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selection rule

noun, Physics.
1.
any of several rules designating allowed transitions between quantum states in terms of the quantum numbers associated with the states.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Encyclopedia Article for selection-rule

selection rule

in quantum mechanics, any of a set of restrictions governing the likelihood that a physical system will change from one state to another or will be unable to make such a transition. Selection rules, accordingly, may specify "allowed transitions," those that have a high probability of occurring, or "forbidden transitions," those that have minimal or no probability of occurring. Selection rules usually are stated as sets of changes in one or more quantum numbers that characterize properties changed by the transition in question.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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