forbidden lines

forbidden lines

Lines that are not found in spectra under normal terrestrial conditions but are observed in certain astronomical spectra. In emission nebulae, for example, atoms can be excited by impact of low-energy electrons. Under normal laboratory conditions such atoms would be de-excited by collisions with other atoms, etc., before they had time to radiate. Such collisions are very infrequent in nebulae and the forbidden transitions between the excited state and a state of lower energy can occur, producing the forbidden lines.

Forbidden Lines


in optics, the spectral lines in the spectra of atoms and molecules for which the so-called selection rules are not satisfied. Upon passing from an excited state to a normal state, an atom emits light of a specific wavelength (a spectral line). During such transitions the selection rules must be satisfied. In reality, transitions that do not satisfy these rules (transitions from the metastable state) may also occur, but with a lower probability. The spectral lines that appear during these “forbidden” transitions are the forbidden lines.

Under ordinary conditions the intensity of forbidden lines is very low, since as a result of collisions with other particles the atom passes from the metastable state into the normal state without radiation. However, in strongly rarefied gases, in which the mean time interval between collisions of particles is comparable to or greater than the lifetime of an atom at the metastable level, an atom may pass into the normal state before a collision, radiating energy in the process. Such transitions give rise to the appearance of intensive forbidden lines in the spectra of gaseous nebulas in space and in the upper layers of the atmosphere.


El’iashevich, M. A. Atomnaia i molekuliarnaia spektroskopiia. Moscow, 1962.
Shpol’skii, E. V. Atomnaiafizika, vol. 2. Moscow-Leningrad, 1950.
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