In Solid-state Physics
Forbidden transitions in rare earth atoms such as erbium and neodymium make them useful as dopants for solid-state lasing media. In such media the atoms are held in a matrix which keeps them from de-exciting by collision, and the long half life of their excited states makes them easy to "optically pump" to create a large population of excited atoms. Neodymium doped glass derives its unusual coloration from "forbidden" f-f transitions within the neodymium atom, and is used in extremely high power solid state lasers.
Read more about this topic: Forbidden Mechanism
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