Two-photon decay of21S0and23S1states of heliumlike ions

Systematic calculations of two-photon decay rates of metastable $2{}^1{S}_0$ and $2{}^3{S}_1$ states are presented for heliumlike ions with nuclear charges in the range $Z=2\mathrm{}–100$. These calculations include retardation and are carried out using relativistic configuration-interaction wave functions that account for the Breit interaction. Photon energy distributions and total rates are given. The relativistic $2{}^1{S}_0$ decay rates agree, to within two standard deviations, with precise measurements for heliumlike Ar, Ni, Br, Kr, and Nb. The calculated $2{}^1{S}_0$ rates are 30% smaller than the corresponding nonrelativistic rates at high $Z$. The $2{}^3{S}_1$ two-photon decay rates remain a factor of about ${10}^{-4}$ of the corresponding $M1\mathrm{}$ rates throughout the helium isoelectronic sequence.