Decay energetics and lifetime measurements of transient negative ions using Rydberg atoms

Abstract

Dissociative electron attachment to CFCl$\sb3$, CF$\sb2$Br$\sb2$, and 1,1,1- and 1,1,2-C$\sb2$Cl$\sb3$F$\sb3$ is studied by measuring the velocity and angular distributions of negative ions produced through electron transfer in collisions with velocity selected K(np) Rydberg atoms. Data analysis is done with the aid of Monte Carlo simulations. Measurements indicate that the mean lifetimes of the transient ions CFCl$\sb3\sp{-*}$ and CF$\sb2$Br$\sb2\sp{-*}$ are $\sim$3.5 ps and $\sim$4.5 ps respectively and that only a small fraction of the excess energy of reaction appears in translation. The 1,1,1-C$\sb2$Cl$\sb3$F$\sb3$ data show one reaction channel resulting in immediate dissociation with an ion lifetime of $\sim$0 ps, and a second channel associated with a longer lived intermediate, $\tau\ \sim$ 0.1 to 1.0 ps. Only limited energy transfer occurs for the 1,1,2 isomer but the operative reaction channel(s) could not be identified. Possible reaction mechanisms are discussed with the aid of ab-initio calculations.