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Electromagnetic Spectrum and Origin of Absorption Spectra

Visible light is one from of the energy generally known as electromagnetic radiation. Other common forms of electromagnetic radiation are X-rays; ultraviolet radiation (UV, the radiation from the sun);infrared radiation (IR, the radiation from a heat source); microwaves (used in radar and in the microwave open); and radio waves (used to carry radio and television signals). Electromagnetic radiation can be characterised by their frequency or wavelength.The wavelength is a more convenient description of waves with higher frequencies.The wavelength (Λ) of a wave is defined by the equation

                 λ= C/υ                 ……..(i)

in which c = the velocity of light = 3 x 1010 cm/sec, υ = frequency of radiation
The energy associated with a particular electromagnetic radiation is given by the expression:

 E = hυ = hc/ λ = hcυ                  ……(ii)
 E = energy of radiation in ergs
 h = Plank’s constant ( 6.62 x 10-27 ergs sec)  and,  λ= wavelength of electromagnetic radiation ( in hertz or cps )

 λ= is expressed in number of units such as Angstron (); micron (µ) ; millmicron (mµ)
                   and nanometer (nm).

                   1 Å =10-8  cm =10-10 m
                   1 µ = 10-4 cm =10-6 m
                   1 µm = 10-7 cm =10-9 m
                   1 nm = 10-7 m =10-9 m
                             = 10 x 10-8 cm =10 

Wave number simply as the reciprocal of the wavelength. Obviously units of  υ = (= 1/ λ) will be cm-1 or m-1.

Equation(ii) shows that the energy, frequency and wavelength of electromagnetic radiation are simply related.

 Thus, when the frequency or wavelength of electromagnetic radiation is known,    its energy ia also known. The arrangement of all types of electromagnetic radiation in the order of increasing wavelength or decreasing frequencies is called the electromagnetic spectrum.
The types of radiation within the electromagnetic spectrum are shown in Fig. 1.1. All electromagnetic radiation are fundamentally the same, the various forms differ only in energy.

Fig. 1.1 Electromagnetic Spectrum

The energy (E) of molecule, besides translational and nuclear energies ( which do not interface in spectroscopic analysis) can be considered to be made up of:

1)  Electronic or transitional energy: This is energy which associated with motion of electron in the                                                                 molecule.

2) Vibrational energy: This is the energy which is associated with the vibration of the Constituent                                            atom in the molecule.

3) Rotational energy: This is the energy which is associated with the rotation of the Molecule as a                                          whole.

Thus the total energy of the molecule for spectroscpic purpose can be written as:

       E(total) = E(electronic) +E(vibrational)  +                        E(rotational)

Each of the energies is quantized and can exist with some discrete values .This means that these energies have to follow quantum restrictions. The discrete (quantized) of these energies depend on the properties of the molecules, i.e., shape ,size, flexibility as well as on the type of motion.

If a molecule is placed in an electromagnetic radiation, a transfer of energy from the electromagnetic radiation to the molecule occurs when,

                                 E = hυ                    

        where,        h = Plank’s constant
                              υ = frequency of light
                            E = Difference in energy.                                          between two quantized                                      states.
The molecule absorb energy, when it is excited from the lower energy state E1 to higher state E2 . Hence,
                          ∆E = E1 E2

When molecule absorbs energy, there may be three types of changes in the molecules.

i)   There may changes in rotational levels.

ii) There may be a change in vibrational level on which rotational change may be superimposed.

iii)  An electronic state may be change simultaneously with changes in both vibrational and rotational       energies.

As a result even with a simple diatomic molecule, there is possibility of  a large number of changes in energy states producing many spectral line.

The various energy level in the molecule are shown in Fig. 1.2. Suppose A and B are two electronic states (energy states  n) of a molecules. In each electronic states there are vibrational energy level,

FIg. 1.2. Molecular energy level digram

indicated  by vibrational quantum number V (= 0,1, 2 ,3, 4, 5,…..). Again foer each vibrational states (energy level), there exist several rotational energy levels, indicated by quantum number J (= 0, 1 ,2, 3, 4, 5,..). A possible transition in any one of these energy level will lead to change in energy content and hence give rise to spectral lines. 

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