Energy dependence of angular momentum capture states in charge exchange collisions between slow highly charged argon ions and argon neutrals

K-shell x-ray emission measurements of electron capture into Rydberg states of Ar17+ and Ar18+ ions are carried out as a function of collision energy in order to investigate the energy dependence of angular momentum capture states in slow charge exchange collisions. The highly charged ions are produced using an electron beam ion trap and extracted onto an external argon gas target. A retardation assembly in the beamline allows projectile energies from similar to 2 keV/amu down to similar to 5 eV/amu to be selected. For decreasing collision energies a shift to electron capture into low orbital angular momentum capture states is observed. Comparative K-shell x-ray emission measurements of electron capture by Ar17+ and Ar18+ ions from background gas in the trap are also presented. The results of the trapping experiments are in discrepancy with those obtained using extracted ions and possible explanations are discussed.