This document provides an overview of infrared spectroscopy. It discusses the electromagnetic spectrum, infrared regions, molecular vibrations including different types of vibrations. It also discusses fundamental and overtone bands, degree of freedom, instrumentation including sources, monochromators, sample cells, and detectors such as thermocouples, bolometers, Golay cells, thermistors, and pyroelectric substances. It provides details on different types of detectors used in infrared spectroscopy.

1. INFRARED SPECTROSCOPY
PRESENTED BY: DR.VIJAYA BARGE
(Vice Principal & Professor)
Pune District Education
Association’s Shankarrao Ursal
College of Pharmaceutical
Sciences & Research Centre.

2. LEARNING
OBJECľIVES
Afteí completing this sub-unit the students should be able:
1. ľ o study the theoíy of Infíaíed absoíption spectíoscopy .
2.ľo study the modes of vibíations in IR spectíoscopy
3.ľo study the factoís affecting vibíations in IR spectíoscopy
4.ľo study the instíumentation involved in IR spectíoscopy
5.ľo study the applications of IR spectíoscopy

6. ELCETROMAGNETIC SPECTRUM
The electromagnetic (EM) spectrum is the range of all types of EM radiation

8. IR SPECTROSCOPY

9. IR REGIONS

13. MOLECULAR VIBRATIONS

16. Wagging;

18. A fundamental band is the spectral band that occurs in a vibrational
spectrum of a molecule when the molecule makes a transition from the ground
state (v=0) to the first excited state (v=1)
An overtone band is the spectral band that occurs in a vibrational spectrum of
a molecule when the molecule makes a transition from the ground state (v=0) to
the second excited state (v=2)
FUNDAMENTAL AND OVERTONE BANDS

19. DEGREE OF FREEDOM

20. The IR spectrum is a plot of % transmittance (or absorbance)
of the radiation through the molecule versus wave number of
the radiation

21. IR INSTRUMENTATION

23. SOURCE

29. MONOCHROMATORS

31. SAMPLE CELLS

32. Sampling of solids

33. Solid films

34. Press pellet technique

35. Sampling of liquids

36. Sampling of gases

37. DETECTORS

38. Thermocouple
A thermocouple consists of a pair of junctions. Two junctions are formed by
connecting two pieces of a similar metal like bismuth ( Bi) to either ends of a
dissimilar metal such as antimony (Sb) or a semiConducting alloy. One junction is
used as a reference junction and another junction is used as a detector junction.
A potential difference is developed across the two junctions depending on the
difference between their temperatures.

39. Temperature of a reference junction need not be kept constant since the difference between
the temperatures of the junctions is required to determine the intensity of a signal. Still, most
of the times, to get the accurately measurable difference between the temperatures of the
two junctions, the temperature of the reference junction is kept more or less constant by
cooling it in ice. The greater the difference in the temperatures of the two junctions,
the higher is the sensitivity of the method and the higher is the accuracy of the
measurement. The reference junction has relatively large heat capacity. i.e. a large amount
of heat is associated with the minute change in the temperature of the reference junction.
2. A detector junction
A detector junction or the junction for detecting intensity of IR radiation transmitted after
the interaction with an analyte should be designed so as to have complete absorption of
heat from the IR radiation incident on it. Hence, special care is taken to design this junction
from fine wires of platinum, silver, antimony or bismuth. Fine wires increase the surface area
of the junction and help in complete absorption of heat from the IR radiation
1. Reference junction

40. Bolometer

42. Golay cell
It is also called as gas thermometer. It consists of a cylindrical chamber, filled with
xenon gas. Walls of this chamber are blackened for the complete absorption of
heat supplied by the IR radiation incident on it. One end of this cylinder is fitted
with an IR transparent window. Another end contains a flexible diaphragm which is
silvered from outside. The chamber also contains a phototube.When the IR
radiation enters this chamber. the blackened walls absorb heat provided by the
radiation. This in turn heats the xenon gas inside the chamber. The xenon gas
expands and exerts The curvature pressure on the flexible diaphragm depends on
the increase in the temperature of the xenon gas, which in turn depends on the
heat absorbed by it, which further dépends on the intensity of the incident
IR radiation. When the IR radiation falls on the silvered diaphragm, they reflect
back in the chamber and fall on the photosensitive surface of a phototube.,
Fending on the curvature of the flexible diaphragm, a definite fraction
of reflected beam strikes the active surface ot a phototube. This leads to a change
in the photocurrent. which can related to the power of IR.

44. Thermistors

45. Pyroelectric substances are the dielectric materials having special thermal
and electrical properties. Certain such as lithium Iantatate, barium titanate
and triglycine sulfate possess pyroelectric property. They
produce temperature sensitive dipole moments.
To construct a pyroelectric detector, a pyroelectric substance is placed
between two electrodes, one of which has an IR transparent window.
Two electrodes are connected to each other via a voltmeter . Molecular
dipoles in a pyroelectric substance are randomly oriented at
room temperature. When the IR radiation falls upon a pyroelectric
substance, it absorbs energy from the radiation. This energy is utilized in
aligning the molecular dipoles of the pyroelectric substance.
Pyroelectric detectors

47. 1.Infrared light at wavelengths sometimes beyond 5 μ
mcan be detected
with lead salt detectors, e.g. containing lead selenide (PbSe) or lead sulfide
(PbS). They are photoconductors , not photodiodes, i.e., they do not contain
a p–n junction but exhibit a reduction in electrical resistance caused by
incident light, which induces intraband transitions.
2.Another detector technology, which is widely used, is based on
photoresistors containing mercury cadmium telluride. Detection at rather long
wavelengths (partly beyond 12 μ
m
) is possible.
PHOTOCONDUCTIVITY DETECTORS

48. REFERENCES