Browsing by Author "Popple, Richard Allen"

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    Determination of transient negative ion lifetimes using Rydberg atoms
    (1996) Popple, Richard Allen; Dunning, F. B.
    Potassium Rydberg atoms are used to investigate the lifetimes, on a picosecond timescale, of transient negative ions formed during dissociative electron attachment to CCl$\sb4$ and CF$\sb3$I. The transient negative ions are formed during Rydberg electron transfer reactions of the type$${\rm K}(n{\rm p})+AB\to{\rm K}\sp{+}+AB\sp{-*}\to{\rm K}\sp{+}+A\sp{-}+B$$For low values of principal quantum number $n\ (n\ \sbsp{\sim}{<}\ 20)$ the electrostatic interaction between the ${\rm K}\sp{+}-AB\sp{-*}$ and ${\rm K}\sp{+}+A\sp{-}$ ion pairs perturbs the ion trajectories. As a consequence, the final angular and velocity distributions of the ${\rm K}\sp{+}+A\sp{-}$ ions are dependent on the lifetime of the $AB\sp{-*}$ intermediate ion. The $AB\sp{-*}$ lifetime is determined by comparing product ion distributions, determined using position sensitive detection techniques, with the results of a Monte Carlo simulation that models the kinematics of the reaction. The data show that the lifetime of the excited $\rm CF\sb3\Gamma\sp{*}$ intermediate formed by Rydberg electron capture is short, $\sbsp{\sim}{<}2$ ps, and that the lifetime of the CCl$\sbsp{4}{-*}$ intermediate is $7.5\pm2.5$ ps.
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    Investigation of the velocity dependence of free ion production in Rydberg atom collision processes
    (1991) Popple, Richard Allen; Dunning, F. B.
    Experiments to verify a semiclassical model of Rydberg atom collision processes revealed discrepancies between the model predictions and experimental results for the reaction$$\rm K\sp{**}(np)+SF\sb6\to K\sp{+}+SF\sbsp{6}{-*}$$at low to intermediate values of the principal quantum number n (n ${<}{\approx}$ 20). Initial measurements using non-velocity selected Rydberg atoms yielded ion angular distributions that did not agree with model calculations. To explore this further, experiments were initiated using velocity selected Rydberg atoms. These data show that the discrepancy between the model and experiment is due to transfer of energy from the SF$\sbsp{6}{-*}$ ion into translational energy of the product ion pair through a close collision. This process is not included in the semiclassical model. This conclusion is substantiated by study of the dissociative electron transfer reaction$$\rm K\sp{**}(np)+CH\sb3I\to K\sp{+}+CH\sb3I\sp{-*}\to K\sp{+}+I\sp-+CH\sb3$$for which similar energy transfer cannot occur. The data agree well with model calculations.