Temperature dependence of negative ion lifetimes

The autodetachment lifetimes of SF6-* and C6F6-* ions formed by charge transfer in K(np)/SF6, C6F6 collisions are measured as a function of target temperature over the range of similar to 300-600 K with the aid of time-of-flight techniques and a Penning ion trap. At room temperature only formation of long-lived SF6-* ions with lifetimes tau greater than or similar to 1 ms is seen. As the temperature is increased the lifetime of these long-lived ions is reduced, some having lifetimes as short as similar to 0.4 ms. The appearance of a short-lived, tau less than or similar to 10 mu s, SF6-* signal is also observed. Rydberg electron transfer to room temperature C6F6 leads predominantly to formation of short-lived, tau less than or similar to 10 mu s, C6F6-* ions, although a small number of longer-lived anions with lifetimes of similar to 50-100 mu s is also evident. This signal disappears as the temperature is raised and the lifetime of the short-lived ions decreases dramatically. The measured lifetimes are compared to those predicted using quasiequilibrium theory and updated values of the input parameters, including calculated vibrational frequencies for the anions. For C6F6, the calculated anion lifetimes are in good agreement with those measured experimentally. While similarly good agreement is obtained for SF6 at room temperature, the predicted temperature dependence of the lifetime is very different to that observed. This suggests that the excitation energy is not completely randomized in the anion and the measurements point to formation of at least two quasi-independent groups of anion states that have very different lifetimes. The present results are compared to those of earlier work. (C) 2007 American Institute of Physics.