Strong Modification of Radiative Transition Rates due to a Breit-Interaction-Induced Avoided Crossing

We present the observations of x-rays emitted from the $1s2s^{2}2p_{1\mathrm{/}2}2p_{3\mathrm{/}2}$ inner shell excited state of B-like W and Bi ions. The relative transition rates are obtained for two dominant radiative transitions to $1s^{2}2s^{2}2p_{1\mathrm{/}2}$ and $1s^{2}2s^{2}2p_{3\mathrm{/}2}$. The experimental results and the comparison with rigorous relativistic calculations show that the rates of the strong electric dipole allowed $1s^{2}2s^{2}2p$ – $1s2s^{2}2p^2$ transitions are strongly modified due to a drastic change in the wavefunction caused by the Breit interaction.