Relativistic many-body perturbation theory applied ton=2 triplet states of heliumlike ions

Relativistic many-body perturbation theory calculations of the energies of the 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_0$, 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_2$, and 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{S}}_1$ states of heliumlike ions with nuclear charges Z in the range 10–36 are presented. These calculations include Coulomb correlation corrections through third order, correlation corrections of first order in the Breit interaction and zeroth, first, and second order in the Coulomb interaction, together with second-order Breit-Breit corrections and first-order corrections for retardation of the Breit interaction. At Z=10, the calculated energies of the 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_0$, 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_2$, and 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{S}}_1$ states differ by 2.9, 0.02, and 0.3 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively, from benchmark calculations based on variational wave functions. For higher Z, the difference between the present calculations and the benchmark calculations increases smoothly. Radiative and mass-polarization corrections are added to the many-body calculations and comparisons are made with measured 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_0$–2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{S}}_1$ and 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_2$–2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{S}}_1$ intervals.