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

Abstract

The negative muonium ion ${\mathrm{Mu}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\equiv}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$ 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 ${\mathrm{Mu}}^{\mathrm{\ensuremath{-}}}$ beam at the focal plane of our spectrometer had an intensity 1.4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ times that of the incident ${\ensuremath{\mu}}^{+}$ beam or 2 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, which corresponds to a ${\mathrm{Mu}}^{\mathrm{\ensuremath{-}}}$/${\ensuremath{\mu}}^{+}$ conversion probability of 0.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ at the foil in agreement with the calculated value. The kinetic energies of ${\mathrm{Mu}}^{\mathrm{\ensuremath{-}}}$ at the foil are less than several keV as expected.