$K$-shell excitation studied for H- and He-like bismuth ions in collisions with low-$Z$ target atoms

The formation of excited projectile states via Coulomb excitation is investigated for hydrogenlike and heliumlike bismuth projectiles $\mathrm{}(Z=83\mathrm{})\mathrm{}$ in relativistic ion-atom collisions. The excitation process was unambiguously identified by observing the radiative decay of the excited levels to the vacant $1s$ shell in coincidence with ions that did not undergo charge exchange in the reaction target. In particular, owing to the large fine-structure splitting of Bi, the excitation cross sections to the various $L$-shell sublevels are determined separately. The results are compared with detailed relativistic calculations, showing that both the relativistic character of the bound-state wave functions and the magnetic interaction are of considerable importance for the $K$-shell excitation process in high-$Z$ ions such as Bi. The experimental data confirm the result of the complete relativistic calculations, namely, that the magnetic part of the Liénard-Wiechert interaction leads to a significant reduction of the $K$-shell excitation cross section.