Abstract
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 $Z=2--100$. 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 ${10}^{\ensuremath{-}4}$ of the corresponding $M1$ rates throughout the helium isoelectronic sequence.
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