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In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in atomic nuclei, and so on. The selection rules may differ according to the technique used to observe the transition. In the following, mainly atomic and molecular transitions are considered. ==Overview== In quantum mechanics the basis for a spectroscopic selection rule is the value of the transition moment integral〔Harris & Bertolucci, p. 130〕 : , where and are the wave functions of the two states involved in the transition and µ is the transition moment operator. If the value of this integral is zero the transition is forbidden. In practice, the integral itself does not need to be calculated to determine a selection rule. It is sufficient to determine the symmetry of transition moment function, . If the symmetry of this function spans the totally symmetric representation of the point group to which the atom or molecule belongs then its value is (in general) not zero and the transition is allowed. Otherwise, the transition is forbidden. The transition moment integral is zero if the transition moment function, , is anti-symmetric or odd, i.e. ''y(x) = -y(-x)'' holds. The symmetry of the transition moment function is the direct product of the parities of its three components. The symmetry characteristics of each component can be obtained from standard character tables. Rules for obtaining the symmetries of a direct product can be found in texts on character tables.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「selection rule」の詳細全文を読む スポンサード リンク
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