Double ionization of helium is investigated for 60-, 120-, and 420-MeV/u (v/c =0.34-0.72)U +-ion impact. The measured double-to-single ionization ratios indicate that, even for these very high velocities, double ionization of the He target results predominantly from independent interactions of the projectile with both target electrons. It is concluded that the asymptotic high-velocity regime for one-step double ionization (i.e. , "shakeoff") has not yet been reached even for U projectiles at 420 MeV/u, and, in fact, cannot be reached for projectiles with q 7, thereby verifying that the ionic charge is fully as important as the velocity in determining the importance of a given ionization mechanism.PACS number(s): 34.50.Fa

Resonant transfer and excitation (RTE) is investigated for Feq+ ions (q =23, 24, and 25) colliding with H&. For each charge state, cross sections for RTE were obtained from measurements of K x rays, emitted from the doubly excited intermediate state, coincident with single-electron capture by the incident ion. Additionally, for Fe ' cross sections were obtained from measurements of coincidences between the two K x rays emitted from the intermediate state. These latter measurements provide information on the lifetimes of intermediate metastable states formed in the RTE process. In all cases, measured cross sections are in good agreement with calculations based on theoretical cross sections for dielectronic recombination (DR). Since RTE closely approximates DR, the results indicate that dielectronicrecombination cross sections involving K-shell excitation can be accurately predicted for highly charged iron ions. The results for Fe"+ show that metastable states are suSciently short lived to be observable in the RTE (or DR) process for these hydrogenlike ions.

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