Applying the selection rule, as the transitions can take place between vibration levels whose vibration quantum numbers different by unity (i.e.) ,t he energy absorbed by a diatomic molecule, taking it as a simple harmonic oscillator, when the transition takes place form to level will be given by

       
Thus only one absorption line will be obtained the vibration spectrum whose wave number is equal to the equilibrium vibration are equally spaced, any transition form any to will give rise to the Further, at room temperature, as most of the molecules are in the lowest (ground state) vibration level with , therefore the transitions taking place are from to . The frequency thus observed is called the fundamental frequency. These spectra are observed into he infrared (IR) region.

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