IR Spectroscopy

An introduction to IR spectroscopy

1. Fourier Transform Infrared
(FT-IR) Spectroscopy
Theory and Applications
THE ELECTROMAGNETIC SPECTRUM
INFRAREDGAMMA RAYS X RAYS UV VISIBLE
Prof. V. Krishnakumar
Professor and Head
Department of Physics
Periyar University
Salem – 636 011, India

2. Introduction to FTInfrared
Spectroscopy
•What is infrared
spectroscopy?
•Theory of FT-IR
•FT-IR Advantages?

3. What is Infrared?
•Infrared radiation lies between the visible and microwave portions
of the electromagnetic spectrum.
•Infrared waves have wavelengths longer than visible and shorter
than microwaves, and have frequencies which are lower than
visible and higher than microwaves.
• The Infrared region is divided into: near, mid and far-infrared.
–Near-infrared refers to the part of the infrared spectrum that is
closest to visible light and far-infrared refers to the part that is
closer to the microwave region.
–Mid-infrared is the region between these two.
•The primary source of infrared radiation is thermal radiation. (heat)
•It is the radiation produced by the motion of atoms and molecules
in an object. The higher the temperature, the more the atoms and
molecules move and the more infrared radiation they produce.
•Any object radiates in the infrared. Even an ice cube, emits
infrared.

4. What is Infrared? (Cont.)
Humans, at normal body temperature, radiate most
strongly in the infrared, at a wavelength of about 10
microns (A micron is the term commonly used in
astronomy for a micrometer or one millionth of a
meter). In the image to the left, the red areas are the
warmest, followed by yellow, green and blue
(coolest).
The image to the right shows a cat in the
infrared. The yellow-white areas are the
warmest and the purple areas are the coldest.
This image gives us a different view of a
familiar animal as well as information that we
could not get from a visible light picture.
Notice the cold nose and the heat from the cat's
eyes, mouth and ears.

5. Infrared region

LIMIT OF RED LIGHT: 800 nm, 0.8 mm, 12500 cm
-1
NEAR INFRARED: 0.8 -2.5 mm, 12500 - 4000 cm
-1
MID INFRARED: 2.5 - 25 mm, 4000 - 400 cm
-1
FAR INFRARED: 25 - 1000 mm, 400 - 10 cm
-1
Divisions arise because of different optical materials and
instrumentation.

6. Principle of Infrared spectroscopy
· E-vector in EMR has frequency n
·Energy transferred to molecule
by resonance
·Oscillating electric field begins in
molecule
· Vibration must have change in DM
· Vibration frequency n

7. Vibrations in molecules

· Energy in range 20-4000 cm
-1
·  vibrational levels quantized,
labelled by v.
·molecule vibrational energy
E = hn(v+1/2)

8. Vibrating Diatomic molecule
When two atoms combine to form a stable covalent molecule
– internal electronic arrangement
Repulsion – Positively charged two nucleus and between two
electron clouds
Attraction – Between Positively charged nucleus and electron
clouds
Spring – compression and extension
Spring obeys Hooke’s law
f = - k(r-r
eq
)
F – restoring force
K – force constant
R – internuclear distance
R
eq
- equilibrium distance

10. Simple harmonic oscillator
0
3
1
2
5
4
w
( )
21
2
eq
E K r r= -
1
2
osc
k
w
p m
=
1
2
osc
k
v
p m
=
Energy
Oscillation
Wavenumbers
1
2
osc
E h
n
n w
æ ö
= +
ç ¸
è ø
1
2
osc
E
h
hc
n
n
e n w
æ ö
= = +
ç ¸
è ø
Permitted energy levels of
the harmonic oscillator
n - Vibrational quantum number
If n = 0,
1
2
osc
E
h
hc
n
n
e w
æ ö
= =
ç ¸
è ø
Zero-point energy
Selection rule for the harmonic oscillator undergoing vibrational changes Dn = ± 1

11. 1
1
1 1
1
2 2
osc osc osc
cm
n n
e n v n v v
-
+ ®
æ ö æ ö
= + + - + =
ç ¸ ç ¸
è ø è ø
Applying the selection rule
For emission
1
1 osc
cm
n n
e v
-
+ ®
=
The vibrating molecule will absorb energy only from radiation which it
can coherently interact and this must be radiation of its own oscillation
frequency

12. •Bond – not a perfect Elastic
nature
•Does not obey exactly simple
harmonic motion
•Dissociates
•Not a ideal parabola
( ){ }
2
1 exp
eq eq
E D a r ré ù= - -
ë û
A – constant; D
eq
– Dissociation energy
Morse curve
Anharmonic Oscillator

13. Anharmonic Oscillator
1 1
1
2 2
e e
x
n
e v n n
ì üæ ö æ ö
= - + +í ýç ¸ ç ¸
è ø è øî þ
1
1
2
osc e e
xv v n
ì üæ ö
= - +í ýç ¸
è øî þ
Allowed vibrational energy levels
v - oscillation frequency; xe – anharmonicity constant
Anharmonic oscillator frequency
1
0
1
1
2
e e
x cme v
-ì ü
= -í ý
î þ
Ground state energy
Selection rule Dn = ±1, ±2, ±3…

14. Infrared Spectroscopy
The bonds between atoms in the molecule
stretch and bend, absorbing infrared energy
and creating the infrared spectrum.
Symmetric Stretch Antisymmetric Stretch Bend
A molecule such as H
2
O will absorb infrared light when the vibration
(stretch or bend) results in a molecular dipole moment change

15. Energy levels in Infrared
Absorption
Infrared absorption occurs among the ground vibrational states, the
energy differences, and corresponding spectrum, determined by the
specific molecular vibration(s). The infrared absorption is a net
energy gain for the molecule and recorded as an energy loss for the
analysis beam.
hn
Excited
states
Ground
(vibrational)
states
h(n
1
-
n
0
)
h(n
1
- n
0
)
h(n
2
- n
1
)
(overtone)
Infrared Absorption and
Emission
n
1
n
2
n
0
n
3

17. Number of vibrations

· Vibration: centre of gravity
unchanged
· N nuclei, each move along x,y,z
· Nonlinear molecule: 3N-6 modes
· Linear molecule: 3N-5 modes

18. Infrared Spectroscopy
A molecule can be characterized (identified) by its
molecular vibrations, based on the absorption and
intensity of specific infrared wavelengths.

19. •All molecules can be
identified on the basis of
their characteristic
absorption spectrum
(except diatomic
elements such as O2 and
noble gases)
•Each molecule absorbs
infrared radiation at its
characteristic frequencies
•IR absorption spectrum is
a fingerprint unique to
each molecule
•Beer’s law: Absorption
strength i.e absorbance
is directly proportional to
concentration

IR spectrum
of HCl
Wavenumbers
A
b
s
o
r
b
a
n
c
e
All gases except O
2
, N
2
, H
2
, Cl
2
, F
2
, H
2
S, and noble gases can be measured
HCl molecule stretching vibration at
2880 cm
-1

20. Interferogram
is made by an interferometer.
Interferogram
is transformed
into a spectrum using a FT.
BKG
SB
3000 2000 1000
[cm-1]
Sample
SB
Sample
3000 2000 1000
[cm-1]
Sample/BKG
IR spectrum
%T
3000 2000 1000[cm-1]
The Principles of FTIR Method

21. FTIR seminar
Interferometer
He-Ne gas laser
Fixed mirror
Movable mirror
Sample chamber
Light
source
(ceramic)
Detector
(DLATGS)
Beam splitter
FT Optical System Diagram

22. Fourier transformation
Fourier
transformation
pair
•The interferogram signal is recorded as a function of optical path
difference
•The interferogram is comparable to a time domain signal (eg. a
recorded sound) and the spectrum represents the same information
in frequency domain (eg. the frequency of the same sound)
•Fourier transformation is the mathematical relation between the
interferogram and the spectrum (in general, between time domain
signal and frequency signal)
•A pure cosine wave in the interferogram transforms to a perfectly
sharp narrow spike in the spectrum
OPD / cm
I
n
t
e
n
s
it
y
I
n
t
e
n
s
it
y
Wave number / cm
-1

23. Capabilities of Infrared
Analysis
•Identification and quantitation of organic solid, liquid
or gas samples.
•Analysis of powders, solids, gels, emulsions, pastes,
pure liquids and solutions, polymers, pure and mixed
gases.
•Infrared used for research, methods development,
quality control and quality assurance applications.
•Samples range in size from single fibers only 20
microns in length to atmospheric pollution studies
involving large areas.

24. Infrared Spectroscopy
For isopropyl alcohol, CH(CH3)2OH, the
infrared absorption bands identify the various
functional groups of the molecule.

25. Applications of Infrared
Analysis
•Pharmaceutical research
•Forensic investigations
•Polymer analysis
•Lubricant formulation and fuel additives
•Foods research
•Quality assurance and control
•Environmental and water quality analysis
methods
•Biochemical and biomedical research
•Coatings and surfactants
•Etc.

26. 1.Better sensitivity and brightness
- Allows simultaneous measurement over the entire wavenumber range
- Requires no slit device, making good use of the available beam
2.High wavenumber accuracy
- Technique allows high speed sampling with the aid of laser light
interference fringes
- Requires no wavenumber correction
- Provides wavenumber to an accuracy of 0.01 cm-1
3. Resolution
- Provides spectra of high resolution
4. Stray light
- Fourier Transform allows only interference signals to contribute to
spectrum.
Background light effects greatly lowers.
- Allows selective handling of signals limiting intreference
5. Wavenumber range flexibility
- Simple to alter the instrument wavenumber range
CO
2
and H
2
O sensitive
FT-IR Advantages and Disadvantages