Absorption edge

In <a href="/facts/Physics/a7aH2y08">physics</a>, an absorption edge (also known as an absorption discontinuity or absorption limit) is a sharp discontinuity in the <a href="/facts/Absorption_spectroscopy/GH7RC14D">absorption spectrum</a> of a substance. These discontinuities occur at <a href="/facts/Wavelength/kcJU0ymz">wavelengths</a> where the energy of an absorbed <a href="/facts/Photon/mFNhAEzA">photon</a> corresponds to an <a href="/facts/Molecular_electronic_transition/EeRT45sP">electronic transition</a> or <a href="/facts/Ionization_energy/lezUajfA">ionization potential</a>. When the <a href="/facts/Quantum_energy/66x51Khc">quantum energy</a> of the incident radiation becomes smaller than the work required to eject an electron from one or other <a href="/facts/Quantum_state/7jbxd7Dx">quantum states</a> in the constituent absorbing <a href="/facts/Atom/FoQ4kGN5">atom</a>, the incident radiation ceases to be absorbed by that state. For example, incident radiation on an atom of a wavelength that has a <a href="/facts/Compton_wavelength/og2h1J2q">corresponding energy</a> just below the binding energy of the K-shell electron in that atom cannot eject the K-shell electron.
<a href="/facts/Siegbahn_notation/FYk5MPEn">Siegbahn notation</a> is used for notating absorption edges.
In compound semiconductors, the bonding between atoms of different species forms a set of <a href="/facts/Dipole/G1vRqruD">dipoles</a>. These dipoles can absorb energy from an electromagnetic field, achieving a maximum coupling to the radiation when the frequency of the radiation equals a <a href="/facts/Normal_mode/XCsvQpuI">vibrational mode</a> of the dipole. When this happens, the absorption coefficient gets a peak yielding the fundamental edge. This occurs in the far <a href="/facts/Infrared_spectrum/44XdJNcc">infrared region of the spectrum</a>.

Absorption edge open-in-new

Absorption edge