Fine-structure quenching and multiplet distortion in a screened Coulomb atom

Theoretical results presented in this paper reflect that the relativistic fine-structure due to the mass-velocity, spin-orbit and Darwin terms is sensitive to the screening strength parameter in an exponential screened Coulomb hydrogen atom, that is sometimes used to model a plasma-embedded atom. With stronger screening the fine-structure correction undergoes a gradual suppression in magnitude, but contributes to the total binding energy in an increasing proportion, indicating that the relativistic contribution to binding may become quite significant in the ultra-low binding regime under large screening strength. In the presence of screening the l-independence of the fine-structure correction as predicted by the Dirac theory progressively disappears, and a departure from the Z⁴-scaling law of the correction occurs along the H-isoelectronic sequence of ions - both the effects become accentuated with growing screening strength. In conjunction with screening-induced removal of the Coulomb degeneracy of non-relativistic levels, these result in a deformed multiplet structure for the screened Coulomb atom. Received 31 May 1999 and Received in final form 20 September 1999