This document provides an overview of infrared (IR) spectroscopy and IR spectrophotometers. It discusses how IR spectroscopy works by detecting the absorption of IR radiation by molecules as they undergo transitions between vibrational and rotational energy levels. The key components of an IR spectrophotometer are described, including the IR radiation source, sample cells, monochromators to select wavelengths, detectors to measure absorption, and recorders to display the spectra. Common molecular vibrations that can be observed in IR spectra are also outlined.

1. Spectroscopy
By
Dr. Krishna
Assistant Professor in Biotechnology
Tumkur University, Tumakuru

2. IR Spectrophotometer
• Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) involves the
interaction of infrared radiation with matter. It covers a range of techniques,
mostly based on absorption spectroscopy.
• To identify Chemicals
• Samples may be solid, liquid, or gas
• IR Spectrophotometer instrument – to produce infrared spectrum
• An IR spectrum - visualized in a graph of infrared light absorbance (or
transmittance) on the vertical axis vs. frequency or wavelength on the horizontal
axis.
• units of frequency - in IR spectra are reciprocal centimeters (sometimes called
wave numbers), with the symbol cm−1.
• Units of IR wavelength are commonly given in micrometers (formerly called
"microns"), symbol μm, which are related to wave numbers in a reciprocal way.

3. • Fourier transform infrared (FTIR) spectrometer, 2d IR – commonly used lab instrument
• The infrared portion of the electromagnetic spectrum is usually divided into three
regions;
• 1. the near-, 2. mid- and 3. far- infrared, named for their relation to the visible spectrum
• The primary source of infrared light is the thermal radiation. This is produced by the
motion of atoms and molecules in an object.
• Every object which has a temperature greater than absolute zero (-273°C), radiates in the
infrared.
• All atomic and molecular motion ceases at absolute zero temperature.
• The wavelength at which an object radiates most intensely depends on its temperature.
In general, as the temperature of an object cools, it shows up more prominently at
farther infrared wavelengths.
• When an object is not quite hot enough to radiate visible light, it will emit most of its
energy in the infrared.

5. Molecular Transitions:
• Molecular transitions: The energy levels of a molecule in the order of decreasing energy are:
electronic, vibrational and rotational.
• It is based on the assumptions that the nuclear motion is much slower than electron motion and
the nuclear motion (e.g., rotation, vibration) occurs in a smooth potential from the speedy
electrons.
• The quantized energy (quantized energy states is that only certain photon energies are allowed
when electrons jump down from higher levels to lower level) stored in a molecule thus can be
thought of as the sum of energy stored in distinct modes of rotation, vibration, and electronic.
• Electronic transition occurs so rapidly that the internuclear distance can't change much in the
process.
• Vibrational transitions occur between different vibrational levels of the same electronic state.
• Rotational transitions occur mostly between rotational levels of the same vibrational state,
although there are many examples of combination vibration-rotation transitions for light
molecules.
• Molecular transitions result in emission or absorption of photons: the electronic transitions in UV
or optical, vibrational transitions in infrared, rotational transitions in microwave range.

6. Energy-levels in a molecule: electronic, vibrational and rotational levels

7. • Periodic motion, in physics, motion repeated in equal intervals of
time. Periodic motion is performed, for example, by a rocking chair, a
bouncing ball, a vibrating tuning fork, a swing in motion, the Earth in
its orbit around the Sun, and a water wave.
• Translational motion is the motion by which a body shifts from one
point in space to another. One example of translational motion is the
the motion of a bullet fired from a gun.
• A molecular vibration occurs when atoms in a molecule are in
periodic motion while the molecule as a whole has constant
translational and rotational motion.
• A molecular vibration is excited when the molecule absorbs a
quantum of energy, E, corresponding to the vibration's frequency.

8. • Vibrational energy levels of the diatomic HCl molecule is presented in
Figure. as an anharmonic oscillator, the electronic states can be
represented as a function of internuclear distance.
The HCl molecule as an anharmonic oscillator vibrating at
energy level E3. D0 is dissociation energy here, r0 bond length, U
potential energy (Morse potential). Energy is expressed in
wavenumbers. The hydrogen chloride molecule is attached to
the coordinate system to show bond length changes on the
curve.

9. • Since the vibrational transitions of molecules result in infrared
photons, Simple diatomic molecules with only 2 nuclei perform a
vibration that resembles to a harmonic oscillator. There are several
ways the relative positions of atoms may change in a more
complicated molecule, these are:
• Stretching: a change in the length of a bond
• Bending: a change in the angle between two bonds
• Rocking: a change in angle between a group of atoms
• Wagging: a change in angle between the plane of a group of atoms
• Twisting: a change in the angle between the planes of two groups of
atoms
• Out-of-plane: a change in the angle between any one of the bonds
and the plane defined by the remaining atoms of the molecule

11. -Each shell made of many sub shells which are
themselves composed of atomic orbitals.
-First shell K have 1 sub shell = s
-Second shell L = 2 sub shells = s , p
-Third shell M = 3 sub shells =s , p, d
-fourth shell N = 4 sub shells = s , p, ,d, f
N
o.
Sub
shell
Name
1 s Sharp
2 p Principa
l
3 D Diffuse
d
4 f fundam
ental
Shells Sub shells No. of electron Type of sub shells
found in shell
Distribution electron
in sub shells
K s 2 only s 1s
L P 6 s ,p, 2s , 2p,
M d 10 s, p, d, 3s , 3p, ,3d
N f 14 s, p, ,d, f , d, 4s , 4p , 4d , 4f
… g and so on 18… s , p, d, f, g,… Alphabetically so
on

13. Principle of IR
• In the context of infra red spectroscopy the term "infra red" covers the
range of the electromagnetic spectrum between 0.78 and 1000 mm.

14. Regions of wavelength range
• Wavenumber range (cm-1)
• The most useful I.R. region lies between 4000 - 670cm-1.
• The infrared portion of the electromagnetic spectrum is usually divided
into three regions; the near-, mid- and far- infrared, named for their
relation to the visible spectrum. The higher-energy near-IR, approximately
14000–4000 cm−1 (0.8–2.5 μm wavelength) can excite overtone or
harmonic vibrations. The mid-infrared, approximately 4000–400 cm−1 (2.5–
25 μm) may be used to study the fundamental vibrations and associated
rotational-vibrational structure. The far-infrared, approximately 400–
10 cm−1 (25–1000 μm), lying adjacent to the microwave region

15. Theory of infra red absorption
• IR radiation does not have enough energy to induce electronic
transitions as seen with UV.
• Absorption of IR is restricted to compounds with small energy
differences in the possible vibrational and rotational states.
• For a molecule to absorb IR, the vibrations or rotations within a
molecule must cause a net change in the dipole moment of the
molecule.

16. Molecular rotations
• Rotational transitions are of little use to the spectroscopist.
• Rotational levels are quantized, and absorption of IR by gases yields
line spectra.
• However, in liquids or solids, these lines broaden into a continuum
due to molecular collisions and other interactions.

17. Molecular vibrations
• The positions of atoms in a molecules are not fixed;
they are subject to a number of different vibrations.

18. Spectra
• The twodimensional plot
• The dependece of transmitance on wavenumber
• Each compound has its own specific spectra

19. Examples of spectra

20. Examples of spectra

21. • Infrared spectroscopy exploits the fact that molecules absorb frequencies that
are characteristic of their structure. These absorptions occur at resonant
frequencies, i.e. the frequency of the absorbed radiation matches the vibrational
frequency.
• The infrared spectrum of a sample is recorded by passing a beam of infrared light
through the sample. When the frequency of the IR is the same as the vibrational
frequency of a bond or collection of bonds, absorption occurs.
• Examination of the transmitted light reveals how much energy was absorbed at
each frequency (or wavelength)
• This measurement can be achieved by scanning the wavelength range using a
monochromator
• This technique is commonly used for analyzing samples with covalent bonds.

22. • The main parts of IR spectrometer are as follows:
• radiation source
• sample cells and sampling of substances
• Monochromators
• detectors
• recorder

23. • IR radiation sources
• IR instruments require a source of radiant energy which emit IR
radiation which must be steady, intense enough for detection and
extend over the desired wavelength. Various sources of IR radiations
are as follows.
• a) Nernst glower - Nernst Lamp uses a small ceramic rod that was heated to
incandescence. The lamp is also called a incandescent "glower".
• b) Incandescent lamp - The incandescent lamp was the second form of electric light to
be developed for commercial use after the carbon arc lamp. It is the second most used
lamp in the world today behind fluorescent lamps
• c) Mercury arc
• d) Tungsten lamp
• f) Nichrome wire - Nichrome is any of various alloys of nickel, chromium, and often iron

24. Incandescent lamp
Sources: The IR spectrometer consists of a source of infrared light, emitting radiation throughout the
whole frequency range of the instrument. An inert solid is electrically heated to a temperature in the range
of 1500-2000K. This heated material will then emit IR radiation. Following are some of the sources:

25. • sample cells and sampling of substances
• IR spectroscopy has been used for the characterization of solid, liquid
or gas samples.
• i. Solid - Various techniques are used for preparing solid samples such as
pressed pellet technique, solid run in solution, solid films, mull technique etc.
• ii. Liquid – samples can be held using a liquid sample cell made of alkali
halides. Aqueous solvents cannot be used as they will dissolve alkali halides.
Only organic solvents like chloroform can be used.
•
• iii. Gas – sampling of gas is similar to the sampling of liquids.

26. • Monochromators – Various types of monochromators are prism,
gratings and filters. Prisms are made of Potassium bromide, Sodium
chloride or Caesium iodide. Filters are made up of Lithium Fluoride
and Diffraction gratings are made up of alkali halides.
• Detectors – Detectors are used to measure the intensity of
unabsorbed infrared radiation. Detectors like thermocouples,
Bolometers, thermisters, Golay cell, and pyro-electric detectors are
used.
• Recorders – Recorders are used to record the IR spectrum

27. • The Nerst Glower: It is a cylinder of rare earth oxides. Platinum wires are
sealed to the ends and a current is passed through the cylinder and can
reach temperatures of around 2200K.
•
The Globar source: It is a silicon carbide rod which is electrically heated to
around 1500K. The spectral output is comparable with the Nerst glower,
except at short wavelengths (less than 5mm) where it’s output becomes
larger.
•
The Incandescent wire source: This is a tightly wound coil of nichrome
wire, which is electrically heated to 1100K. It produces a lower intensity of
radiation than the above mentioned Nerst or Globar sources, but it has a
longer working life.

29. Optical choppers
Optical choppers, usually rotating disc mechanical shutters,
are widely used in science labs in combination with lock-in
amplifiers. The chopper is used to modulate the intensity
of a light beam, and a lock-in amplifier is used to improve
the signal-to-noise ratio.
Chopper: The two beams are reflected to a chopper which is rotating at
a speed of 10 rotations per second. This chopper makes the reference
and the sample beam to fall on the monochromator grating alternately.

30. • Monochromator grating: The grating also rotates, though slowly. This rotation sends individual
fre
• Detector: At the wavelength where the sample has absorbed, the detector will receive a weak
beam from the sample while the reference beam will retain full intensity. This leads to a pulsating
or alternating current to flow from detector to amplifier. On the other hand, at the frequencies
where the sample doesn’t absorb, both the beams will have equal intensities and the current
flowing from the detector to the amplifier will be direct and not alternating. The amplifier is
designed to amplify only the alternating current.
There are three different types of detectors.
• Thermocouples: They consist of a pair of junctions of different metals. The potential difference
(i.e.; the voltage) between the junction changes according to the difference in temperature
between the junctions.
•
Pyroelectric detectors: They are made from a single crystalline wafer of a pyroelectric material
(eg; triglycerine sulphate). The properties of a pyroelectric material are such that when an electric
field is applied across it, electric polarisation occurs. In a pyroelectric material, when the field is
removed, the polarisation persists. This degree of polarisation is temperature dependent.
•
Photoelectric detectors: They comprise a film of semiconducting material deposited on a glass
surface, sealed in an evacuated envelope (such as mercury cadmium telluride detector).

31. IR Spectrophotometer