Isotope Effects in Low Energy Ion-Atom Collisions

Abstract

Isotope effects for charge transfer processes have recently received increased attention. 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 and is therefore well suited to investigate isotope effects. 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. The intense higher velocity multi‐charged ion beams allow, for the first time, measurements with both H and D from keV/u down to meV/u collision energies in the center‐of‐mass frame. 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 Si4++H(D) below 0.1 eV/u. However, little or no difference is observed for N2++H(D). Recently, strong effects have been predicted for the fundamental system He2++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.