Selective Removal of Electrons from a Penning Trap for Negative Ion Autodetachment Studies

It is shown that free electrons can be selectively removed from a Penning trap used to store heavy autodetaching negative ions by application of a series of small positive extraction pulses to one of the end electrodes of sufficient amplitude to extract the electrons yet not significantly perturb the heavy ion motion. This precludes electron reattachment processes in the trap, allowing accurate measurement of negative ion lifetimes. This has been used to determine the lifetimes of SF$_6^-$ and C$_2$Cl$_4^-$ ions produced by electron transfer in collisions with K(np) Rydberg atoms. The data for C$_2$Cl$_4^-$ point to ion lifetimes in the range 3 to 500 $\mu$s. Those for SF$_6^-$ range from 1 to 10 ms. The measurements for SF$_6^-$ also provide evidence of radiative stabilization. For both targets the effects of post-attachment interactions involving the K$^+$ core ion become increasingly important for values of n $\leq$ 20, leading to a sizable increase in the product ion lifetimes.