Rydberg Ionization: From Field to Photon

Atoms where one of the electrons is in a highly excited state, the so-called Rydberg atoms, have proven to be a useful tool for studying the atom-radiation interaction. In the last decade a large variety of radiation sources have been used to study the ionization mechanisms of Rydberg atoms. The physical mechanism of ionization depends on the radiation frequency. For very low frequency the field amplitude determines the ionization, while for higher frequencies the photon energy is most essential. In this article we will review the ionization mechanism of Rydberg atoms exposed to radiation ranging from DC fields to optical frequencies.