An absorption edge, absorption discontinuity or absorption limit is a sharp discontinuity in the
absorption spectrum of a substance. These discontinuities occur at
wavelengths
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
where the energy of an absorbed
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
corresponds to an
electronic transition
A quantum jump is the abrupt transition of a quantum system (atom, molecule, atomic nucleus) from one quantum state to another, from one energy level to another. When the system absorbs energy, there is a transition to a higher energy level (ex ...
or
ionization potential
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
. When the
quantum energy of the incident radiation becomes smaller than the work required to eject an electron from one or other
quantum states
In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution i ...
in the constituent absorbing
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, ...
, the incident radiation ceases to be absorbed by that state. For example, incident radiation on an atom of a wavelength that has a
corresponding energy just below the binding energy of the K-shell electron in that atom cannot eject the K-shell electron.
["The Penguin Dictionary of Physics", 3rd ed., Longman Group Ltd. (2000), p. 3.]
Siegbahn notation The Siegbahn notation is used in X-ray spectroscopy to name the spectral lines that are characteristic to elements. It was introduced by Manne Siegbahn.
The characteristic lines in X-ray emission spectra correspond to atomic electronic transiti ...
is used for notating absorption edges.
In compound semiconductors, the bonding between atoms of different species forms a set of dipoles. These dipoles can absorb energy from an electromagnetic field, achieving a maximum coupling to the radiation when the frequency of the radiation equals a vibrational mode of the dipole. When this happens, the absorption coefficient gets a peak yielding the ''fundamental edge.'' This occurs in the far infrared region of the spectrum.
See also
*
K-edge
*
Siegbahn notation The Siegbahn notation is used in X-ray spectroscopy to name the spectral lines that are characteristic to elements. It was introduced by Manne Siegbahn.
The characteristic lines in X-ray emission spectra correspond to atomic electronic transiti ...
References
Electromagnetic radiation
Radiation
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