In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atoms undergoing nominally "forbidden" energy transitions not normally allowed by the selection rules of quantum mechanics. In formal physics, this means that the process cannot proceed via the most efficient (electric dipole) route. Although the transitions are nominally "forbidden", there is a small probability of their spontaneous occurrence, should an atom or molecule be raised to an excited state. More precisely, there is a certain probability that such an excited atom 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 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 rather than choosing the forbidden route. Nevertheless, "forbidden" transitions are only relatively unlikely: states that can only decay in this way (so-called meta-stable states) usually have lifetimes of order milliseconds to seconds, compared to less than a microsecond for decay via permitted transitions.
Read more about Forbidden Mechanism: In Solid-state Physics, In Astrophysics
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