Electron capture in small-angleAr2++ Ar collisions

Electron capture in ${\mathrm{Ar}}^{2+}$ + Ar is studied at low keV energies. Using ${\mathrm{Ar}}^{2+}$ + He collisions as an energy reference we show that the dominant single-electron-capture process is endothermic and of the type ${\mathrm{Ar}}^{2+}{}_{}{}^3{}_{}{}^{}P+\mathrm{Ar}\to {\mathrm{Ar}}^{+}{}_{}{}^2{}_{}{}^{}P+{\mathrm{Ar}}^{+}3s^{2}3p^4\mathrm{nl}$. Detailed studies at 2.4 keV show $\mathrm{nl}$ to be predominantly $3d$ and $4p$. Weaker exothermic processes are found and due mainly to long-lived, highly excited states of ${\mathrm{Ar}}^{2+}$. The direct scattering and double-electron-capture channels are also studied. The direct scattering occurs with no Ar-target excitation. Differential cross sections are presented for the collision processes. The single-electron-capture results are interpreted with the use of a simple model in which a strongly attractive intermediate state couples the incident and outgoing channels.