Effective intermolecular relaxation in (C₆F₆)_n^- clusters: mechanism of C₆F₆^- formation on low energy electron impact

Electron attachment to C₆F₆ molecules and clusters is studied in a beam experiment in the energy range 0-8 eV. In single molecules, a long-lived parent anion C₆F₆^- is observed within a narrow peak near 0 eV. In clusters, a comparatively broader attachment feature yielding the monomeric anion C₆F₆^- and the larger cluster ions (C₆F₆)_n^- is observed with a peak maximum distinctly above 0 eV. The present results indicate that the narrow width of the attachment peak in isolated molecules is essentially controlled by the autodetachment lifetime of the transient anion and hence by the observation time window of the experiment. In clusters, intermolecular stabilization processes strongly compete with autodetachment, resulting in an attachment profile which mirrors the primary shape of the C₆F₆^- resonance. In addition to the low energy process, C₆F₆^- (and the products (C₆F₆)_n^-) are generated from clusters with considerable intensity also at energies above 3 eV. The associated energy profiles strongly suggest that these ions are formed via inelastic scattering by one molecule and capture of the slow electron by another molecule of the same cluster (autoscavenging). The present results are compared with previous electron transmission experiments on gaseous C₆F₆ and discussed in terms of the molecular orbitals involved.