Formation of heavy-Rydberg ion-pair states in Rydberg atom collisions with attaching targets*

Electron transfer in collisions between K($n$p)Rydberg atoms and electron attaching molecules can lead to formation of heavy-Rydberg ion-pair states comprising a weakly-bound positive-negative ion pair orbiting at large internuclear separations. In the present work ion-pair states are created in a small collision cell and allowed to exit into an analysis region where their binding energy and velocity distributions are determined with the aid of electric-field-induced dissociation and a position sensitive detector. Ion pair production is analyzed using a Monte Carlo collision code that models both the initial Rydberg electron capture and the subsequent behavior of the product ion pair. The data demonstrate that collisions with SF$_{6}$ and CCl$_{4}$ lead to formation of long-lived ion pair states with a broad distribution of binding energies whose velocity distribution is strongly peaked in the forward direction.