Stabilization of transient negative ions by vibrational energy transfer: Acluster and thin film study on SF6 and C6F6

Resonant low energy electron attachment to SF6 and C6F6 in argon clusters a nd on a multilayer krypton film is presented. The energy dependence of the absolute attachment cross sections for the condensed molecules is determine d in the range 0-8 eV by measuring charge localization on the film. We find a cross section maximum of (3.0 +/- 0.8)x10(-15) cm(2) at 60 meV for SF6- formation, which is close to the gas phase value. For C6F6, we obtain a max imum value (4.8 +/- 1.2)x10(-15) cm(2) at 75 meV. In contrast to isolated g as phase molecules, where both ions are only observed in a very narrow ener gy range around 0 eV, SF6- resulting from electron attachment to clusters i s observed up to 1.5 eV. The same is true for the condensed submonolayers, where charge localization is observed up to 1.5 eV for SF6. The extension o f stable SF6- formation to much higher energies in condensed media is attri buted to the transfer of intramolecular vibrational energy from SF6- to pho non vibrational modes of the Ar cluster or the Kr multilayer film. C6F6- fo rmation is also observed up to 1.7 eV from clusters, but on the film, charg e localization is only detected up to 0.7 eV. This difference is explained by the orientation of the C6F6 molecules on the rare gas film, which dimini shes vibrational energy transfer to the phonon bath of the Kr lattice. Simi lar results are also obtained for the dimeric configurations of these molec ules seeded into Ar clusters. (C) 2000 American Institute of Physics. [S002 1-9606(00)70120-6].