Electron-impact ionization ofAr2, ArKr,Kr2, KrXe, andXe2

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Relative cross sections for electron-impact ionization of the rare-gas Van der Waals dimers ${\mathrm{Ar}}_{2}$, ArKr, ${\mathrm{Kr}}_{2}$, KrXe, and ${\mathrm{Xe}}_{2}$ have been measured with a mass spectrometer in the electron energy range from threshold to 180 eV. The following appearance potentials for direct ionization from the corresponding neutral dimers have been found: ${\mathrm{Ar}}_{2}^{+}$: 15.2\ifmmode\pm\else\textpm\fi{}0.2 eV, Ar${\mathrm{Kr}}^{+}$: 14.0\ifmmode\pm\else\textpm\fi{}0.2 eV, ${\mathrm{Kr}}_{2}^{+}$: 13.45\ifmmode\pm\else\textpm\fi{}0.3 eV, Kr${\mathrm{Xe}}^{+}$: 12.2\ifmmode\pm\else\textpm\fi{}0.2 eV, and ${\mathrm{Xe}}_{2}^{+}$: 11.75\ifmmode\pm\else\textpm\fi{}0.3 eV. These values agree with those obtained in recent photoelectron studies. The appearance potential of ${\mathrm{Ar}}_{2}^{+}$ formation via associative ionization of argon atoms is found to be 14.6\ifmmode\pm\else\textpm\fi{}0.2 eV.