Isotope Effects in Low Energy Ion-Atom Collisions

Isotope effects for charge transfer processes have recently received increased attention Stolterfoht et al., PRL 99 (2007) 10301. The ion-atom merged-beams apparatus at Oak Ridge National Laboratory is used to measure charge transfer for low energy collisions of multi-charged atomic and molecular ions with H and D and is therefore well suited to investigate isotope effects. When charge transfer occurs at relatively large inter- nuclear distances (via radial couplings) the ion-induced dipole attraction can lead to trajectory effects, causing differences in the charge transfer cross sections for H and D. A strong isotope effect (nearly a factor of two) has been observed in the cross section for Si$^{4+}$ + H(D) below 0.1 eV/u. However, little or no difference is observed for N$^{2+}$ + H(D). Recently, strong effects have been predicted by Stolterfoht et al. for the fundamental system He$^{2+}$ + H(D,T) at collision energies below 200 eV/u where charge transfer occurs primarily through united-atom rotational coupling. We are currently exploring systems where rotational coupling is important and isotopic differences in the cross section can be observed.