Recombination measurements at low energies with Ar16+ and Ar18+ ions in a dense, cold electron target

Recombination of multiply charged ions with electrons at very low relative energies has become a major topic of interest, due to the observation of rates which are enhanced beyond the expectations for radiative recombination. We present results for Ar16+ and Ar18+ ions from systematic measurements along the argon isonuclear sequence using a high density cold electron beam target (ne = 7 × 109 cm-3) at the UNILAC of GSI. The transverse and longitudinal temperatures of the electron beam were determined from DR resonance features observed with metastable Ar16+ (23S) ions. The rate at Erel = 0 for radiative recombination of completely stripped Ar18+ calculated with electron beam temperatures kT∥ = 0.002 eV, kT⊥ = 0.2 eV amounts to α = 10-9 cm3 s-1. This is exceeded by nearly a factor of 10 by the rate measured in experiments with Ar18+ ions.