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.

REFERENCES

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