Fundamental and incidental limits on the spectroscopy of single electron ions

Precision measurements of spectra from one-electron ions are principally focused on tests of QED corrections to the energy levels implied by the Dirac equation. Even through spectroscopic test in atomic hydrogen itself and determination of the anomalous moment of the electron have reached impressive levels of refinement and demonstrate equally impressive consistency between experiment and theory, exploration of the Z-dependence of such comparisons remains of interest. Fundamentally, such ''Lamb-shift'' experiments are characterized by and are limited by some ''Q-value'' determined by the magnitude of the QED shift, S, in relation to a line-width parameter, γ. Such Q-values are rather small for the traditional Δn=0 experiments, regardless of Z. Substatial improvement in this regard is available if one studies Δn=1 transitions, but in these cases there is substantial, though largely incidental, penalty in loss of ''leverage'' in the measurement. Additionally, and also incidentally, the earliest example of such Δn=1 experiments have suffered from various combinations of Doppler troubles and spectator electron pertubations. Only in one very recent effort has it been possible to bring both of these problems under simultaneous control thereby inviting consideration of a still more refined level at which fundamental limitations again appear dominant. One can further deal with the dominant intrinsic limitation occuring at this stage in principle but at the cost of still more refined measurement technology which appears to be practical but to lie in the future. (orig.)