Lifetime of heliumlike metastable ions measured

To an atomic physicist the $2^3\mathrm{S}$ state of helium represents the classic metastable state. Lying nearly 20 eV above the $1^1\mathrm{S}$ 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.

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