Infrared Spectroscopy - Techniques of spectroscopy

IR

1. INFRARED SPECTROSCOPY


BY PRIYANKA YADAV

2. Introduction:-

Wave number:- defination
Wave length range:
0.8-2.5μm-near IR
2.5-50 μm-middle IR
50-400μm-far IR
here wave length is specified in wave no.
1cm=10
4
μm

 Wavelength of radiation is inversely proportional to energy
,while wavenumber is directly proportional to energy.

IR is imp for
-detection of functional group present in molecule
-Which bonds are present in molecule
-So known as finger print of molecule.

3. It is plot of %T vs wave length/wave number.
Ideally it should be plot of frequency vs. %T. but
here, frequency is not used because its unit is
very large.
here inverted band is obtain because at 0
absorbance, there is 100%T.
Below 100cm
-1
only rotation occur known as
rotational spectra
100-10000cm
-1
vibrational and rotational
transition occur

5. Where absorption occur?:-
(1)if frequency of electrical field associated with
incident radiation exactly matches to frequency
of vibrational and rotational spectraIR
radiation is absorbed.
(2)any moleculewhen change in dipole
momentthen only IR band is observed
If dipole not change and rotation/vibration
occurknown as IR inactive rotation
Relative dipole moments:-
C-O > C-Cl > C-N > C-C-OH > C-C-H

6. For change in dipole molecule must be
assymetric.
For monoatomic species,homonuclear atomic
molecule(no change in dipole) no IR spectra
observed.
Dipole is recorded bymagnitude of charge
difference and distance
When IR fall on moleculeincrease in distance
of bondtherefore change in dipoleso IR
active
2 types of molecular vibration occur:-
(1)streching
(2)bending

7. 1.Stretching:-
Definition:-it is that in which 2 bonded atoms oscillate
continously,without altering bond axis or bond angle.
 Here no change in bond angle but continuously
change in interatomic distance along the bond axis.
-Here energy absorbed is high.
Again it is of 2 types:-
(1)symetricaway/toward
(2)assymetricaway/toward

8. MODES OF VIBRATIONS
1. Stretching
o o


o
symmetric
o o


o
assymmetric

9. 2.bending:-
Definition:-characterised by continuous changing in
bond angle and axis with common atom
-Less energy is absorbed.
Mainly it is of 2 types:-
(1)inplane:-here bending occur in
same plane
(1)Rocking (2) cissoring
(2) out of plane:-here back and forth
rotation but in
other plane
(1)Wagging (2)twisting

10. 2.bending or deformations
o o

o
scissor
o o

o
rock
In-plane deformations
out-of-plane deformations
o o

o
o o

o
twist
wagging

11. Hook’s law:-
The vibratory motion of the nuclei of diatomic
molecule may be similar to the vibration of a linear
harmonic oscillator.
“In the oscillator, the force tending to restore an
atom to its original state is proportional to the
displacement of the vibratory atom from the
original position” is Hook’s law.
Wave no=1/2cf/μ
Where f=bond strength
μ= reduced mass
μ=m
1.m
2/(m
1+m
2)
m
1=atomic wt*no.of atom/avogadro no

12. Vibrational/fundamental modes/peaks:-
For any atom total no. of degree of freedom
is 3(x,y,z)
So, a molecule of n atoms has 3n degree of
freedom
-for nonlinear molecule,3 degree of freedom
describe translation and 3 describe rotation.
so, for any nonlinear molecule no. of peaks for
vibrational transition
=3n-6

13. For, linear molecule degree of freedom for
rotational transition is only 2.
So, for linear molecule degree of freedom=3n-5
E.g. of nonlinear molecules:-benzene,
hexane,
methane,
H
2O
.
E.g. of linear molecules:-CO
2
HCl.
Sometimes in spectra greater no. of peaks appear
i.e. due to appearance of combination or
overtone peak.

14. Fundamental peak:-if molecule vibrates from 0
ground level to 1
st
vibrational level.
Overtone peak:-if molecule vibrates from 0 ground
level to 2
nd
vibrational level.
-here, frequency is twice or thrice than fundamental
peak.

15. Reasons for increase in no. of peaks:-
(1)if single photon exciting 2 vibrator
simultaneously than results in combination
peak.
-frequency of this combination peak is addition or
differentiation of 2 fundamental peaks.
(2)due to same vibrator that jumps from 2
nd
to
3
rd
or so on.
(3)appearance of overtone peakfrequency of
which is twice or trice than fundamental peak.

16. Reasons for decrease in no. of peaks:-
(1)if molecule absorbs very less radiation which
is difficult to detect by detector.
(2) fundamental frequency that fall outside of
the 2.5-15 μm region.
(3)fundamental bands that are too weak to be
absorbed.
(4)if 2 fundamental peaks are close together 
coalescence of peaks.

17. (5)if molecule is symmetric
(6)Where energy of absorption of 2 vibrator are
same  then, only single peak appear in spectra
-this kind of peak is known as degenerate peak.

18. Factors influencing vibrational
frequency
(1)Vibrational coupling:-
Here, interaction of 2 vibrational level which
vibrate at same frequency and near by in
molecule  known as vibrational coupling
Occur only when bonds are located closely to
each other.

19. Here,2 more peaks appear in spectra.
(1)symmetric
(2)Antisymetric
Requirements:-
-2 Stretching vibrational coupling occur when it is
separated through common atom.
-2 bending vibrational coupling occur when it is
separated by common bond.
-Coupling can occur between stretching and
banding if bond involved in stretching is
involved in bending also.

20. -Both vibrator should vibrate with same
frequency.
-If no.of bonds will increase or seperated by more
than 2 atomsvibrational coupling is negligible.
E.g;-(1)-C-H show streching vibration only.

C H  appear 2 peaks
H

21. Antisymmetric:-at 3000cm
-1

symmetric>:-at 2900cm
-1

If H
C Hhere,antisymmetric:-at 3100cm
-1
H

symetric:-at 3000cm
-1
(2)CO
2
it is linear molecule so no. of peaks should be 3n-5
= 3*3-5= 4
But here only 2 peaks appear in spectra that is anti-
symmetric and bending.
Antisymmetric peak appear at 2350cm
-1

22. Here in symmetric peak doesn’t appear because of there is
no change in dipole moment
- Therefore it is IR inactive
And instead of 2 bending peaks only one bending peak
appear because energy of absorption is same.
(3) Acetaldehyde (CH
3=CH0)
Here n=7 and molecule is non linear so no. of vibrational
peaks should be 15 according to 3n-6
But here only 5 peaks appear in spectra
(4) H
2O
Here 3 peaks appear according to 3n-6
 Symmetric peak appear at 3650 cm
-1

 Antisymmetric peak at 3760 cm
-1

 Bending peak at 1595 cm
-1

23. (5) anhydride R-C-O-C-R
O O
These give rise to 2-C=O streching absorption,1
antisymmetric and 1 symmetric peak.
Here,coupling between 2 carbonyl groups which are
indirectly linked through –O–.
(6) Amide R-C-N-H
O H
Shows 2 absorption bands around 1600-1700cm
-1
i.e. due
to –C=O str and –N-H def.
but due to coupling original character is changed.
Here coupling between –C-N stretching and –N-H bending
vibrational level takes place
 Here amide I peakdue to –C=O stretching
 and amide II peak  due to coupling.

24. Fermiresonance:-interaction between
fundamental vibrations and overtones known as
Fermiresonance.
E.g(1) aldehyde(R-C-H)
O
Here coupling between –C-H stretch and
overtone of –C-H str occur
(2)in organic structure :-
appearance of doublet of –C=O stretch of
cyclopentanone ,Fermiresonance with an
overtone or combination band of an alfa-
methylene group shows 2 absorption in the
carbonyl stretch region
.

25. (2)Hydrogen bonding:-
Possible only if any system contain 1 proton
donor and 1 proton acceptor groups in system.
Here,”s” orbital of proton

donor overlaps ”p” or
“”orbital of proton acceptor group.
E.g of donors:--COOH
-OH
-NH
2
-CONH
2
E.g: of acceptor:-any system containing “=“

26. “=“acts as proton

acceptor i.e. it acts as Lewis
base.
Whenever H-bond is present in system
lengthening of the bond occur.
e.g:-R-O-H---------O-R
H
this bond length is increased.
so decreased bond strength.
so “f” value is decreased.
so decreased in wave no.
so decreased in vibrational frequency
So change in shape and position of IR peak.

27. H bonding is denoted by “-----”
2 types of H-bonding
(1)inter molecular
(2)intra molecular
H-bond occur at how much extent depends on
geometry, nature of proton

donor/acceptor ,ring
strain existing in molecule.
At low concentration 2 peaks appear for
alcohol.
-the sharp band is of –O-H str in free alcohol
-the broad band is due to H-bonded –O-H
str

28. E.g:-in which intermolecular H-bonding occur
-alcohol,
- phenol,
-carboxylic acid,
etc.
E.g:-in which intramolecular H-bonding
occuro-chloro and o-alkoxy
phenols,
-beta-hydroxy amino/nitro compounds
Hydrogen bonding is strongest when
bonded structure is stabilized by
resonance.

29. E.g:-(1)salicylic acid (o-hydroxy benzoic acid/p-
hydroxy benzoic acid):-here, both shows different
peaks because OHBA shows intramolecular H-
bond
(2)phenol shows inter/intramolecular H-bonding
(3)enols/chelates:-here, H-bonding is so strong
that even though diluting solution it can’t break
-C- and –OH:-here, H-bond with O of –C=O
O so, decreased double bond
characteristic.
Depends on basicity of –C=O group.
If more basic stronger H bond.
-COOH also shows H-bonding

30. (3)aromatic compounds posses pie-system.
-so. posses conjugation.
-act as Lewis base.
-so. decreased ‘=‘characteristic
-so, deceased F
(4)-NH
2:-shows 2 peaks, at 3000cm
-1
and 3600cm
-1
-if free amino group  shows peak at 3600cm
-1
-if H-bonded amino group shows peak at 3000cm
-1
- H-bond in –N-H is more weaker than that of in
–O-H.

31. (3)Electronic effects-
depends on presence of substituent.
(a)conjugation:-cause delocalization of electron,
-so, decreases “=“characteristic
increases bond length.
decreases bond strength
so decrease on vibrational frequency.


O here,delocalization of -electron
between –C=O and ring increases double bond
character of bond joining them.

32. (b)Resonance effect(mesomeric effect):-means single
molecule can be represented in 2 or more than 2
forms
-if electron releasing group present increases in
delocalization decreases in “=“ characteristic
increases in bond length decreases bond
strength so, decreases vibrational frequency.

33. (c)Inductive effect:-
-Inductive and resonance both type of effects are
existing in molecule.
-finally which type of effect is shown by molecule
depends on which effect is predominant.
E.g:-amidesR-C-NH
2:-NH
2 is electronreleasing
O group so, more
resonance effect
so decreases F
so, decreases vibrational
frequency

34. (2)R-C-Cl:--C=O is electron withdrawing
O groupso,decreases in
delocalization of electron
so, increases bond strength
so, increases in vibrational
frequency
(3)Esters:-R-C-0-R
1:-if alkyl estersresonance
O effect is predominant
If R=benzeneinductive effect is predominant.

35. (4)Field effect:-
If 2 functional group present and 1 affect the
vibration of otherknown as field effect.

36. OCH3
C=O:-here, Cl
-
present in
equatorial position
Cl 2 functional groups are near
each other
 so, repulsion occur
When –C=O group is in axial position no field
effect
(5)Bond angle:-normal bond angle is of 120
0.
-if decrease in angle strain occur bond length is
decreased and “=“characteristic is
increased.increased Fincreadsed vibrational
frequency.

37. If ring expands increased length of bond so
decreased F so, decreased vibrational
frequency
(6)Interaction between solute and solvent:-due to
this change in position and shape of IR peak.
(7)Concentration of analyte :-increased
concentration of analyte so, more interaction
so, band broadening
(8)Nature of solvent:-should be transparent to IR
-should be free from water
-should be inert.

38. (9)Temperature effect:-
-if increased temperature
increased interaction
so band broadening
also, there may be change in position and shape
of IR spectra

39. INFRARED
SPECTROSCOPY
INSTUMENTATION

40. Source Sample Monochromator
Detector Recorder

41. Schematic Diagram of IR Spectrometer

42. Instrumentation:-
Source
Monochromator
Sample compartment
Detector
Recorder

43. •Source(thermal source):-
 Single inert solid rod is heated to high temp.
between 1500-2200
0
K.
Heating is carried out by passing current.
Requirement:-
continuous emission of IR Radiation
all sources used in IR region are thermal
sources
Should emit high intensity radiation.

44. (1)Nernst glower source:-

 Cylindrical rod is prepared by fusing mixture of
oxide of 4 metal i.e. yttrium, cerium, thorium &
zirconium.
Rod is 2 cm long &have 1 mm diameter.
Rod is heated to high temperature between
1200-2200
0
K by passing electrical current.
Emit radiation in range between 1000-
10000cm
-1

45. Disadvantage:-
1. Large negative temperature co-efficient of
electrical resistance. It is not much conductive & so
not easily heated & so Preheating is required which
is carried out by auxillary heater
2 High heat is generated,so ventilation is require to
remove surplus heat and evaporated oxides and
binder.
3. Emits radiation in range of 100-10000cm
-1
.
4. heated upto certain temp. range only
5. frequent mechanical failure.

46. (2) Glober source:-

It is single silicone carbide rod which is 50 mm
long & 5 mm in diameter & heated to 1300 -
1500 K by passing electrical current.
Maximum radiation at 5200 cm
-1
Advantage over Nernst:-
 Possess large positive temperature co-efficient.
So no need of auxillary heater.
And not even preheating required.
Disadvantage:-
Radiation intensity is less than nerst glower.

47. (3) Nichrome wire source:-

It is heated to 1100
0
K which emits IR radiation.
It is simple wire of nichrome which is spirally
wounded.
Advantage:-
Simple, rugged and has longer life time and
requires less care compared to other sources.
Disadvantage:-
 It emits lower intensity radiation compared to
other source.

48. (4) Tungsten filament lamp:-
Used for near IR only.
Also for visible IR.
Emits radiation in range of 0.78-2.5µm.

(5) Hg arc lamp:-
For far IR region.
Hg vapour at high pressure > 1 atm filled in quartz
envelop.
Electrical current is passed which emits radiation.

49. (6) CO2 laser source:-
Emits radiation in range of 900-1100cm
-1
used for determination of
C
6H
6,NH
3,ethanol,NO
2,trichloro ethylene.
Also used for determination of atmospheric
pollution.
Advantage:-
 Emits radiation of very high intensity compared to
thermal source.
(7)Rhodium wire:-
Incadecent wire source
Heated at temp.at 1100ºk

50. 2. Monochromator:-
Convert polycromatic light to monochromatic light
(1) Prism:-
 Quartz prism can be used between 0.8-3 µm
& can not be used above 4 µm because above 4
µm, it starts absorbing radiation strongly.
NaCl prism is widely used in range of 5-15 µm & can
not be used above 15 µm because it absorbs
radiation strongly.
CsBr-KBr prism is used for far IR (15-40 µm)
 LiF
2 prism is used for near IR(1-5 µm).

51. Disadvantage of prism:-
Resolution is not good as that of grating
monochromator.
they absorbs moisture & so fogging occur.
so it require polishing which can be done by
polishing agent i.e. slurry of aloxite & bansite which
is prepared in ethanol-ethylene glycol water.
Slurry is spreaded on silk cloth stretched on flat
surface.
Prism is polished by rubbing on flat surface &
wipped by dry cloth.

52. 2 types of monochromator:-
(1)single pass
(2)double pass/ double monochromator:-
It has high resolution because here radiation
passes 4 times
Here 2 plain mirror are to be used.

53. (2) Grating:-

Either made up of glass or plastic coated with
aluminum.
Advantage:-
Covered with Alluminium so strong enough so
life-time is high.
Produce linear dispersion
Not affected by moisture
High resolution

54. 3.Sample compartment and
sample Preparation:-

Sample handling is most difficult & most time
consuming part of IR analysis. So it is most
important.
Sample cuvette made up of quartz or halide
salts.
Sample handling depend on physical state of
sample (gas, liquid or solid).

55. (A)GAS:-

Choice depends on
-nature of gas
-concentration
Path length should be long
-if highly absorbing gas10cm sufficient
-if less absorbing gasmore than 10cm length
is required.
(1) single path cells:-
It posses halide window .
It possess two pots -entrance pot & exit pot

56.  Both are fitted with valve which is open and
connected to tubes which allows entry of gas
from entrance valve till it recover all space & exit
from exit pot.
Gas is filled at high pressure, so more amt of gas
come out of exit pot.
(2) Multipath cell:-
It is used for gas which required longer path
length in cell.
Internal surface is reflective so gas beam can
possess no. of paths.
 Internal surface coated with Ag/Au.
 Low boiling point liquid sample allow to
expand.

58. (B) Liquid:-


For low viscous liquid  filled in couvette.
For highly viscous liquid sandwitched
between 2 halide plats.
(1) For low viscous liquid:
Liquid sample is filled in couvette.
Sample is taken by use of syring.
Dimension of couvette is in between 0.01-1 mm.
Solvent should be transparent to IR
 But no solvent is available which is
transparent in IR region.

59. Desirable characteristic of
solvent:-

transparent in wave length range
Not interact with solute,i.e it should be inert.
Adequate solubility property (sample should be
soluble in solvent.)
Refractory index of solvent should match to salts
which is used to prepare cell.
We can’t use polar solvents ,as they absorb
moisture.
For that add 2,2-dimethoxypropane,so if any
moisture is present that will react with it.

60. Examples of solvents used:-
(1) Combination of n-heptane & tetrachloro ethylene
 n-heptane --- transparent in IR region of 250-1000
cm
-1
 tetrachloro ethylene--- transparent in IR region of
1000-4000 cm
-1.

Range of mixture in IR region---250-4000 cm
-1.

(2)CCl
4 and CS
2 combination
CCl
4 transparent in 1350-4000cm
-1
CS
2 transparent in 625-1350cm
-

So range of mixture is 625-4000cm-1

61. CS
2 not used in sample containing
primary/secondary amine or alcohol.
Dioxane,CHCl
3, and DMF(dimethyl formamide)-
polar solvents are used.
Now a days,ultra microcavity as a sample holder
is available. For that microlitre sample is
required

62. (2)For highly viscous liquid:-
Two flat plates of halide salt is used & in
between sample solution is placed.
Plates are made up of CsBr/CsI.
Thickness should be very very less which should
be in range of 0.01-0.1 mm.
Plats are held together by capillary action.
For,volatile liquid AgCl plates are used.

63. Sampling of Liquid

64. (C) Solid:-
(1)Prepared as mulls:-
Preparation of paste by addition of mulling oil
(mineral oil)
2 to 5 mg drug is taken & grinded in mortar.
Grinding should be such that particle size should be
<2 µm. Otherwise excessive scattering of radiation
occurs.
A drop of mulling oil is added for preparation of
paste.
Paste is sandwitched between 2 halide plats.
 Mulling oil is nujol (high boiling point petroleum
oil,hydrocarbon) which is widely used because
characteristic of it in IR region is known.

65.  4 different peak for nujol oil-2915,1462,1376,719 cm
-1
If any sample absorbing at this range,we can’t use
nujol.
For that other halogenated polymer like flourolube is
used.
Other mulling oil is Hexachlorobutadiene
Nujol and flurolube scan sample in range
250-4000cm
-1

66. (2)Pressed disc:-
Also known as pellet disc method.
It is prepared by mixing 1-2 mg of drug with 100 mg
of KBr & grinding & mixing is done in mortar or
small vibrating ball-mill is used until particle size is <
2 µm which avoid scattering.
It is pressed under high pressure 10000-15000 psi in
vaccum to remove water.
Otherwise 2 additional peaks appear at 1640-
3450cm
-
 Special dyes &punch are used to press the powder
in specific shape.

67. Pressed Pellet Technique

68. (3)Deposited film:-

Solid film method
Sample is prepared in volatile solvent.
Here sample is spread as thin film on halide
platethen allow to evaporate solventso thin
film deposition
For solution which on cooling or evaporation
produces thin film of solid material.
Used for obtaining IR spectra of plastic & resin.
Disadvantages:-cause excessive scattering

69. 4.Detectors:-




(1)Thermal detector:-
Important & used in simple IR.
Used over wide range of wavelength
It depends on its heating effect.
It is small black body material which is heated.
Temperature rise due to IRwhich is converted to
potential differenceand that is measured.

70. Disadvantage:-
Low sensitivity.
thermal noisei.e due to surrounding of
detector.which is reduced by
<1> enclosing the detector in evacuated housing with
KBr/CsI window & operated in presence of vacuum.
<2> By placing chopper
Here, radiation come out of analyte will pass
through chopper which is operated at low frequency
because sensitivity of detector is less.
 extraneous (Unwanted) radiation will be
differentiated by analyte signal which has frequency
of chopper.

71. Type of thermal detector:-

(1) Thermocouple
(2) Bolometer
(3) Thermister
(4) Golay cell
They are less sensitive then pyroelectric and
photon detector.

72. (1) Thermocouple:-

Most widely used due to simple construction.
It works on principle of Peltier effect i.e.
temperature difference between 2 junction rises
potential difference which depends upon the
amount of radiation falling upon the hot junction.
Constructed by 2 junction which is prepared by
fusing 2 similar metal to dissimilar metal
Metals areBismuth-Antimony
 Junctions are coated with metallic oxide film.

73. Placed in evacuated steel housing having KBr /
CsI window.
One junction is cold junction & other is hot
junction.
Potential difference between 2 junction is 6-
8μV
Cold junction is kept at constant temperature &
hot junction is exposed to IR radiation So
temperature of it increases.
here, temperature difference between Cold
junction & hot junction is calculated & converted
to potential difference

74. Potential difference of thermocouple is directly
proportional to intensity of IR radiation.
Sensitivity is low& have slow response time.
It can detect temperature difference of 10-6 K
Prepared by making series of thermocouple by
fusing & this is called as thermopile for increasing
sensitivity.
Advantage:- independence of the response with
changes in wavelength

75. Thermocouple Detector

76. (2) Bolometer(resistance thermometer):-

Consist of single metallic strip like Pt or Ni or small
strip of semiconductor material.
When IR is absorbed strip is heated decrease in
electrical resistance.
Change in electrical resistant is converted to
current.
Constructed in form of whiston bridge. On one arm
of it, bolometer is placed and on other arm, same
type of strip is placed which is not exposed to IR.
Two arm is joined by Galvanometer.

77. Bolometer Detector

78. When no IR radiation, then bridge remain balanced,
no current flow .
current is registered by galvanometer.
Placed in evacuated still housing with KBr/CsI
window to minimize thermal noise. Most widely
used is germanium bolometer which is operated at
1.5 K temperature and it is used in range 5 to 400
cm
-1

(3) Thermister:-
•Similar to bolometer but instead of metal,
metalic oxide is used.
•Prepared by fusing mixture of metallic oxide
such as cobalt, manganese or nickel. Change in
electrical resistance is converted to electrical
current.

79. (4) Golay cell
Principle:-It responds to change in volume of
non absorbing gas as a function of temperature
rise. As a result of expansion of the gas , the
pressure increases which is then converted to an
electrical signal.
 consist of small metallic cylinder in which inert
gas is filled. one side consist of black metallic
plate.
Opposite side consist of flexible metallic
diaphragm.

80. When no IR Diaphragm is in contracted position.
When contracted then the radiation is reflected
on opaque surface.
IR fall on metallic plate and so heated up .so
temperature transfer to gas . Diaphragm is
pushed in outward direction . As soon as
Diaphragm pushes outward visible optical
system is placed .

81. and when pushed then radiation is reflected and
passed through metallic grid and passes from
lense and detected by PMT due to change in
angle of reflection and gas expansion is
converted to electrical current.
In other type-no need of visible optical system
-here 2 capacitor plates are used.
Here, capacitance between 2 capacitor plates is
measured as electrical current.
 It require less response time.

82. (2)Pyroelectric Detector:-

Constructed from pyroelectic material
(dielectric material)
Here,polarised material is used  polarization
depends on temperature and it is continuous
after removal of electrical field.
Here,2 electrode are used and in between them
the plate of pyroelectic material is placed. and
when IR fall change in charge it is
registered as electrical current.

83. E.g. of pyroelectic material:-
-Triglycine sulfate
-Barium titanate
-Lithium niobate
-lithium tantalate
-deuterated triglycine sulphate
these all have both properties i.e. thermal and
electrical.
They should not heated above temperature
which is known as curie point.

84. Above curie point,polarisation capacity is lost.
e.g:-diglycerine sulfate loose its polarization
capacity above 47
0
C
Advantage:-responding time is less i.e. upto
1milisec or <1milisec
Current depends on surface area of crystal

85. (3)Photon detector:-

Advantage: 10-100 times more sensitive than thermal
detector.
Response is more rapid than that of thermal detectors.
Disadvantage: useful over a narrow wavelength range (1 to
6 micrometer)
Principle:-as temperature increases decreases in electrical
resistance.
Constructed from thin film of both intrinsic and extrinsic
semiconductor material
e.g.:-lead sulphide
-indium antimonyl
-indium arsenyl
-lead selenile
-Hg/Cd teluride

86. It is placed on glass surface which is non-
conducting at lower energy state.
When IR radiation fall and energy of valance
electron rise, they posses highest energy and
become conductor.
Here, electrical current is measured.
Change in electrical resistance is converted to
electrical current.

87. THANK YOU