Forbidden mechanism

As forbidden transition or forbidden line is called in physics a transition from one energy level - or more generally of a ( quantum ) state - to another, if he does not, or if it occurs much less frequently than other transitions in the same system.

Delimitation of different uses of the term

Often the term is used for transitions, which rarely occur under laboratory conditions. The reason for the rarity of the observation of the transition may be due to a different mechanism of the state change, for example in the "competition " of another target state with a higher transition probability or, as in the example below, the " Nebuliums " in the competition.

However, the transition can be actually prohibited by a law of nature, and a violation would be a hint to its validity limits. Which transitions are allowed and which are prohibited, is formulated in the selection rules.

Accordingly, it is forbidden decays, for example, during beta decay.

" Nebulium " as an example

Some astronomical objects, such as NGC 6543 ( Cat's Eye Nebula ), green emission lines were observed, caused by radiative de-excitation of metastable energy levels. As they usually can not be produced in the laboratory on Earth, is first assumed a hitherto unknown on earth element called nebulium source, which would cause these lines.

The real mechanism was elucidated by Ira Sprague Bowen 1927. Usually atoms are de-excited by collisions before they can emit their excitation energy. Only below a particle density of 108/cm ³ the time between collisions of the particles with one another is larger than the occupation time of the metastable state. Forbidden lines therefore occur only in very dilute gases, such as in the upper atmosphere or in the interstellar medium. In describing a spectrum of forbidden lines in square brackets around the emitting atom or ion.

Some of the most prominent emission lines from H II regions and planetary nebulae are those forbidden lines, such as the [O III ] line of doubly ionized oxygen at 500.7 nm or the [ N II ] line of singly ionized nitrogen at 658, 3 nm solar corona shows particularly strong forbidden lines of highly charged ions, such as the "green" line of 13 -fold charged Fe XIV ( FE13 ion ) at 530.3 nm

Also green aurora is an example of the forbidden green oxygen lines.

• Quantum physics