STRUCTURE AND LAMB SHIFT OF 2S(1/2)-2P(3/2) LEVELS IN LITHIUM-LIKE TH87+ THROUGH NEON-LIKE TH80+

We report the measurement of thirteen 2s(1/2)-2p(3/2) dipole-allowed transitions in the highly ionized tho ions Th80+ through Ths(87+). Th, transitions have been measured with a high-resolution crystal spectrometer on the Livermore electron beam ion trap in which highly charged thorium ions were produced and excited in the interaction with a 100-keV electron beam. The observed 2s(1/2)-2p(3/2) transitions are readily identified from a qualitative analysis of the dipole-allowed radiative decay channels. The identifications are confirmed by detailed modeling calculations of the line excitation processes, which account for all radiative processes, including radiative cascades among levels in the n=2 and n=3 shells, and all electron-impact excitation processes. The transition energies have been determined with an overall accuracy of 35 ppm. A detailed discussion of the measurement uncertainties is given, including the effects of beam-induced polarization on the hydrogenic reference transitions used for calibration. Measurements of the 1s(2)-1snp (n less than or equal to 10) transitions in heliumlike Ar16+, which fall into the same wavelength region as the 2s(1/2)-2p(3/2) transitions in thorium, are presented that validate the estimate of our experimental uncertainties. These measurements achieved twice the accuracy of previous measurements of the Ar16+ lines. A value of 4025.23 +/- 0.14 eV is found for the 2s(1/2)-2p(3/2) transition in lithiumlike Th87+. The value is sensitive to the effect of nuclear polarization and tests predictions of the 36-eV contribution from quantum electrodynamics to within 0.39%. A similar test of quantum electrodynamics calculations is given by the twelve 2s(1/2)-2p(3/2) transitions measured in the neighboring charge states. A comparison of the measured energies of the 2s(1/2)-2p(3/2) transitions in berylliumlike Th86+ through neonlike Th80+ with theoretical predictions from multiconfiguration Dirac-Fock calculations demonstrates the need for a better treatment of the electron-correlation terms in these highly charged multielectron ions. The measurements thus provide benchmarks for the development of theoretical approaches, such as relativistic many-body perturbation theory and large-scale configuration-interaction calculations, that attempt to account for electron correlations in very highly charged ions.