The 1s2s (3)S(1)→1s(2) (1)S(0) relativistic magnetic dipole transition in heliumlike argon, emitted by the plasma of an electron-cyclotron resonance ion source, has been measured using a double-flat crystal x-ray spectrometer. Such a spectrometer, used for the first time on a highly charged ion transition, provides absolute (reference-free) measurements in the x-ray domain. We find a transition energy of 3104.1605(77) eV (2.5 ppm accuracy). This value is the most accurate, reference-free measurement done for such a transition and is in good agreement with recent QED predictions.

We investigate the asymptotic properties of higher-order binding corrections to the one-loop selfenergy of excited states in atomic hydrogen. We evaluate the historically problematic A60 coefficient for all P states with principal quantum numbers n ≤ 7 and D states with n ≤ 8 and find that a satisfactory representation of the n-dependence of the coefficients requires a three-parameter fit. For the high-energy contribution to A60, we find exact formulas. The results obtained are relevant for the interpretation of high-precision laser spectrocopic measurements.PACS numbers: PACS numbers 12.20. Ds, 31.30.Jv, 06.20.Jr, Bound-state quantum electrodynamics (QED) occupies a unique position in theoretical physics in that it combines all conceptual intricacies of modern quantum field theories, augmented by the peculiarities of bound states, with the experimental possibilities of ultra-high resolution laser spectroscopy. Calculations in this area have a long history, and the current status of theoretical predictions is the result of continuous effort. The purpose of this Letter is twofold: first, to present improved evaluations of higher-order binding corrections to the bound-state self-energy for a large number of atomic states, including highly excited states with a principal quantum number as high as n = 8, and second, to analyze the asymptotic dependence of the analytic results on the bound-state quantum numbers. Highly-excited states (e.g, with n = 4 to 12) are of particular importance for high-precision spectroscopy experiments in hydrogen (for a summary, see for instance [1, p. 371]). In the analytic calculations, we focus on a specific higher-order binding correction, known as the A 60 coefficient or "relativistic Bethe logarithm." We write the (real part of the) one-loop self-energy shift of an electron in the field of a nucleus of charge number Z aswhere F (nl j , Zα) is a dimensionless quantity. In this Letter, we use natural units withh = c = m = 1 and e 2 = 4πα (m is the electron mass). The notation nl j is inspired by the usual spectroscopic nomenclature: n is the level number, j is the total angular momentum and l is the orbital angular momentum. The semi-analytic expansion of F (nl j , Zα) about Zα = 0 for a general atomic state with quantum numbers n, l ≥ 1 and j gives rise to the expression, F (nl j , Zα) = A 40 (nl j ) + (Zα) 2 A 61 (nl j ) ln(Zα) −2