Relativistic calculation of the 23S1–11S0magnetic dipole transition rate and transition energy for heliumlike argon

A calculation of the magnetic dipole transition 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{S}}_1$–1 ${}_{}{}^1{}_{}{}^{}$ ${\mathit{S}}_0$ in heliumlike argon is presented. The calculation is performed in a relativistic framework and includes the Coulomb interaction, in the no-virtual-pair approximation, to all orders. Contributions from the exchange of virtual photons, i.e., from the Breit interaction, are of the same formal order of magnitude as contributions from the Coulomb interaction and are treated on the same footing. The combination of a relativistic treatment and the inclusion of correlation in the calculation results in an improved accuracy compared with previous calculations. The lifetime of the 2${\mathrm{}}^3$ ${\mathit{S}}_1$ state is predicted to be 209.4±0.4 nsec, which includes an estimate of neglected effects, ±0.2 nsec, and numerical errors, ±0.2 nsec. This result agrees with experiment, 202±12 nsec, within present error bars. A detailed breakdown of the contributions to the transition energy is also presented. The contribution to the ground-state energy of ${\mathrm{Ar}}^{16+}$ from the mixture of the Breit and the Coulomb interaction is calculated to all orders. This contribution is 9% smaller than the result in the Pauli approximation.