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Molecular Spectra :

Spectra emitted by molecules containing two or more atoms are called molecular spectra. 
Molecular spectra under low dispersion appears as continuous band & therefore called spectra. 
Usually a band has a sharp intense edge on one side called bond head & gradually decreases in intensity toward the other side.
With high resolving power spectroscope, bond spectra are found to have following structure:
  • Each band is seen to be composed of a large number of lines which are crowded together at long wavelength side called band head & they recede from th band head,  the line become more widely separated and also weaker. The line however being so close together as to appear under low resolution like a continuous spectra.
  •  There is a regular sequence in which several bond follow one another & thus constitute a group of bands. 
  • These different group of bands are quite close & thus because of their regular arrangement from a group of bands. 
Fig. Graphical representation
of molecular spectra. 

Molecular spectra are of three types :

(i) Electronic Spectra:
These spectra are observed, both in emission and absorption, in the visible and the ultraviolet regions. Each spectrum consists of quite a large number of bands. Each individual band has a sharp edge, called band-head, where the intensity falls suddenly to zero; and from this edge the intensity falls off gradually to the other side of the band.
 Under high resolution, a band is found to be composed of a series of lines which draw farther apart as the distance from the band-head increases. The band is said to be degraded’ toward the side opposite to the band-head. 
The electronic spectra are observed for the heteronuclear as well as for the homonuclear diatomic molecules.
(ii) Vibrational-Rotational Spectra :
These spectra are observed in absorption, in the near infra-red region (1 μ-102 μ ; 1 μ 10-6 m 104 A). Each spectrum consists of an
intense band, called “fundamental” band which is accompanied by a few weak bands.
These bands, like electronic bands, have a fine structure. They are observed only for the
heteronuclear molecules. The homonulcear molecules like H2, N2, 02 do not produce
(iii) Pure Rotational Spectra :
These spectra are observed in absorption in the far infra-red (102 μ-103 μ) 01 in the microwave (103 μ-104 μ) region. Each spectrum is composed of a series of nearly equidistant lines.
 Like vibrational-rotational spectra, the pure rotational spectra are observed only for the heteronuclear diatomic molecules.The origin of different molecular spectra lies in the different energy states of the molecule.