Temperature dependence of negative ion lifetimes

• Published in: The Journal of chemical physics 2007-08, Vol.127 (6), p.064314-064314-10
• Main Authors: Cannon, M., Liu, Y., Suess, L., Dunning, F. B., Steill, J. D., Compton, R. N.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.2756528

The autodetachment lifetimes of S F 6 − * and C 6 F 6 − * ions formed by charge transfer in K ( n p ) ∕ S F 6 , C 6 F 6 collisions are measured as a function of target temperature over the range of ∼ 300 - 600 K with the aid of time-of-flight techniques and a Penning ion trap. At room temperature only formation of long-lived S F 6 − * ions with lifetimes τ ≳ 1 ms is seen. As the temperature is increased the lifetime of these long-lived ions is reduced, some having lifetimes as short as ∼ 0.4 ms . The appearance of a short-lived, τ ≲ 10 μ s , S F 6 − * signal is also observed. Rydberg electron transfer to room temperature C 6 F 6 leads predominantly to formation of short-lived, τ ≲ 10 μ s , C 6 F 6 − * ions, although a small number of longer-lived anions with lifetimes of ∼ 50 - 100 μ 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 C 6 F 6 , the calculated anion lifetimes are in good agreement with those measured experimentally. While similarly good agreement is obtained for S F 6 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.