Determination of Transient Negative Ion Lifetimes Using Rydberg Atoms.

Potassium Rydberg atoms are used to investigate the lifetimes, on a picosecond timescale, of transient negative ions formed during dissociative electron attachment to CCl_4 and CF_3 I. The transient negative ions are formed during Rydberg electron transfer reactions of the type {rm K}(n{rm p})+AB to{rm K}^{+}+AB^ {-*}to{rm K}^{+ }+A^{-}+BFor low values of principal quantum number n (n _sp {~}{<} 20) the electrostatic interaction between the {rm K}^{+}-AB^{-*} and {rm K}^{+}+A ^{-} ion pairs perturbs the ion trajectories. As a consequence, the final angular and velocity distributions of the {rm K }^{+}+A^{-} ions are dependent on the lifetime of the AB^ {-*} intermediate ion. The AB ^{-*} 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_3Gamma^{*} intermediate formed by Rydberg electron capture is short, _sp {~}{<}2 ps, and that the lifetime of the CCl_sp{4} {-*} intermediate is 7.5+/-2.5 ps.