Vg. Palchikov et G. Vonoppen, THE SPIN-SPIN MIXING OF 1S3S S-3(1) AND 1S3D D-3(1) STATES IN THE HELIUM ISOELECTRONIC SEQUENCE, Physica scripta. T, 52(4), 1995, pp. 366-371
Both relativistic acid nonrelativistic analytical calculations of the
off-diagonal matrix elements between excited \1s3s S-3(1)> and \1s3d D
-3(1)) states of He-like atoms have been performed, taking into accoun
t the spin-spin interaction of the first order of perturbation theory.
The considered values of nuclear charge Z range from Z=2 to Z=100. Th
e nonrelativistic results for neutral helium agree well with earlier n
umerical estimates. The relativistic approach of evaluation of the off
-diagonal matrix elements of He-like multicharged ions is analyzed in
some details, using the hydrogenlike approximation for the basis wave
functions. A precise evaluation of the one-photon exchange correction
to the \1s3s S-3(1) > and \1s3d D-3(1)> energies is made.