RETARDATION (CASIMIR) ENERGY SHIFTS FOR RYDBERG HELIUM-LIKE LOW-Z IONS - AN EXPLORATORY-STUDY

The development of storage rings and electromagnetic traps for heavy charged particles is opening up new regimes of atomic physics, including, in particular, spectroscopic studies of Rydberg helium-like ions - with nuclear charge Z, one electron in the Is state, and one electron in a near-hydrogenic state of high n and l < n, with n and l the principal and orbital quantum numbers, respectively. We consider the possibility of detecting energy shifts due to retardation, DELTA-E(ret) (n, l), Casimir-like effects. These are quantum electrodynamic (QED) retardation effects associated with the finite speed of light. (As opposed to basically kinematic and dynamic QED effects for small quantum numbers n and 1, the appropriate expansion parameter for n and l large for retardation QED corrections is not Z(e2/hc) but [(Z - 1)/n2Z2](h c/e2).) We wish to provide some orientation to those planning experiments in the area, with regard to the choices of n, l, and Z most likely to be able to generate a high-precision confirmation of a retarded interaction. To do so, we provide extensive tables of estimates, for 1 s, nl states, of DELTA-E(ret)(n, l), of radiative widths, and of E, the spin-independent ("electric" fine structure) energy in the absence of retardation shifts, for (nuclear spin zero) ions with Z = 2, 6, 8, 16 and 20. These ions might be experimentally accessible in storage rings, and the Z's are low enough that virtual pair production effects may not yet be significant. There is also a brief survey of possible experimental techniques.