SPECTROSCOPY
Spectroscopy :
It is a branch of science which deals with the study of interaction of matter
with electromagnetic radiation.
Electromagnetic Radiation :
Electromagnetic Radiations are a certain amount of energy
depending upon its wavelength. Hence these radiations are the sources of energy
so called as electromagnetic energy or radiant energy.
Electromagnetic Radiations are of
many types like radio waves, UV rays, IR rays, visible light etc. these are all
electromagnetic radiations with different energy, wavelength and frequency. All
types of electromagnetic radiations travel with the speed of light but differ
in wavelength and frequency.
Energy of electromagnetic radiations
can be expressed as
Where E = Energy
n = Frequency
l = Wavelength
c
= Velocity of light = 3*108 m/s.
h
= planks constant
Types of electromagnetic radiations
Type
|
Frequency (Hz)
|
Wavelength (1/m)
|
Radio wave (low energy)
|
3*105
|
103
|
Microwave
|
3*109
|
10-1
|
Infrared
|
3*1010
|
10-2
|
Visible light
|
3*1014
|
10-6
|
Ultraviolet
|
3*1016
|
10-8
|
X-rays
|
3*1018
|
10-10
|
Gamma rays (high energy)
|
>3*1018
|
<10-10
|
Type of spectra :
Spectra are of two types
1. Emission spectra :
When a substance is exposed to
intense heat or light its atoms or molecules absorbs energy and get excited.
When these atoms or molecules return to their initial state they emit
radiations which when pass through a prism it produce a spectrum. This spectrum
is known as a emission spectrum. When this type of spectrum is recorded on a
photographic plate then bright lines are formed on a black background in case
of atoms and bands are formed in case molecules
2. Absorption Spectra :
When a substance or matter is exposed
to intense heat or light it absorbs energy. Due to this intensity of absorption
varies as function of the frequency. This variation is known as absorption
spectrum. This type of spectra contains dark (black) lines or bands on light
coloured background as some radiations are absorbed by the medium.
In this
process only those photons of radiation are absorbed whose energy is equal to
energy difference (DE) between two energy levels of molecules of substance.
Where h =
planck’s constant = 6.63 * 10-34 Js
n = Frequency
c = Velocity of light = 3 * 108
m/s
l = Wavelength of radiation
NA
= Avogadro’s number = 6.02 * 1023 mol-1
Difference between Emission and Absorption Spectrum :
In Emission Spectrum molecules come
back to lower energy state from higher energy state while in Absorption
Spectrum The molecule goes to Higher energy state from lower energy state.
Absorption Spectrum and its types :
in addition to the nuclear energy, total internal
energy of a molecule consist of three types of energies.
Einternal = Eelectronic
+ Evibrational + Erotational
Since, electromagnetic radiation is a
form of energy so its absorption by a molecule, increases the internal energy
of the molecule. Also when a molecule is exposed to electromagnetic radiation,
molecule does not absorbs all the radiations rather it absorb a particular
portion of radiation depending upon the structure of the molecule and amount of
absorption of energy depend upon the frequency of the radiation also. So it is
clear that different molecules absorb different type of energies and undergo
different excitations. Depending upon the absorption of energy molecule may go
under Electronic excitation, Vibrational excitation and/or Rotational
excitation. So produces different Spectra.
you read this post at Chemistry Notes Info http://chemistrynotesinfo.blogspot.in
Table : Some Components of the Electromagnetic Radiation and Absorption
Spectra
Absorbed
Radiation and Type of Spectra
|
Wavelength
|
Energy
(kJ/mol)
|
Types
of Excitation
|
Effect
on Molecule
|
Application
|
Microwave
|
1 cm
|
0.01 - 1
|
Rotational
|
Changes in the Rotational energy levels of the
molecule
|
Calculation of bond distance and bond length
|
Infrared
|
2 – 15 mm
|
1 – 100
|
Vibrational and Rotational
|
Changes in Vibrational and rotational energy levels
|
Identification of functional groups, Calculation of
Bond length, Bond angle and Qualitative analysis
|
Electronic
(i)
Visible
(ii)
Ultraviolet
|
200-400 nm
400-800 nm
|
150-300
300-600
|
Electronic
Vibrational and rotational transitions also take
place but their resolution is not measurable
|
Change in Electronic energy level
|
Qualitative and quantitative analysis
|
The Absorption energy is measured
with the help of Spectrophotometer and expressed as Frequency, Wavelength or
Wave-number.
Law of Light Absorbance :
According to the Lambert Beer’s Law “The Absorption of light
is directly proportional to the concentration of the solution and the length of
the cell containing sample.
Where, I0 = Intensity of
incident Radiation
I = Intensity of transmitted radiation
c
= Molar concentration
l
= Length of the cell in centimetre
e(epcylon) = Molecular extinction
coefficient
The Wavelength at which molecule has maximum
Absorption coefficient e (max) is expressed as l (max). in above
expression log I0/I is
known as absorbance or optical density of the solution. And log I/I0 is known as
Transmission of the solution.
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