ir spectroscopy
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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
Introduction to FTInfrared Spectroscopy
• What is infrared spectroscopy?
• Theory of FT-IR
• FT-IR Advantages?
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.
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.
Infrared region
LIMIT OF RED LIGHT: 800 nm, 0.8 m, 12500 cm-1
NEAR INFRARED: 0.8 -2.5 m, 12500 - 4000 cm-1
MID INFRARED: 2.5 - 25 m, 4000 - 400 cm-1
FAR INFRARED: 25 - 1000 m, 400 - 10 cm-1
Divisions arise because of different optical materials and instrumentation.
Principle of Infrared spectroscopy
E-vector in EMR has frequency
Energy transferred to molecule
by resonance
Oscillating electric field begins in molecule
Vibration must have change in DM
Vibration frequency
Vibrations in molecules
Energy in range 20-4000 cm-1
vibrational levels quantized,
labelled by v.
molecule vibrational energy
E = h(v+1/2)
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 extensionSpring obeys Hooke’s law f = - k(r-req)F – restoring forceK – force constantR – internuclear distance Req- equilibrium distance
Simple harmonic oscillator
0
3
1
2
5
4
21
2 eqE K r r
1
2osc
k
1
2osc
k
Energy
Oscillation
Wavenumbers
1
2 oscE h
1
2 osc
Eh
hc
Permitted energy levels ofthe harmonic oscillator
- Vibrational quantum number
If = 0,1
2 osc
Eh
hc
Zero-point energy
Selection rule for the harmonic oscillator undergoing vibrational changes = ± 1
11
1 11
2 2osc osc osccm
Applying the selection rule
For emission
11 osccm
The vibrating molecule will absorb energy only from radiation which it can coherently interact and this must be radiation of its own oscillation frequency
• Bond – not a perfect Elastic nature
• Does not obey exactly simple harmonic motion
• Dissociates
• Not a ideal parabola
2
1 expeq eqE D a r r
A – constant; Deq – Dissociation energy
Morse curve
Anharmonic Oscillator
Anharmonic Oscillator
1 11
2 2e ex
11
2osc e ex
Allowed vibrational energy levels
- oscillation frequency; xe – anharmonicity constant
Anharmonic oscillator frequency
10
11
2e ex cm
Ground state energy
Selection rule = ±1, ±2, ±3…
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 H2O will absorb infrared light when the vibration (stretch or bend) results in a molecular dipole moment change
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.
h
Excited states
Ground (vibrational)
states
h(1 - 0 )
h(1 - 0)
h(2 - 1) (overtone)
Infrared Absorption and Emission
1
2
0
3
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
Infrared Spectroscopy
A molecule can be characterized (identified) by its molecular vibrations, based on the absorption and intensity of specific infrared wavelengths.
• 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
Abs
orba
nce
All gases except O2, N2, H2, Cl2, F2, H2S, and noble gases can be measured
HCl molecule stretching vibration at 2880 cm-1
Interferogram is made by an interferometer.
Interferogramis transformedinto 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
FTIR seminar
Interferometer
He-Ne gas laser
Fixed mirror
Movable mirror
Sample chamber
Light source
(ceramic)
Detector
(DLATGS)
Beam splitter
FT Optical System Diagram
Fourier transformation
Fourier transformationpair
• 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
Inte
nsi
ty
Inte
nsi
ty
Wave number / cm-1
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.
Infrared Spectroscopy
For isopropyl alcohol, CH(CH3)2OH, the infrared absorption bands identify the various functional groups of the molecule.
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.
1.Better sensitivity and brightness- Allows simultaneous measurement over the entire wavenumber range- Requires no slit device, making good use of the available beam2.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-13. Resolution- Provides spectra of high resolution4. Stray light- Fourier Transform allows only interference signals to contribute to spectrum. Background light effects greatly lowers.- Allows selective handling of signals limiting intreference5. Wavenumber range flexibility- Simple to alter the instrument wavenumber rangeCO2 and H2O sensitive
FT-IR Advantages and Disadvantages
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