Selection rules usually are stated as sets of changes in one or more quantum numbers that characterize properties changed by the transition in question. Atoms, for example, radiate light or other electromagnetic energy whenever they make a transition from a higher to a lower energy. 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. Of the multitude of transition lines one might draw, most are forbidden by selection rules. Google has many special features to help you find exactly what you're looking for. Thus, according to Sect.
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