The energy states and spectra of molecules are much more complex than those of isolated atoms. In addition to the energies associated with molecular electronic states, there is kinetic energy associated with vibrational and rotational motions. The total energy, E, of a molecule (apart from its translational55 and nuclear energy) can be expressed as the sum of three terms:

E=Eelectronic+Evibrational+Erotational  (9.6.1)

Absorption of electromagnetic radiation by molecules occurs not only by electronic excitation of the type described for atoms, but also by changes in the vibrational and rotational energies.

Both rotations and vibrations of molecules are quantized. This means that only particular values of rotational angular momentum or vibrational energy are possible. We speak of these permitted values of the energies as the vibrational and rotational energy levels.

Translational energy is not very important in connection with spectroscopy and will not be considered here.