An absorption edge, absorption discontinuity or absorption limit is a sharp discontinuity in the
absorption spectrum
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating f ...
of a substance. These discontinuities occur at
wavelengths where the energy of an absorbed
photon corresponds to an
electronic transition or
ionization potential. When the
quantum energy
A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical particles, which can have any amount of energy. The te ...
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 in quantum mechanics, measurement on a system. Knowledge of the quantum state together with the rul ...
in the constituent absorbing
atom, 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 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 In X-ray absorption spectroscopy, the K-edge is a sudden increase in x-ray absorption occurring when the energy of the X-rays is just above the binding energy of the innermost electron shell of the atoms interacting with the photons. The term is ba ...
*
Siegbahn notation
References
Electromagnetic radiation
Radiation
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