Energy level diagram showing electronic, vibrational, and rotational states

Now it becomes possible, from a study of the frequencies and intensities of the light absorbed or emitted by a molecule in going from one state to another, to gain some information concerning the interatomic distances and atomic configurations in the molecule. 

We need only consider changes in / of 1, since there is a selection rule which forbids any other transitions within a single electronic 

If no change other than rotational takes place, i.e. if a molecule in a specific vibrational state of a given electronic state changes from one rotational state to another, the resulting spectrum, known as a pure 

only molecules possessing a permanent dipole moment would 

1) state. The frequencies in a vibration-rotation band will 

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Figure 12. Energy level diagram showing electronic, vibrational, and rotational states.

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