First observation of the negative muonium ion produced by electron capture in a beam-foil experiment

The negative muonium ion ${\mathrm{Mu}}^{\mathrm{-}}$≡${\mathrm{\mu }}^{+}$ ${\mathrm{e}}^{\mathrm{-}}$ ${\mathrm{e}}^{\mathrm{-}}$ has been produced by double electron pickup as a beam of positive muons passes through a foil. A clean signal was detected by electrostatically accelerating and analyzing the ions in a magnetic time-of-flight spectrometer. The ${\mathrm{Mu}}^{\mathrm{-}}$ beam at the focal plane of our spectrometer had an intensity 1.4×${10}^{\mathrm{-}5}$ times that of the incident ${\mu }^{+}$ beam or 2 ${\mathrm{s}}^{\mathrm{-}1}$, which corresponds to a ${\mathrm{Mu}}^{\mathrm{-}}$/${\mu }^{+}$ conversion probability of 0.5×${10}^{\mathrm{-}4}$ at the foil in agreement with the calculated value. The kinetic energies of ${\mathrm{Mu}}^{\mathrm{-}}$ at the foil are less than several keV as expected.