INTRODUCTION
TO
SPECTROSCOPY.
The study of interaction of Electromagnetic radiation with matter is
called as spectroscopy.
The study of spectroscopy deals with emission and absorption
spectra.
When the
electric discharge (Radiation) is passed through the
vapours of the substance, then by the absorption of energy the electrons
in the ground state are promoted to higher energy states. When the
electron jumps from higher energy state to lower energy state, some
energy is released in the form of radiation. If these radiations are analysed
we get emission spectrum.
If the Electromagnetic radiations are passed through the substance,
then the radiations of certain wavelengths are absorbed by the substance.
The wavelengths which are absorbed characterize some particulars
functional group present in the compound. The dark pattern of lines
which corresponds to the wavelengths absorbed is called as Absorption
spectrum. Hence is a Absorption spectroscopy.
ELECTROMAGNETIC RADIATION :
It is a form of energy that is transmitted through the space at an
Enormous velocity. Visible light (Light) is a part of electromagnetic
radiation.
NATURE OF ELECTROMAGNETIC RADIATION :
Light is a form of Electromagnetic radiation having dual nature i.e.
it has a particle as well as wave like properties.
Light radiations are produced by the oscillations of electric charge
and the magnetic field residing on the atom. The electric and Magnetic
components are perpendicular to each other and are coplanar.
These radiations are characterized by their wavelengths. The
energy carried by radiations is directly proportional to its frequency.
The entire range of wavelength over which electromagnetic
radiations extends is called as electroimagnetic spectrum. A wave is
characterized by the following parameters which are released to each
other <> NITs.
WAVELEGNTH :
It is the distance between the two adjacent crests or troughs in a
wave. It is denoted by
(lambda) & expressed in Ao or mll or nm.
WAVE NUMBER:
The number of waves which can pass through a space (distance) of
one cm is called as wave number. It is expressed as . . . . or it is the
reciprocal of wavelength.
FREQUENCY :
It is defined as the number of waves which can pass through a
point in one second. It is expressed as . . . (nu) in cycles per second or in
Hz (Hertz)
AMPLITUDE (A)
The maximum displacement of wave from x-axis is called as
amplitude of radiation.
VELOCITY : (C) :
It is the distance travelled by a wave in unit time. It is expressed in
centimeter per second or meter per second. The velocity of radiation is
dependent on the medium through which the radiation passes. In vacuum
its velocity is 3.0 x 10-8 meter per second.
C=
ENERGY (E)
Electromagnetic radiations carries the energy in the form of energy
packets or particles called as quanta or photons. Radiations have energy
due to oscillating Electric and magnetic fields. Each photon or quanta has
discrete (its own) amount of energy & is given by Einsterin planks
equation.
E=
:. h = planks constant.
ELECTROMAGNETIC SPECTRUM :
The entire range of wavelength over which electromagnetic
radiations extends is called as electromagnetic spectrum.
Or.
When all types of electromagnetic radiations are arranged in their
increasing wavelength or decreasing wavelength is called as
electromagnetic spectrum.
Radia tv Radar Microwave Far near visible UAV X ray Gamma
rays.
decrease in Wavelength.
DIFFIERENT REGIONS OF ELECTROMAGNETIC
RADIATIONS
In a spectrum the electromagnetic radiations are arranged
according to their increasing or decreasing wavelengths. A small region
of this electromagnetic spectrum is visible (seen) to human eyes and is
called as visible region or visible light or light.
The various spectral regions are as below.
1)
RADIO WAVES :
These are the radiations having longest (higher) wavelength (lower
frequency). They have a wavelength of 10 m to 1 cm. Radio waves are
used in NMR spectroscopy. Radio, TV and mobile phones work using
radio wave signals.
2)
MICROWAVES :
Microwaves are the second higher wave - length or lower
frequency radiations. They have a wavelength of 0.1 mm to 1 cm.
Microwaves are used in rotational spectroscopy and for radar,
landline phone and the transmission of computer data, cooking ovens etc.
3)
INFRA RED WAVES / RADIATIONS :
The radiations in this region have a wavelength 0.4 u to 200 u these
(IR) radiations are associated with the changes in the vibration of
molecules and hence used in vibrational spectroscopy and in remote
controls etc.
4)
VISIBLE LIGHT :
These radiations can be seen by human naked eye hence called
visible light or light. They have a wavelength of 400 – 750 nm. These
radiations are used in colorimetric analysts.
5)
ULTRAVIOLET WAVES / RADIATIONS :
These are the radiations having shorter wavelength in the range 20
to 400 nm, these are the radiations used in UV spectroscopy.
6)
X-RAY WAVES :
These are the radiations having very short wavelength & hence
high energy. These radiations have a wavelength of 0.1 to 1 nm. These
are used to image bone structure of body and in X-ray spectroscopy used
the analysis of crystal structure.
7)
GAMMA RAYS :
These are the radiations having very shorter wavelength than X-
rays having a range of 0.02 to 0.1 nm. Gama rays are emitted by
lightning, nuclear explosions and radioactive decay.
* RELATIONSHIP BETWEEN WAVELENGTH AND ENERGY
OF ELECTROMAGNETIC RADIATION :
According to Einstein-Plank relation the energy of Electronic
radiation depends only on the frequency or wavelength of waves and not
on the intensity of its beam.
When a photon of Energy (E) is absorbed or emitted, then the
transition takes place between the energy states of a system.
The frequency ( ) absorbed or emitted radiation is given by
We know that the frequent of radiation is given as –
Substituting in
E
=
E
=
Thus, Energy is directly proportional to Frequency of a wave.
Shorter is the wavelength, greater is the frequency and greater is the
energy of the radiation.
* FREQUENCY OF ELECTROMAGNETIC :
RADIATION WITH MATTER :
When the Electromagnetic radiations interact with matter (sub.),
the radiations ma be absorbed, transmitted, reflected and diffracted. This
interaction is a Quantum phenomenon, which depends on the nature of
matter and wavelength of radiation used.
Organic Spectroscopy is essentially the study of how photons are
absorbed by organic molecules and which ones are absorbed. By knowing
the kinds of photons that an organic molecule can absorb, we can get
information about the shape, size, structure etc. of org. molecule.
When, a organic comp. is exposed to the electromagnetic radiation.
Then the radiations having an energy equal to the energy required for
excitation of a molecule (Electronic or vibrational states) is present, then
and then only the radiations are absorbed by the substance and changes in
electronic, vibrational, rotational states occur.
Thus for example excitation of an electron in a molecule from
lower energy level (E1) to higher energy level (E2) takes place if the
energy (photon or qunta) of the radiations is equal to the energy
difference of two electronic levels.
(i.e.
E=
E2
-
E1)
So,
Where
E
=
Energy required for transition.
E1 & E2
=
Energy states or Levels.
E1 & E2
=
Energy of that level.
The E transition energy is characteristic of etomic or molecular
structure of the substance. Thus the peaks in the spectrum are due to type
of atoms, type of bonds, stereochemistry of molecule etc. So the intensity
and amount of radiation absorbed gives information relating to the
structure of molecule and conc. of it.
The energy absorbed is given out in the form of neat or re-emitting
electromagnetic radiation. The instrument which records the variation in
intensity of incident radiation with respect to emitted radiation is known
as spectrometer.
The inactions can be observed in two ways. The spectrum obtained
by recording radiations emitted by sample is known as emission
spectrum. Where is spectrum obtained by recording radiation absorbed
from continuous source is known as absorption spectrum.
ENERGY TYPES AND ENERGY LEVELS OF ATOMS AND
MOLECULES :
Energy Types :
There are many forms of Energies, but all of them can be put into
two categories 1) Potential energy and 2) Kinetiz energy.
Potential energy comes in forms that are stored including chemical,
gravitation, mechanical and nuclear.
Kinetic energy forms are doing work, like electrical, neat, light,
motion and sound.
All the above forms of energies are related and are inter
convertible.
LEVELS :
Electrons of atoms or molecules are distributed between many
different energy levels called as atoms or molecular energy levels.