Rotational spectroscopy

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Rotational spectroscopy studies the absorption of electromagnetic radiation (typically in the microwave region of the spectrum) by molecules. Rotational spectroscopy is only really practical in the gas phase where the rotational motion is quantized. In solids or liquids the rotational motion is usually quenched due to collisions.

For a molecule to have a rotational spectrum it is necessary (to first order) that it have a dipole moment, that is a difference between the center of charge and the center of mass, or equivalently a separation between two unlike charges. It is this dipole moment that enables the electric field of the light (microwave) to exert a torque on the molecule causing it to rotate more quickly (in excitation) or slowly (in de-excitation).

The simplest rotational spectra belong to diatomic molecules such as CO (carbon monoxide).

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