MCDF calculations of the specific mass shift in helium-like ions

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An extension of the GRASP92 (Parpia F A, Froese Fischer C and Grant I P 1996 Comput. Phys. Commun. 94 249) multi-configuration Dirac-Fock (MCDF) program described previously (Perger W F and Idrees M 1995 Phys. Commun. 85 389-97) is used for the calculation of the specific mass shift (SMS) of the helium 1S ground state isoelectronic sequence. We also employ a multi-configuration Hartree-Fock (MCHF) method to calculate the ground state SMS for comparison with MCDF results. The SMS matrix elements for two-electron systems obtained from the relativistic program are shown to exhibit a trend: the larger the atomic number Z, the larger the relativistic contributions to the SMS matrix elements for the ions. The SMS matrix elements approximately vary as Z3 along the isoelectronic sequence from Z = 2 to 92. In addition, it is shown that the relativistic effects increase approximately as Z3/A2 in the SMS values for all the ions considered confirming some previous observations (Parpia F A, Tong M and Froese Fischer C 1992 Phys. Rev. A 46 3717-24). Excellent agreement is found between the present ab initio calculations and the available semi-relativistic calculations for the small values of Z along the helium-like ions. Furthermore, a large set of configuration state functions used in the calculations has revealed larger disagreements for high-Z ions between both relativistic (MCDF-optimized-level) and nonrelativistic (MCHF) calculations suggesting that the SMS for helium-like ions with Z > 40 relativistic and correlation effects are increasingly important.

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Journal of Physics B: Atomic, Molecular and Optical Physics