Measurements and modeling of electric-dipole-forbidden2p1/2−2p3/2transitions in fluorinelikeU81+through berylliumlikeU88+

Abstract

We report the observation of electric-dipole-forbidden ${2p}_{1/2}{\ensuremath{-}2p}_{3/2}$ transitions in berylliumlike, boronlike, carbonlike, nitrogenlike, oxygenlike, and fluorinelike uranium. Our measurement identified nine magnetic dipole transitions, as well as one strong electric quadrupole transition in carbonlike ${\mathrm{U}}^{86+}$ that has not been observed in lower-Z ions because of collisional quenching. The measurement was carried out with a high-resolution crystal spectrometer, and an accuracy as good as 35 ppm was obtained in the determination of the transition energies. The transitions are significantly less affected by quantum electrodynamical effects than analogous electric-dipole-allowed transitions in these ions. The data thus provide benchmarks for theoretical approaches of electron-electron correlation effects in the high-Z limit $Z\ensuremath{\alpha}\ensuremath{\approx}1$ that complement earlier measurements of $2s\ensuremath{-}2p$ transitions in such highly charged ions. The accuracy of the present measurements, however, was sufficient to determine the residual contributions from quantum electrodynamics (about 2 eV) with an accuracy of 5%, i.e., with an accuracy comparable to that of the best measurements of such contributions to the $1s$ ground level in hydrogenic ${\mathrm{U}}^{91+}.$ The contributions from quantum electrodynamics are in large part due to the vacuum polarization terms. The ${2p}_{1/2}{\ensuremath{-}2p}_{3/2}$ transition energies thus provide a handle for testing the accuracy of vacuum polarization terms nearly independent of the terms arising from the electron self energy. Results from collisional-radiative calculations are presented that show that the forbidden lines are almost exclusively produced by indirect processes, i.e., radiative cascades, radiative electron capture, and the ionization of ${2p}_{1/2}$ electrons. This is in stark contrast to the electric dipole-allowed transitions, which are mostly populated by direct electron-impact excitation.