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forbidden mechanism
In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atomic nuclei, atoms, or molecules undergoing nominally "forbidden" energy transitions not normally "allowed" by the selection rules of quantum mechanics. Obviously, any process which is truly forbidden will not happen. The typical usage of this term, is in a situation where, according to usual approximations (such as the electric-dipole approximation for the interaction with light), the process cannot happen, but at a higher level of approximation (''e.g.'' magnetic dipole, or, electric quadrupole) the process is allowed but at a much lower rate.
A simple everyday example is "glow in the dark" materials. They absorb light and form an excited state whose decay involves a spin flip, and is therefore forbidden by electric dipole transitions. The result is emission of light slowly over minutes or hours. This process is known technically as phosphorescence.
Although the transitions are nominally "forbidden", there is a small probability of their spontaneous occurrence, should an atomic nucleus, atom or molecule be raised to an excited state. More precisely, there is a certain probability that such an excited entity will make a forbidden transition to a lower energy state per unit time; by definition, this probability is much lower than that for any transition permitted or allowed by the selection rules. Therefore, if a state can de-excite via a “permitted” transition (or otherwise, e.g. via collisions) it will almost certainly do so before any transition occurs via a “forbidden” route. Nevertheless, most “forbidden” transitions are only relatively unlikely: states that can only decay in this way (so-called meta-stable states) usually have lifetimes on the order milliseconds to seconds, compared to less than a microsecond for decay via permitted transitions. In some radioactive decay systems, multiple levels of “forbiddenness” can stretch life times by many orders of magnitude for each additional unit by which the system changes beyond what is most allowed under the selection rules. Such excited states can last years, or even for many billions of years (too long to have been measured).
== In radioactive decay ==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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