MEASUREMENTS AND MODELING OF ELECTRIC-DIPOLE-FORBIDDEN 2P(1 2)-2P(3/2) TRANSITIONS IN FLUORINELIKE U81+ THROUGH BERYLLIUMLIKE U88+/

We report the observation of electric-dipole-forbidden 2p(1/2)-2p(3/2) transitions in berylliumlike, boronlike, carbonlike, nitrogenlike, ox ygenlike, and fluorinelike uranium. Our measurement identified nine ma gnetic dipole transitions, as well as one strong electric quadrupole t ransition in carbonlike U86+ that has not been observed in lower-Z ion s because of collisional quenching. The measurement was carried out wi th a high-resolution crystal spectrometer, and an accuracy as good as 35 ppm was obtained in the determination of the transition energies. T he transitions are significantly less affected by quantum electrodynam ical effects than analogous electric-dipole-allowed transitions in the se ions. The data thus provide benchmarks for theoretical approaches o f electron-electron correlation effects in the high-Z limit Z alpha ap proximate to 1 that complement earlier measurements of 2s-2p transitio ns in such highly charged ions. The accuracy of the present measuremen ts, however, was sufficient to determine the residual contributions fr om quantum electrodynamics (about 2 eV) with an accuracy of 5%, i.e., with an accuracy comparable to that of the best measurements of such c ontributions to the Is ground level in hydrogenic U91+. The contributi ons from quantum electrodynamics are in large part due to the vacuum p olarization terms. The 2p(1/2)-2p(3/2) transition energies thus provid e a handle for testing the accuracy of vacuum polarization terms nearl y independent of the terms arising from the electron self energy. Resu lts from collisional-radiative calculations are presented that show th at the forbidden lines are almost exclusively produced by indirect pro cesses, i.e., radiative cascades, radiative electron capture, and the ionization of 2p(1/2) electrons. This is in stark contrast to the elec tric dipole-allowed transitions, which are mostly populated by direct electron-impact excitation.