From AMS Glossary
The study of the interaction between matter and radiation.
Since the energy levels in all atoms and molecules are discrete (quantized), the absorption or emission of radiation occurs at distinct energies (frequencies) characteristic of a given chemical. The energy levels probed may be rotational (microwave region), vibrational (infrared region), or electronic (visible or ultraviolet). In mass spectroscopy, molecules are ionized and the different masses selected by use of an electric or magnetic field. Many spectroscopic techniques have been used to identify molecules or atoms in the atmosphere.