Fast-Electron-Ion Coincidence Measurements of the Argon Atom-Ion Collision

The triple-valued structure in the inelastic energy loss in certain violent ${\mathrm{Ar}}^{+}$-on-Ar collisions is commonly thought to be due to the creation of $L$-shell vacancies. These should de-excite through an Auger process that results in the emission of fast electrons whose energy is of the order of 180 eV. The present experiment detects such electrons in coincidence with a scattered ion appearing at a known angle $\theta$ with a known final charge state $m$. Large-angle scattering in the reaction ${\mathrm{Ar}}^{+}+\mathrm{Ar}\to {\mathrm{Ar}}^{+m}+{\mathrm{Ar}}^{+n}+(m+n-1\mathrm{})\mathrm{}{e}^{-}$ is studied for incident energies between 10 and 30 keV, where the ${\mathrm{Ar}}^{+m}$ is scattered to $\theta =21\mathrm{}°$. The electron-energy spectrum measured in coincidence shows the expected peak, though its location in energy decreases as $m$ increases. The number of fast electrons agrees fairly well with the number expected according to a statistical model developed earlier.