Two-photon decay of 2 S-1(0) and 2 S-3(1) states 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 = 2-100. 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 rates throughout the helium isoelectronic sequence.