Observation of forbidden transitions from highly charged ions in a Kingdon trap

We have reproduced the solar wind charge exchange collision of hydrogen-like O⁷⁺ ions with He gas at collision energies of 42 keV in the laboratory. According to the classical over barrier model, the dominant electron capture level in the collision of O⁷⁺ with He is the principal quantum number n=4. The populations of 1s2s and 1s2p states become large due to cascades from the higher excited states, so the main emission lines following the charge exchange are short-lived allowed transition of 1s^{2 1}S₀-1s2p ¹P₁ and long-lived forbidden transition of 1s^{2 1}S₀-1s2s ³S₁. By using a Kingdon ion trap, we succeeded in observing of the for bidden transitions from the helium-like O⁶⁺ ions produced by single electron capture. The measured X-ray spectrum had a peak at 560 eV which correspond to the forbidden emission of 1s^{2 1}S₀-1s2s ³S₁ in O⁶⁺ ion. (author)