12
2 12sin ( / )
The penetration depth is larger at smallwavenumbers. The intensity varies!
Reflection methodsHarrick’s formula
Jyväskylä 2008
Reflection methodsUsually multiple reflection ATR
α
IRE
Sample
Sample
M 2
M 1
M 3
M 4
Jyväskylä 2008
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.