This document discusses the history and theory of infrared spectroscopy. Infrared spectroscopy was first developed in the 1880s and became a routine analytical technique between 1903-1940. Fourier transform infrared spectroscopy, which provides greater sensitivity and resolution, became common in the 1980s. Infrared spectroscopy is based on the absorption of infrared light by molecules and excitation of vibrational and rotational energy levels. It can be used to identify functional groups and study hydrogen bonding based on how infrared frequencies shift. Fourier transform infrared spectroscopy uses an interferometer to encode the infrared spectrum as an interference pattern that can then be converted to a spectrum through Fourier transformation.

1. Lecturer: Mr. Kipkemoi
Infrared and Microwave Spectroscop
y

2. The History of Infrared Spectroscopy
 Infrared (IR) Spectroscopy:
– Herschel first recognized the existence of I
R and its relation to the heating of water
– First real IR spectra measured by Abney an
d Festing in 1880’s
– IR spectroscopy became a routine analytica
l method as spectra were measured and ins
truments developed from 1903-1940 (espec
ially by Coblentz at the US NBS)
– IR spectroscopy through most of the 20th ce
ntury is done with dispersive (grating) instru
ments, i.e. monochromators
– Fourier Transform (FT) IR instruments beco
me common in the 1980’s, led to a great inc
rease in sensitivity and resolution
W. Abney, E. R. Festing, Phil. Trans. Roy. Soc. London, 1882, 172, 887-918.
J. F. W. Herschel
W. Coblentz

3. Infrared Spectral Regions
 IR regions are traditionally sub-divided as follows:
Region Wavelength
(), m
Wavenumber
(), cm-1
Frequency (), Hz
Near 0.78 to 2.5 12800 to 4000 3.8 x 1014 to 1.2 x 1014
Mid 2.5 to 50 4000 to 200 1.2 x 1014 to 6.0 x 1012
Far 50 to 1000 200 to 10 6.0 x 1012 to 3.0 x 1011
After Table 16-1 of Skoog, et al. (Chapter 16)

4. What is a Wavenumber?
 Wavenumbers (denoted cm-1) are a measure of frequency
– For an easy way to remember, think “waves per centimeter”
 Relationship of wavenumbers to the usual frequency and w
avelength scales:
Image from www.asu.edu


10000
1 

cm
 Converting waveleng
th () to wavenumbe
rs:

5. Vibrational Spectroscopy: Theory
 In IR spectroscopy, IR photons is absorbed and converted
by a molecule into energy of molecular vibration
 A simple harmonic oscillator is a mechanical system con
sisting of a point mass connected to a massless spring.
The mass is under the action of a restoring force proporti
onal to the displacement of the particle from its equilibriu
m position and the force constant k of the spring (under t
he classical Hooke’s law)
m1 m2
r

6.   m
h
v
E 
2
1


m is the natural frequency of the oscillator (a.k.a. the fundamental vibrational wavenumber)
k is the Hooke’s law force constant (now for the chemical bond)
u
k
m


2
1

v is the vibrational quantum number
h is Planck’s constant
 v must be a whole number, so:
 The potential energy function is:
2
2
1
)
(
)
( e
HO r
r
k
r
E 
 or 2
2
2
1
)
(
)
2
(
)
( e
m
HO r
r
c
r
E 
 





k
h
h
E m
2





k
12
10
3
.
5 


and
(wavenumbers)
r is the distance (bond distance)
re is the equilibrium distance
2
1
2
1
m
m
m
m



Vibrational Spectroscopy: Harmonic Oscillator
 The quantum version of the classical oscillator (spring):

7. Vibrational Spectroscopy: Theory
Figure from Skoog et al.
 The potential energy of vibrations fits the parabolic functio
n fairly well only near the equilibrium internuclear distance
.
 The anharmonic oscillator model is a more accurate descr
iption for the overall motion

8. Anharmonic Corrections
 Anharmonic motion: when the restoring force is not propor
tional to the displacement.
– More accurately given by the Morse potential function than by the
harmonic oscillator equation.
– Primarily caused by Coulombic (electrostatic) repulsion as atoms
approach
 Effects: at higher quantum numbers, E gets smaller, an
d the ( = +/-1) selection rule can be broken
– Double ( = +/-2), triple ( = +/-3), and higher order transitions
can occur, leading to overtone bands at higher frequencies (NIR)
2
)
(
)
1
(
)
( e
r
r
a
e
Morse e
hcD
r
E 


 De is the dissociation energy
e
m
hcD
c
a
2
)
2
( 2





9. Rotational Spectroscopy: Theory
 Vibrational spectra of condensed phases appear as bands
rather than lines.
 When vibrational spectra of gaseous molecules are observ
ed under high-resolution conditions, each band can be fou
nd to contain a large number of closely spaced component
s resulting from rotational energy levels.
m1 m2
r0

10. Rotational Spectroscopy: Theory
 Rotational energy levels can be de
scribed as follows:
R. Woods and G. Henderson, “FTIR Rotational Spectroscopy”, J. Chem. Educ., 64, 921-924 (1987)
D
J
B
J
J 3
)
1
(
)
1
(
)
( 




c
r
h
B 2
0
2
8
/ 


2
3
/
4 c
B
D 

Where:
c is the speed of light
k is the Hooke’s law force constant
r0 is the vibrationally-averaged bond length
The rotational constant:
The centrifugal distortion coefficient:
u
k
c
c


2
1

Example for HCl:
B = 10.4398 cm-1
D = 0.0005319 cm-1
r0 = 1.2887 Å
 is the reduced mass
h is Planck’s constant
c = 2990.946 cm-1 (from IR)
k = 5.12436 x 105 dyne/cm-1
For J = 0, 1, 2, 3…

11. The selection rules allow
transitions with  = +1 a
nd J = ±1 (the transition
with J = 0 is normally n
ot allowed except those
with an odd number of el
ectrons (e.g. NO)).
P R
Rovibrational Spectroscopy: Theory
 A vibrational absorption transition from  to +1 gives ris
e to three sets of rotational lines called branches:
 Lower-frequency P branch: =1, J=-1
 Higher-frequency R branch: =1, J=+1
 Q branch: branch: =1, J=0

12. Molecular Vibrations: Total Modes
 How many vibrational modes are possible in a molecule?
A molecule has as many degrees of freedom as the total d
egree of freedom of its individual atoms. Each atom has thr
ee degrees of freedom (corresponding to the Cartesian coo
rdinates), thus in an N-atom molecule there will be 3N degr
ees of freedom.
 Translation: the movement of the entire molecule while the
positions of the atoms relative to each other remain fixed.
There are 3 degrees of translational freedom.
 Rotational transitions: interatomic distances remain consta
nt but the entire molecule rotates with respect to three mut
ually perpendicular axes. There are 3 degrees of rotational
freedom in a nonlinear molecule and 2 degrees in a linear
molecule.

13. Vibrational Modes and IR Absorption
 For a molecule with N atoms the
number of vibrational modes is:
– Linear: 3N – 5 modes
– Non-linear: 3N – 6 modes
 Types of vibrations:
– Stretching
– Bending
 Examples:
– CO2 has 3 x 3 – 5 = 4 normal modes
Symmetric
No change in dipole
IR-inactive
Asymmetric
Change in dipole
IR-active
Scissoring
Change in dipole
IR-active
 IR-active modes require molecular dipole moment change
s during rotations and vibrations.

14. Vibrational Coupling
 Vibrations in a molecule may couple – changing each oth
er’s frequency.
– In stretching vibrations, the strongest coupling occurs between vi
brational groups sharing an atom
– In bending vibrations, the strongest coupling occurs between gro
ups sharing a common bond
– Coupling between stretching and bending modes can occur when
the stretching bond is part of the bending atom sequence.
– Interactions are strongest when the vibrations have similar freque
ncies (energies)
– Strong coupling can only occur between vibrations with the same
symmetry (i.e. between two carbonyl vibrations)

15. Vibrational Modes: Examples
 IR-activity requires
dipole changes du
ring vibrations!
 For example, this i
s Problem 16-3 fro
m Skoog (6th editi
on):
Inactive
Active
Active
Active
Inactive
Inactive
Active

16. IR Spectra: Formaldehyde
 Certain types of vibrations have distinct IR frequencies – hence the ch
emical usefulness of the spectra
 The gas-phase IR spectrum of formaldehyde:
Formaldehyde spectrum from: http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm#ir2
Results generated using B3LYP//6-31G(d) in Gaussian 03W.
 Tables and simulation results can help assign the vibrations!
(wavenumbers, cm-1)

17. IR Frequencies and Hydrogen Bonding Effects
 IR frequencies are sensitive to hydrog
en-bonding strength and geometry (pl
ots of relationships between crystallo
graphic distances and vibrational freq
uencies):
G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford, 1997.

18. Instrumentation for Vibrational Spectroscopy
 Two IR Absorption methods:
– Dispersive methods: Scanning of wavelengths using a grating (c
ommon examples are double-beam, like a spectrometer discusse
d in the optical electronic spectroscopy lecture).
– Fourier-transform methods: based on interferometry, a method of
interfering and modulating IR radiation to encode it as a function
of its frequency.
Radiation
Source
Sample
Wavelength
Selector
Detector
(transducer)
Radiation
Source
Interferometer Sample
Detector
(transducer)

19. IR Emission Spectroscopy
 Emission is seldom used for chemical analysis
 The sample must be heated to a temperature mu
ch greater than its surroundings (destroying mole
cules)
 IR emission is widely used in astronomy and in re
mote sensing applications on heated materials.

20. Fourier Transform IR Spectroscopy: Rationale
 Advantages of FT methods:
– The Jacqinot (throughput) advantage: FT instruments have fe
w slits, or other sources of beam attenuation
– Resolution/wavelength accuracy (Connes advantage): achieve
d by a colinear laser of known frequency
– Fellgett (multiplex) advantage: all frequencies detected at onc
e, signal averaging occurs and thermal noise grows more slowl
y than signal (good with IR detectors)
 These advantages are critical for IR spectroscopy
 The need for FT instruments is rooted in the detector
– There are no transducers that can acquire time-varying signals
in the 1012 to 1015 Hz range – they are not fast enough!
 Why are FT instruments not used in UV-Vis?
– The multiplex disadvantage (shot noise) adversely affects sign
al averaging – it is better to multiplex with array detectors (suc
h as the CCD in ICP-OES)
– In some cases, there are technical challenges to building interf
erometers with tiny mirror movements

21. Inteferometers for FT-IR and FT-Raman
 The Michelson interf
erometer, the produ
ct of a famous physi
cs experiment:
 Produces interfere
nce patterns from
monochromatic an
d white light
Figures from Wikipedia.org

22. Inteferometers
 For monochromatic radia
tion, the interferogram lo
oks like a cosine curve
 For polychromatic radiati
on, each frequency is en
coded with a much slow
er amplitude modulation
 The relationship betwee
n frequencies:
 Example: mirror rate = 0.3 cm/s modulates 1000 cm-1 light at 600 Hz
 Example: mirror rate = 0.2 cm/s modulates 700 nm light at 5700 Hz

c
v
f M
2

Where:
 is the frequency of the radiation
c is the speed of light in cm/s
vm is the mirror velocity in cm/s

23. FTIR Spectrometer Design
Michelson
Interferometer
IR Source
Sample
Moving Mirror
Fixed Mirror
Beamsplitter
Detector
Interferogram
Fourier Transform - IR Spectrum
 It is possible to build a detector that detects multiple frequen
cies for some EM radiation (ex. ICP-OES with CCD, UV-Vis
DAD)
 FTIR spectrometers are designed around the Michelson inte
rferometer, which modulates each IR individual frequency wi
th an additional unique frequency well suited to the time res
ponse of IR detectors:

24. The Basics of the Fourier Transform
 The conversion from time- to frequency domain:
50 100 150 200 250
-1
-0.5
0.5
1
50 100 150 200 250
0.5
1
1.5
2
FT
50 100 150 200 250
-1.5
-1
-0.5
0.5
1
1.5
2
50 100 150 200 250
0.5
1
1.5
2
2.5





1
0
/
2
1N
k
N
ikn
k
n e
d
N
f 



b
a
d
t
f
t
K
g 

 )
(
)
,
(
)
( 1
)
t
exp(
)
,
( i
ω
t
K 


Continuous:
Discrete:
FT
Time domain Frequency domain

25. IR Sampling Methods: Absorbance Methods
 KBr/CsI pellet: a dilute (~1%) amount of sample in the halide matrix i
s pressed at >10000 psi to form a transparent disk.
– Disadvantages: dilution required, can cause changes in sample
 Mulls: Solid dispersion of sample in a heavy oil (Nujol)
– Disadvantages: big interferences
 Salt plates (NaCl): for liquids (a drop) and small amounts of solids.
Sample is held between two plates or is squeezed onto a single plate
.
 Cells: For liquids or dissolved samples. Includes internal reflectance
cells (CIRCLE cells)

26. The horizontal lines indicate regions where solvent transmits at least 2
5% of the incident radiation in a 1 mm cell.
Common IR Solvents

27. IR Sampling Methods: Reflectance Methods
 Specular reflection: direct refle
ction off of a flat surface.
– Grazing angles
 Attenuated total reflection (AT
R): Beam passed through an I
R-transparent material with a hi
gh refractive index, causing int
ernal reflections.
 Depth is ~2 um (several w
avelengths)
 Diffuse reflection (DRIFTS): a
technique that collects IR radia
tion scattered off of fine particle
s and powders. Used for both
surface and bulk studies.
Figures from http://www.nuance.northwestern.edu/KeckII/ftir7.asp
ATR
DRIFTS

28. Hybrid/Hyphenated Techniques: Interfaces
 Interfaces between vibrational spectrometers and other an
alytical instruments are possible
 Example: In GC-FTIR, gaseous GC column effluent is pass
ed through “light pipes”
 Another Example: TGA-IR, for identification of evolved gas
es from thermal decomposition
Figure from Skoog et al.

29. Nernst Glower
A rod or cylinder made from sev
eral grams of rare earth oxides,
heated to 1200-2200K by an ele
ctric current.
1-50 µm
(mid- to far-IR)
Globar
Similar to the Nernst glower but
made from silicon carbide SiC, e
lectrically heated. Better perfor
mance at lower frequencies.heat
ed
1-50 µm
(mid- to far-IR)
Tungsten (W) filam
ent lamp
Heated to 1100 K
0.78-2.5 µm
(Near-IR)
Hg arc lamp
High-pressure mercury vapor tu
be, electric arc forms a plasma.
50 - 300 µm
(far-IR)
CO2 laser
High-intensity, tunable radiation
used for quantitation of specific
analytes
9-11 µm
IR Sources

30. IR Detectors
 Thermal transducers
– Response depends upon heating effects of IR radiation (temperat
ure change is measured)
 Slow response times, typically used for dispersive instruments or sp
ecial applications
 Pyroelectric transducers
– Pyroelectric: insulators (dielectrics) which retain a strong electric
polarization after removal of an electric field, while they stay belo
w their Curie temperature.
– DTGS (deuterated triglycine sulfate): Curie point ~47°C
 Fast response time, useful for interferometry (FTIR)
 Photoconducting transducers
– Photoconductor: absorption of radiation decreases electrical resi
stance. Cooled to LN2 temperatures (77K) to reduce thermal nois
e.
– Mid-IR: Mercury cadmium telluride (MCT)
– Near-IR: Lead sulfide (NIR)

31. Interpretation of IR Spectra
 General Features:
– Stretching frequencies are greater (higher wavenumbers) than co
rresponding bending frequencies
 It is easier to bend a bond than to stretch it
– Bonds to hydrogen have higher stretching frequencies than those
to heavier atoms.
 Hydrogen is a much lighter element
– Triple bonds have higher stretching frequencies than double bond
s, which have higher frequencies than single bonds
 Strong IR bands often correspond to weak Raman bands
and vice-versa

32. Interpretation of IR and Raman Spectra
Characteristic Vibrational Frequencies for Common Functional Groups
Frequency (cm-1) Functional Group Comments
3200-3500 alcohols (O-H)
amine, amide (N-H)
alkynes (CC-H)
Broad
Variable
Sharp
3000 alkane (C-C-H)
alkene (C=C-H)
2100-2300 alkyne (CC-H)
nitrile (CN-H)
1690-1760 carbonyl (C=O) ketones, aldehydes,
acids
1660 alkene (C=C)
imine (C=N)
amide (C=O)
Conjugation lowers
amide frequency
1500-1570
1300-1370
nitro (NO2)
1050-1300 alcohols, ethers, esters,
acids (C-O)
See also Table 17-2 of Skoog, et al.
More detailed lists are widely available. See R. M. Silverstein and F. X. Webster, “Spectrometric Identification of Organic Compounds”, 6th Ed., Wiley, 1998.

33. IR Absorption and Chemical Structure

34. IR and Raman Spectra of an Organic Compound
The diamond ATR IR spectrum of tranilast (polymorphic Form I):
T ramilast F orm I
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
Absorbance
1000
1500
2000
2500
3000
3500
4000
Wavenumbers (cm-1)
O
O
N
H
O
OH
O
C1
C6
C2
C3
C4
C5
C7
N1
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
H3C
H3C
O4
O5
O3
O2 O1

35. IR and Raman Spectra of an Organic Compound
The diamond ATR IR spectrum of flufe
namic acid (an analgesic/anti-inflamm
atory drug):
CF3
O OH
F lufenam ic Ac id Aldric h as recd
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24
0.26
0.28
0.30
0.32
Absorbance
1000
1500
2000
2500
3000
3500
4000
Wavenumbers (cm-1)

36. Far IR Spectroscopy in Condensed Phases
 Far IR is used to study low frequency vibrations, like those between m
etals and ligands (for both inorganic and organometallic chemistry).
– Example: Metal halides have stretching and bending vibrations in the 65
0-100 cm-1range.
– Organic solids show “lattice vibrations” in this region
 Can be used to study crystal lattice energies and semiconductor prop
erties in solids via phonon modes.
 The far IR region also overlaps rotational bands, but these are normall
y not detectable in condensed-phase work

37. Terahertz Spectroscopy
 A relatively new technique, addresses an unused portion o
f the EM spectrum (the “terahertz gap”):
– 50 GHz (0.05 THz) to 3 THz (1.2 cm-1 to 100 cm-1)
 Made possible with recent innovations in instrument desig
n, accesses a region of crystalline phonon bands
P. F. Taday and D. A. Newnham, Spectroscopy Europe, , www.spectroscopyeurope.com
G. Winnewisser, Vibrational Spectroscopy 8 (1995) 241-253

38. Applications of FT Microwave Spectroscopy
 Under development for: real-time, sensitive monitoring of
gases evolved in process chemistry, plant and vehicle emi
ssions, etc…
– Current techniques have limits (GC, IR, MS, IMS)
– Normally use pulsed-nozzle sources and high-precision Fabry-Per
ot interferometers (PNFTMW)
Diagram from http://physics.nist.gov/Divisions/Div844/facilities/ftmw/ftmw.html
For more information, see E. Arunan, S. Dev. And P. K. Mandal, Applied Spectroscopy Reviews, 39, 131-181 (2004).
Compound Detection Limit
(nanomol/mol)
Acrolein 0.5
Carbonyl sulfide 1
Sulfur dioxide 4
Propionaldehyde 100
Methyl-t-butyl ether 65
Vinyl chloride 0.45
Ethyl chloride 2
Vinyl bromide 1
Toluene 130
Vinyl cyanide 0.28
Acetaldehyde 1

39. New Methods in FT Microwave Spectroscopy
 A new method using a “chirp” pulse (which excites a wide r
ange of frequencies) has been developed by the Pate grou
p at U. Virginia Charlottesville
– The CP-FTMW (chirped pulse FT microwave) method enhances s
ensitivity by 100 to 10000 times and allows for studies of molecula
r shape changes (occurring on picosecond timescales)
Diagram from B. H. Pate et al., Science, 2008, 320, 924
See C&E News: http://pubs.acs.org/cen/news/86/i20/8620notw1.html

40. Applications of Near IR Spectroscopy
 Near IR (NIR) is heavily used in process chemistry and materials iden
tification
 Amenable to quantitative analysis usually in conjunction with chemom
etrics (calibration requires many standards to be run)
 While not a popular qualitative technique, it can serve as a fast and us
eful quantitative technique especially using diffuse reflectance
 Accuracy and precision in the ~2% range
 Examples:
– On-line reaction monitoring (food, agriculture, pharmaceuticals)
– Moisture and solvent measurement and monitoring
 Water overtone observed at 1940 nm
– Solid blending and solid-state issues

41. Near IR Spectroscopy
Figure from www.asdi.com. For more information see:
1. Ellis, J.W. (1928) “Molecular Absorption Spectra of Liquids Below 3 m”, Trans. Faraday Soc. 1928, 25, pp. 888-898.
2. Goddu, R.F and Delker, D.A. (1960) “Spectra-structure correlations for the Near-Infrared region.” Anal. Chem., vol. 32 no. 1, pp. 140-141.
3. Goddu, R.F. (1960) “Near-Infrared Spectrophotometry,” Advan. Anal. Chem. Instr. Vol. 1, pp. 347-424.
4. Kaye, W. (1954) “Near-infrared Spectroscopy; I. Spectral identification and analytical applications,” Spectrochimica Acta, vol. 6, pp. 257-287.
5. Weyer, L. and Lo, S.-C. (2002) Spectra-Structure Correlations in the Near-infrared, In Handbook of Vibrational Spectroscopy, Vol. 3, Wiley, U.K., pp. 1817-1837.
6. Workman, J. (2000) Handbook of Organic Compounds: NIR, IR, Raman, and UV-Vis Spectra Featuring Polymers and Surfactants, Vol. 1, Academic Press, pp. 77-197.

42. Near IR Spectrum of Acetone
 NIR taken in transmission mode (via a reflective gold plate) on a Fos
s NIRsystems spectrometer
 Useful for quick solvent identification
(nm)

43. Near IR Spectrum of Water (1st Derivative)
 1st derivative (and 2nd derivative) allows for easier identification of ban
ds
(nm)

44. Photoacoustic Spectroscopy
 First discovered in 1880 by A. G. Bell
 When radiation is absorbed, the energy is converted to heat, causin
g expansion and contraction of the sample at the modulation frequen
cy which is transferred to the surrounding air. Can be detected with
a microphone.
 The IR “version” of photoacoustic sampling is generally applied to tw
o types of system
 All gas (or all-liquid) syst
ems:
 The solid-gas system: Solid
IR-Transparent Gas
Gas:
IR Radiation
IR Radiation
A. G. Bell, Am. J. Sci. 20 (1880) 305.
A. G. Bell, Philos. Mag. 11 (1881), 510.

45. The Photoacoustic Effect for Solid-Gas Systems
 The photoacoustic effect is produced when intensity-mod
ulated light hits a solid surface (or a confined gas or liquid
).
Gas
Solid
Modulated IR Radiation
x
PA Cell
Thermal Wave (attenuates rapidly)
J. F. McClelland. Anal. Chem. 55(1), 89A-105A (1983)
M. W. Urban. J. Coatings Technology. 59, 29 (1987).
Microphone
P(x)
P0
IR is absorbed by a vibrational transition,
followed by non-radiative relaxation
P R P e
x
R
P
su
rfa
c
e  



( )
(
1 0
0
+ )
s
u
rfa
c
ere
fle
c
tiv
ity
in
c
id
e
n
tIRb
e
a
mp
o
w
e
r
- a
b
s
o
rp
tio
nc
o
e
ffic
ie
n
t
- th
e
rm
a
ld
iffu
s
io
nle
n
g
th
1





(Psurface)

46. The Thermal Diffusion Length –Solids
Urban, M. W. J. Coatings Technology. 1987, 59, 29
Quintanilla, L., Rodriguez-Cabello, J. C., Jawhari, T. and Pastor, J. M.. Polymer. 1993, 34, 3787.
 The thermal diffusion length  is:
PET
PVF2
0.15 cm/sec IR 1.2 cm/sec IR
 - thermal diffusion length
 =  / 2 
 The thermal diffusivity a is:
 The variable , the modulation frequency of the IR radiati
on, is directly proportional to interferometer mirror velocit
y, and is defined as:
(cm/sec)
eter
interferom
Michelson
of
ocity
Mirror vel
rs)
(wavenumbe
Frequency
IR
4



M
M






a
k
C
k
C






thermal conductivity
density
specific heat



2a

47. The Thermal Diffusion Length
Urban, M. W. and Koenig, J. L. Appl. Spec. 1986, 40, 994.
Quintanilla, L., Rodriguez-Cabello, J. C., Jawhari, T. and Pastor, J. M.. Polymer. 1993, 34, 3787.
 The mirror velocity is therefore inversely related to the ther
mal diffusion length, and therefore can be used to control t
he maximum sampling depth.
 Typical thermal diffusion lengths for the carbonyl band (~1
750 cm-1) of poly(ethylene terephthalate):
Mirror Speed (cm/sec) Thermal Diffusion Length (microns)
0.15 8.9
0.30 6.3
0.60 4.5
0.90 3.6
1.20 3.1
The thermal diffusivity was taken to be 1.3 * 10-3 cm2/sec, and the absorption coefficient of the carbonyl band was assu
med to be 2000 cm-1.

48. A Typical Photoacoustic FTIR Spectrum
A PA-FTIR Spectrum of a silicone sealant:
 The spectrum shows peaks where the IR radiation is being
absorbed due to vibrational energy level transitions.
Paroli, R. M., Delgado, A. H., and Cole, K. C. Canadian J. Appl. Spectr. 1994, 39, 7.
IR Modulation fr
equency is high
IR Modulation f
requency is low
 Differences between a PA-FTIR spectrum and a regular IR
spectrum:
– IR modulation frequency effects (weak CH3 and CH2 bands)
– Saturation of strong bands in the spectrum

49. Photoacoustic Saturation
 Strong bands in PA-FTIR spectra often
show saturation.
 Saturation occurs when the vibrational
transition is being pumped to its excite
d state faster than it can release energ
y.
 A high absorption coefficient coincides
with faster saturation.
A Saturated Band
Rosencwaig, A. Photoacoustics and Photoacoustic Spectroscopy. Wiley: New York, 1980.
Paroli, R. M., Delgado, A. H., and Cole, K. C. Canadian J. Appl. Spectr. 1994, 39, 7.

50. Depth-Profiling Studies with PA-FTIR
Urban, M. W. and Koenig, J. L. Appl. Spec. 1986, 40, 994.
Crocombe, R. A. and Compton, S. V. Bio-Rad FTS/IR Application Note 82. Bio-Rad Digilab Division, Cambridge, MA, 1991.
 Thermal diffusion length
allows for IR depth profili
ng with PA-FTIR
 Example: a layer of poly(
vinylidine fluoride (PVF2)
on poly(ethylene terepht
halate) (PET)
PET
PVF2
PVF2 top layer is 6 micrometers thick.
The carbonyl band, due to the PET, is marked with a red dot ().
Data acquired with a Digilab FTS-20E with a home-built PA cell.
0.15 cm/sec IR 1.2 cm/sec IR
 - thermal diffusion length
 =  / 2 

51. IR Microscopy
 Most FTIR microscopes image using array detectors
 IR spectra from a region are acquired at once, better S/N
– However, this is at the expense of resolution (limited to ca. 10 um), i
n contrast with scanning techniques. Resolution in FTIR imaging is
of course limited by the diffration limit, which is even worse for IR wa
velengths.
Figure from J. L. Koenig, S. Q. Wang, and R. Bhargava, Anal. Chem., 73, 361A-369A (2001).

52. IR Microscopy: Image Analysis
 Extraction of data from FTIR micrographs is done by co
lor-coding peaks based on their IR frequency (a)
 Suitable IR frequencies can be ch
osen via a scatter plot (c) of every
point in the image vs. two (or mor
e) frequencies, followed by locatio
n of the center-of-gravity and pos
sible statistical analysis
 False color images can then be c
onstructed (b)
Figure from J. L. Koenig, S. Q. Wang, and R. Bhargava, Anal. Chem., 73, 361A-369A (2001).

53. IR Microscopy: Polymer Chemistry Applications
 FTIR microscopy can analyze compositional differences in m
aterial science, chemical and biochemical applications
 Example – the study of time-dependent processes like dissol
ution of a polymer by a solvent
Figure from J. L. Koenig, S. Q. Wang, and R. Bhargava, Anal. Chem., 73, 361A-369A (2001).

54. IR Microscopy: Polymer Chemistry Applications
 A complex, solvent-dependent dissolution, diffusion and mol
ecular motion process is observed for polymers (e.g. polymet
hylstyrene) above their entanglement mwt:
Figure from J. L. Koenig, S. Q. Wang, and R. Bhargava, Anal. Chem., 73, 361A-369A (2001).

55. Vibrational Circular Dichroism
 Analogous to electronic (U
V-visible) circular dichroism
(ECD), VCD measures the
differential absorption of rig
ht and left-handed circularly
polarized IR radiation
 Much less sensitive than E
CD, but much higher inform
ation content (many more b
ands show effects linked to
chirality) S-enantiomer
R-enantiomer
VCD of -pinene

56. Further Reading
Optional:
L. J. Bellamy, Advances in Infrared Group Frequencies, Methuen and Co.,
1968.
R. M. Silverstein and F. X. Webster, Spectrometric Identification of Organic
Compounds, 6th Ed., Wiley, 1998.
P. W. Atkins and R. S. Friedman, Molecular Quantum Mechanics, 3rd. Ed.,
Oxford, 1997.