Isotope Effects in Low Energy Ion-Atom Collisions

摘要

Isotope effects for charge transfer processes have recently received increased attention. The ion-atommerged-beams apparatus at Oak Ridge National Laboratory is used to measure charge transfer for low energy collisionsof multi-charged ions with H and D and is therefore well suited to investigate isotope effects. The apparatus has beenrelocated and upgraded to accept high velocity beams from the 250 kV High Voltage Platform at the Multi-Charged IonResearch Facility. The intense higher velocity multi-charged ion beams allow, for the first time, measurements withboth H and D from keV/u down to meV/u collision energies in the center-of-mass frame. When charge transfer occursat relatively large inter-nuclear distances (via radial couplings) the ion-induced dipole attraction can lead to trajectoryeffects, causing differences in the charge transfer cross sections for H and D. A strong isotope effect (nearly a factor oftwo) has been observed in the cross section for Si4+ + H(D) below 0.1 eV/u. However, little or no difference is observedfor N2+ + H(D). Recently, strong effects have been predicted for the fundamental system He2+ + H(D,T) at collisionenergies below 200 eV/u where charge transfer occurs primarily through united-atom rotational coupling. We arecurrently exploring systems where rotational coupling is important and isotopic differences in the cross section can beobserved.