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Selection Rules

We knew that atomic spectra as well as molecular spectra are obtained due to transitions can take place only between definite energy level and not are governed by certain rules which are called the selection rules. If these rules are followed, the transition can take place and it is allied an allowed traction. It these rules are not followed, the transition cannot take place and it is called a forbidden transition. The selection rules to be followed depend upon the type of transition (in case of molecular spectra)*.

The selection rules are generally expressed interims of changes in quantum numbers for the allowed transition. For example, for the pure rotational transition, the selection rules is where J represents rotational quanum number (                corresponds to absorption and corresponds to emmission). similarly, for pure vibrational transition, the selection rule is where represents the vibrational quantum sections.     

Widths and Intensities of the Spectral lines

In any type of molecular spectroscopy the two important characteristics of any spectral line are:

(i)    How sharp (or broad) is the line? This is decided by the width of the line.
(ii)    How strong(or weak)is the line? This is decided by the intensity of the line.

A.    Width of the spectral line.

If spectral line is sharp, , it will have no width as shown in . On the other hand, if the line is not sharp it will have certain width as shown in i.e., it spreads over a range of frequencies (or wave memebers).

                              (a) A sharp spectral line. (b) A spectral line having a width.

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