Helium metastability
= Helium metastability
The low $1s2s$ levels of a <helium atom> lack an allowed direct electric dipole decay to the ground $1s^2\,{}^1S_0$ level. The triplet is excluded by the <spin selection rule>, and the singlet-to-singlet $S$ transition is excluded by <parity> and orbital selection. Weaker radiative channels and collisions can still de-excite these states; metastable does not mean infinitely lived.