Nondissociative low - energy electron attachment to SF 6 , C 6 F 6 , C 10 F 8 , and c - C 7 F 14 : Negative ion lifetimes

Many large polyatomic molecules attach free low-energy electrons to form parent negative ions that subsequently decay by autodetachment, reverting to a neutral molecule and a free electron. Their lifetimes against autodetachment, however, span a broad range. This is consistent with a simple model in which it is assumed that electron capture initially forms an excited intermediate in which only a limited number of the possible vibrational modes interact directly with the captured electron and are initially excited. These ‘‘active’’ modes may then be deexcited by direct autodetachment or by intramolecular vibrational relaxation ~IVR! which transfers the excess energy ~associated primarily with the electron affinity of the target molecule and the kinetic energy of the captured electron! to other ‘‘inactive’’ modes where the energy is no longer available for autodetachment. Once IVR has occurred, the probability that the energy will be reconcentrated in an active mode from which autodetachment can occur is small, leading to the formation of longlived parent anions. The fate of the intermediate negative ion is thus governed by the competition between direct autodetachment and IVR. If the time scale for IVR is long compared to that for direct autodetachment, the intermediate will decay by direct autodetachment, typically within a few picoseconds of its formation. If the reverse is true, the intermediate will be ‘‘stabilized’’ by IVR, leading to the creation of metastable long-lived parent anions with lifetimes measured in microseconds or milliseconds. Earlier work has demonstrated that the competition between IVR and direct autodetachment, and the relative rates associated with these processes, can be investigated using atoms in high Rydberg states through studies of the lifetimes of negative ions created in electron-transfer reactions of the type