Multiple ionization (strong fields)

Article By:

Dörner, Reinhard Institut für Kernphysik, Johann Wolfgang Goethe-Universität, Frankfurt am Main, Germany.

Last reviewed:2005

DOI:https://doi.org/10.1036/1097-8542.YB050470

One of the most important processes that occur when light interacts with matter is the photoelectric effect, in which the light is absorbed and electrons are set free. This process led Albert Einstein to the conclusion that electromagnetic radiation consists of energy packages known as photons. When a photon from the light field is absorbed, an electron is emitted with a kinetic energy given by the photon energy minus the binding energy of the electron in the material. As early as 1931, Maria Göppert Mayer argued that this is not the full story. She predicted that photons are also able to combine their energy to facilitate electron ejection. Such multiphoton processes can be studied with modern short-pulse laser systems. In the focus of such lasers, energy densities beyond 10¹⁵ W/cm² can be routinely reached. This is achieved by combining very short pulses (typically shorter than 10⁻¹³ s) and tight focusing. A laser beam intensity of 10¹⁵ W/cm² at a wavelength of 800 nanometers corresponds to a density of almost 10¹¹ photons in a box the size of the cube of the wavelength. The interaction of matter with such superintense radiation can be best studied by putting a single atom in the focus of the laser beam and examining how it responds. It was soon found that frequently many more photons were absorbed than were necessary to overcome the binding of the electron. This multiphoton process leads to ejection of fast electrons. In addition, an unexpectedly high probability for multiple ionization was observed, that is, ejection of more than one electron from the atom. The key question here is what are the microscopic mechanisms by which many photons are coupled to many electrons in a single atom or molecule. See also: Two-photon emission

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