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<p>which absorb light and form an excited state whose decay involves a <a href="page.php?w=Spin_%28physics%29">spin</a> flip, and is therefore forbidden by electric dipole transitions. The result is emission of light slowly over minutes or hours.</p>

<p>Should an <a href="page.php?w=atomic_nucleus">atomic nucleus</a>, <a href="page.php?w=atom">atom</a> or <a href="page.php?w=molecule">molecule</a> be raised to an excited state and should the transitions be nominally forbidden, then there is still a small probability of their spontaneous occurrence.</p><p>
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