Spectroscopy
The ways in which the measurements
of radiation frequency (either emitted or absorbed) are made experimentally and
the energy levels deduced from these comprise the spectroscopy. It gives qualitative
and quantitative information.
Spectroscopy deals with
interaction of electromagnetic radiation with any compound or atoms. The interaction is measured as energy which
is either absorbed or emitted by the matter in discrete amount called quanta.
Spectroscopy was originally the study of
the interaction between radiation and matter as a function of wavelength
(λ).
Spectrometry is the spectroscopic
technique used to assess the concentration or amount of a given species. In
those cases, the instrument that performs such measurements is a spectrometer
or spectrograph.
Spectroscopy/spectrometry
is often used in physical and analytical chemistry for the
identification of substances through the spectrum emitted from or absorbed by
them.
Spectroscopy/spectrometry
is also heavily used in astronomy and remote
sensing. Most large telescopes have spectrometers, which are used either to
measure the chemical composition and physical properties of astronomical
objects or to measure their velocities from the Doppler shift
of their spectral lines.
There are two types of
spectroscopy
1. Atomic spectroscopy
It deals the
interaction of electromagnetic radiation with atoms which are most commonly in
their lowest energy state called the ground state level.
DE
= h g here
h = Planck's constant
g
= Frequency of radiation
2. Molecular spectroscopy
It deals the interaction
of electromagnetic radiation with molecules which includes rotational, vibrational
and election transitions of the molecule. It gives the information regarding
molecular structure (molecular symmetry, bond distances and angles, and
chemical properties such as electronic distribution, bond strength and intra or
inter molecular processes)
Properties of
electromagnetic radiation
Electromagnetic
radiation is a form of energy that is transmitted through space at an enormous
velocity which is equivalent to the velocity of light in the space. It requires
no supporting media and can travel in vaccum. They have dual nature exhibiting
both wave and particle like properties.
A. Wave properties
Wave properties of
electromagnetic beam are an alternating electrical and associated magnetic
force in space. They possess both the electric component and magnetic component.
Both of these oscillate in plane perpendicular to each other and perpendicular to
the direction of propagation of the radiation. They are coherent and plane
polarized. Phase of one is related to that of other. Velocity of it is
independent of frequency on vaccum (c= 3 x 108 m/s). Some of the
wave properties are:
1. Wavelength
It is a distance
between two successive maxima on an electromagnetic wave.
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Figure: Electromagnetic beam showing wavelength, l
It is denoted by ' l'
lambda (Greek letter). The units of wavelength are m, cm, mm, mm,
nm and A°.
The beam carrying only one discrete wavelength is said to be monochromatic
beam.
2. Frequency
The number of complete
wavelength units passing through a given point in unit time is called frequency
of radiation. It is denoted by 'g'
Gamma
(Greek letter).Its unit is hertz or Fresnel which means per second (s-1).
Further they can be measured in terms of KHz, MHz or GHz etc.
3. Wave number
Frequency is more
fundamental than the wavelength. So it is cumbersome to use in practice because
of large number of frequency in calculation. Therefore in practice, it
expresses frequency in wave number. It is defined as the numbers of waves per
centimeter in vaccum which is denoted by 'n'
(Greek v letter) or bar in frequency.
n
= 1 / l
Its unit is Kaiser or
Kilokaiser (Per centimeter or cm-1).
4. Velocity
It is defined as
distance travelled by electromagnetic radiation per unit time. It is denoted by
'c'.
Velocity=
wavelength x frequency
c
= l
x g
5. Relationship between
frequency, velocity and wave number
c
= l
x g
or
n
= g
/ c here:
n
= 1 / l
\ g
= c x n ………………….equ i
i.e. The
frequency of radiation is mathematically equal to the product of velocity of radiation
and its wave number.
The examples of wave
properties of electromagnetic radiations are refraction, reflection etc.
B. Particle Properties
Electromagnetic
radiation consists of a stream of discrete packets (particles) of pure energy
called photons or quanta. They have definite energy and travel in the direction
of propagation of the radiation beam with the velocity equal to that of light.
E
= h . g Where
h = 6.626 x 10-34 J. s
(Planck's constant )
The example of particle
property of electromagnetic radiation is photoelectric effect.
1. Relationship between
wave and particle properties
We have, E = h . g
Or E = h . c . n
Where c is velocity
and n
is wave number of electromagnetic radiation
Or E = h .
c . 1/ l here; n
= 1/ l
\ E
µ
1/ l since both h and c are constant terms
Velocity is independent
of frequency on vaccum. Finally the above relation shows that the energy
carried by an electromagnetic beam is inversely proportional to the wavelength
of the beam. It means lower the wavelength, higher the energy of the beam and
vice versa. For example: The wavelength for ultraviolet rays and visible rays
ranges from 190-390 nm and 400-750 nm respectively. Since the wavelength of ultraviolet
rays are smaller than that of visible rays the energy carried by the UV rays
are comparatively higher than that of visible rays.
