Absolute partial and total electron impact ionization cross sections for CF4 from threshold up to 180 eV

No abstract

The occurrence and the features of a superasymmetric spontaneous decay reaction C6o'+ (~l)+ + C58 + C2 with z ranging from 3 to 7 are presented. Using isotope-resolved, two-sector-field mass spectrometric techniques it was possible to measure the kinetic energy release and dissociation rate for this reaction in two different time windows after the electron-induced production of the parent ion. The results obtained indicate that this dominant charge separation reaction proceeds via a multistep reaction sequence initiated by unimolecular C2 evaporation followed by a charge transfer process from the ion to the neutral fragment leading to the observed Coulombic repulsion. PACS numbers: 36.40.c, 34.80.Gs, 31.15.Ta Whereas singly charged homogeneous clusters, X"+, areat least at very low temperaturesther modynamically stable with respect to dissociation into any pair of fragments, the stability of multiply charged cluster ion s, X"'+, depends on the delicate balance between Coulombic repulsion and the attractive forces, which in turn depend on (i) the cluster size n,(ii) the charge state z, and (iii) the nature of the cluster constituent X. In general, therefore, it is expected that for a given charge state g and constituent X there will be a transition from unstable small cluster ions to thermodynamically stable ions with large n. This transition defines a critical cluster size n, (z), which is a definite lower size limit to observable multiply charged cluster ions. Mass spectra of cluster distributions indeed indicate lower size limits (appearance sizes), and their interpretation and relationship to bulk properties (in particular, for van der Waals and hydrogen bonded clusters) now appear to be well established [1]. Moreover, the production mechanism, appearance energies, and metastable decay via monomer evaporations have been explored recently in a number of studies which are summarized in Ref.[1].In contrast, only a few investigations have been carried out concerning the mechanisms and the properties of the unimolecular charge separation reaction responsible for the absence of these multiply charged cluster ions below the appearance size. Theoretical descriptions of electronic shell effects in fission barriers and fission dynamics of metal clusters have been summarized recently by Landman and co-workers [2]. Besides collision-induced fission of doubly and triply charged cluster ions [3 -5] and the identification and/or kinetic energy release measurements of very small (diatomic, triatomic, etc. ) dication metastable decay channels [6 -8], the only relevant experimental investigations concerning the size distributions of fission products are those of Kreisle et al. [9 -11) on triply charged molecular cluster ions, of Brechignac et al. [12,13] on doubly charged alkali cluster ions, and of Katakuse, Ito, and Ichihara [14] on doubly charged silver cluster ions. However, aside from these observations per se, no experimental information about the mechanism, the kinetics, and the energetics of the respective charge sep...

Neutralization and delayed ionization in fullerene surface collisions: Fragmentation and ionization rates as a route to activation energies Using recently measured accurate relative partial ionization cross section functions for production of the C 60 fragment ions C 58 ϩ through C 44 ϩ by electron impact ionization, we have determined the respective binding energies BE͑C n ϩ -C 2 ͒, with nϭ58,...,44, using a novel self-consistent procedure. Appearance energies were determined from ionization efficiency curves. Binding energies were calculated from the corresponding appearance energies with the help of the finite heat bath theory. Then using these binding energies we calculated with transition state theory ͑TST͒, the corresponding breakdown curves, and compared these calculated ones with the ones derived from the measured cross sections. The good agreement between these breakdown curves proves the consistency of this multistep calculation scheme. As the only free parameter in this procedure is the binding energy C 58 ϩ -C 2 , we studied the influence of different transition states chosen in the determination of this binding energy via TST theory and iterative comparison with breakdown curve measurements. Based on this study we can conclude that extremely loose transition states can be confidently excluded, and that somewhat looser transition states than those used earlier result in an upward change of the binding energy of less than 10% yielding an upper limit for the binding energy C 58 ϩ -C 2 of approximately 7.6 eV.

No abstract