Optical transition in the negative magnesium ion

A search for the Mg^- ion detected via the 2p⁶3p³ ⁴S{→}2p⁶3s3p² ⁴P transition is described. The Mg⁺{→}Mg^- ⁴S two-step charge-exchange process is investigated and compared with the recently observed Be⁺{→}Be^- ⁴S process. Analysis of the optical spectra shows that the cross section for producing the Be^- ⁴S state by beam-foil or beam-gas collisions is much larger, a factor of 30 or more, than for producing Mg^- ⁴S. The search has been concentrated in the 2860--2950-{} region that covers the two wavelengths 2921 and 2895 {} recently proposed theoretically for the above transition. None of the spectral lines observed within the spectral region studied exhibits the properties of a negative ion. While the possibility of observing the Mg^- ⁴S-⁴P transition at 2921±{}4 {} [D. R. Beck, Phys. Rev. A 40, 2887 (1989)] can be excluded, the 2895±{}3 {} region [C. Froese Fischer, Phys. Rev. A 41, 3481 (1990)] is more complicated since it contains a so far unidentified spectral line. We can assign this line to the quintet spectrum of Mg i. This assignment is supported by four lines forming a closed loop, consistent with optical transitions between the 2p⁵3s3p4s and 2p⁵3s3p² configurations, and by multiconfiguration Hartree-Fock calculations. The lack of experimental observations of the predicted Mg^- ion is discussed.