Isotope Effects in Ion-Atom Collisions

The ion-atom merged-beams apparatus at Oak Ridge National Laboratory is used to measure charge transfer for low energy collisions of multi-charged ions with H and D. The apparatus has been relocated and upgraded to accept high velocity beams from the 250 kV High Voltage Platform at the Multi-Charged Ion Research Facility. Isotope effects for charge transfer processes have recently received increased attention. (Stolterfoht et al , PRL 99, 103201 (2007)). The higher velocity beams allow, for the first time, measurements with both H and D from keV/u down to meV/u collision energies. When charge transfer occurs at relatively large distances (via radial couplings) the ion-induced dipole attraction leads to trajectory effects (Havener et al., ICPEAC XVII Proceedings, Brisbane, 1991) causing differences in the charge transfer cross section for H and D. Such a strong isotope effect has now been directly observed for Si$^{4+}$ + H(D), but not for N$^{2+}$ + H(D). Strong effects have been predicted in the charge transfer cross section for the fundamental system He$^{2+}$ + H(D, T) ( Stolterfoht et al.) at collision energies where charge transfer occurs primarily through united-atom rotational coupling. Currently we are exploring systems where isotope effects in rotational coupling can be measured.