MAR 01, 1971
To an atomic physicist the state of helium represents the classic metastable state. Lying nearly 20 eV above the ground state, it can not emit radiation and drop to the ground state without violating the selection rules that an S state can not decay to an S state and terms of different multiplicities do not combine (that is, triplet states do not decay to singlet states). Usually, helium excited into this metastable state stays there until a collision with another atom or with a wall allows it to drop down; if neither an atom nor a wall obliges soon enough, the state will eventually decay by some “forbidden transition” that violates the selection rules. (Such a situation probably arises for helium atoms in planetary nebulae.) All this is true also for other members of the helium isoelectronic sequence—atoms or ions with but two electrons.
© 1971. American Institute of Physics
