Stark effect in resonant coherent excitation of 2s electron of Li-like Fe23+ ions channeling in a Si crystal

We measured resonant coherent excitation of a 2s electron of 83.3 MeV/u Fe23+ ions planar-channeling in the (2¯20) plane of the silicon crystal. A silicon surface barrier detector (SSD) was used as a crystal target in order to obtain information on the ion trajectory in the channel since the energy deposit (ΔE) to the SSD gives information on the ion trajectories where the resonant transitions occur. For the low ΔE, i.e., near the channel center, optically allowed 2s-3p transitions were much stronger compared with other transitions. Increasing ΔE, i.e., increasing the amplitude of ion trajectory, the optically forbidden 2s-3s transition rapidly became strong. On the other hand, the optically forbidden 2s-3d transitions did not become strong as rapidly as 2s-3s transition. Furthermore, it was found that the transition energies to the n = 3 states changed with ΔE. The shifts of the transition energies were consistent with the estimation for the energy levels of the Stark-mixed n = 3 states depending on the distance from the channel center.