The spin selection rule is a principle in quantum mechanics that dictates the allowed transitions between quantum states based on their spin multiplicities. This rule indicates that transitions are typically allowed only when there is no change in the total spin quantum number, meaning transitions from a singlet state to another singlet state or from a triplet state to another triplet state are permitted. The significance of this rule is paramount in understanding the behavior of atomic and molecular spectra, as it influences the intensity and presence of spectral lines.
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