Fine-structure transitions during charge transfer in argon

Previous measurements of the mobility of atomic argon ions in argon at room temperature have resulted in a single value for ${\mathrm{Ar}}^{+}$ mobility despite attempts to resolve the expected fine structure of the arrival ion currents. Therefore, to understand the momentum exchange that accompanies charge transfer during collisions of the two electronic states (${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$, ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$) of the argon ion with the parent atom, a crossed-atom-ion-beam experiment was undertaken to measure the kinetic energy and laboratory deflection angle of the product ions after charge transfer. The interaction region of the particles was maintained at essentially zero intensity of electric and magnetic fields. The product species was measured by means of a spherical energy analyzer, their identity being determined by a tandem mass analyzer. The ion source used consists of a low-pressure electron-emission sustained-ionization region followed by a magnetic sector for ${\mathrm{Ar}}^{+}$ selection. For laboratory-frame collision energies of 123 eV, two fixed principal scattering peaks are observed as a result of scanning the laboratory angle at low laboratory energy. They correspond to exothermic and endothermic processes having about 0.17-eV energy separation. The argon-ion fine-structure separation is about 0.178 eV. Under low resolution the two peaks appear unresolved and are seen as single peak at about 0° deflection from the neutral-atom beam. The peaks in the intensity distribution can be understood as resulting from an exothermic and endothermic process with scattering peaked at near 0° in the center-of-mass system. Adequate fits to the data require a broad background intensity peak, which would result from isothermic processes.