M. Foltin

On the unimolecular fragmentation of C60+ fullerene ions: The comparison of measured and calculated breakdown patterns

The stability of singly charged C60+ fullerene ions, produced by electron impact ionization of C60, has been studied as a function of the electron energy and the time elapsed from ionization in a Nier‐type ion source/double‐focusing, sector‐field mass spectrometer system. A huge kinetic shift of more than 34 eV (dependent on the observation time) was observed for the dissociation process C60+→C58++C2. The ionization efficiency curves for C58+, C56+, and C54+ fragment ions have been recorded with an energy resolution of approximately 0.5 eV. This allowed us to construct a time‐resolved breakdown graph of the decaying C60+ fullerene ion. Two different methods, i.e., the finite heat bath model of Klots and the Rice–Ramsperger–Kassel–Marcus (RRKM) expression, have been used to calculate the decay rates and the breakdown graph of the C60+ ion, and the results of the calculation have been compared with the experimentally obtained breakdown graph. The best fit leads to a dissociation energy (C58+−C2) of 7.1±0.4 ...

Direct Evidence for the Sequential Decay C60z+-->C58z+-->C56z+ -->.

Using a two sector field mass spectrometer in combination with a crossed beams ion source we have obtained direct experimental evidence that C{sub 60} fragment ions such as C{sub 58},C{sub 56},C{sub 54},...produced by electron impact ionization of C{sub 60} may be formed by unimolecular decay of the C{sub 60} parent ion involving sequential loss of C{sub 2}. Moreover, by comparing experimental and theoretical breakdown graphs the overriding conclusion is that in the case of C{sub 56}{sup {ital z}+} production (with {ital z}=1,2,3) sequential loss of two C{sub 2} units dominates over the loss of a single C{sub 4} unit. {copyright} {ital 1996 The American Physical Society.}

Electron impact ionization of C60 revisited: corrected absolute cross section functions

Abstract We report corrected absolute partial ionization cross sections for the various ions produced by electron impact on C 60 . The correction takes into account ion losses due to metastable fragmentation processes which occur during the flight time of the ions from the ion source to the detector in a two-sector-field mass spectrometer. Due to these ion losses, previously reported absolute partial and total ionization cross-sections were too small by factors ranging from 1.025 to 2. The corrected cross-sections reported here are in good agreement with cross-sections recently reported by two other groups using experimental techniques which are largely insensitive to ion losses caused by metastable decay processes.

Direct Evidence for the Sequential Decay {ital C}{sub 60}{sup {ital z}{bold +}}{bold {r_arrow}}{ital C}{sub 58}{sup {ital z}{bold +}}{bold {r_arrow}}{ital C}{sub 56}{sup {ital z}{bold +}}{bold {r_arrow}}

Using a two sector field mass spectrometer in combination with a crossed beams ion source we have obtained direct experimental evidence that C{sub 60} fragment ions such as C{sub 58},C{sub 56},C{sub 54},...produced by electron impact ionization of C{sub 60} may be formed by unimolecular decay of the C{sub 60} parent ion involving sequential loss of C{sub 2}. Moreover, by comparing experimental and theoretical breakdown graphs the overriding conclusion is that in the case of C{sub 56}{sup {ital z}+} production (with {ital z}=1,2,3) sequential loss of two C{sub 2} units dominates over the loss of a single C{sub 4} unit. {copyright} {ital 1996 The American Physical Society.}

The observation of unusual resonance channels in the electron attachment to mixed argon-oxygen clusters

Abstract An additional electron attachment resonance peak, not observable in pure oxygen cluster anions, has been found at an energy of about 11.5 eV for both the stoichiometric Ar m (O 2 ) n − and the nonstoichiometric Ar m (O 2 ) n −1 O − cluster anions (produced by electron attachment to mixed argon-oxygen clusters). Two possible competing mechanisms are proposed to explain the appearance of these resonance peaks, i.e. the multiple-collision electron-scavenging mechanism and the core-excited resonance attachment mechanism. From the peak-width analysis it follows that in Ar m (O 2 ) 2 − the first mechanism is dominant, while in Ar m (O 2 ) n −1 O − the second mechanism seems to be the only one occurring.

Production and properties of CO2 cluster anions

Stoichiometric and non-stoichiometric negatively charged CO2 cluster ions have been produced in a crossed neutral cluster/electron beam ion source. The abundance and stability of these ions have been studied with a double focussing sector field mass spectrometer. The observed abundance anomalies (“magic numbers”) in the mass spectra of (CO2)n− and (CO2)nO− ions correlate with corresponding small and large metastable fractions of these ions (for loss of one CO2 unit). Variation of the measured metastable fractions as a function ofn are related to corresponding changes in the monomer binding energies. In addition, we have observed for the first time (CO2)nO2− ions (i.e. at electron energies above 8 eV with an energy resonance at about 14 eV) and we discuss possible production mechanisms for these ions. Relative electron attachment cross sections have been determined in the energy regime O<E≦20 eV for (CO2)n−, (CO2)nO− and (CO2)nO2− withn=1 to 20. The shape of the cross section function for (CO2)nO− is strongly dependent on the cluster sizen.

DISSOCIATION OF SINGLY AND MULTIPLY CHARGED FULLERENES : EMISSION OF C4, OR SEQUENTIAL EMISSION OF C2 ?

We have obtained direct mass spectrometric evidence that fullerene ions C60z+ (z=1, 2, or 3) and C58z+(z=1,2) undergo unimolecular dissociation by sequential emission of two C2 units, on a time scale of 10−5 s. Moreover, a comparison of experimental and theoretical breakdown graphs reveals that unimolecular formation of C56+ from the C60+ parent ion within a given observational time window is dominated by successive loss of C2; direct C4 loss does not contribute significantly. This conclusion is not affected by uncertainties in our knowledge of the energetics of C2 vs C4 loss.

Interaction of free electrons with C60: ionization and attachment reactions

Abstract Electron impact ionization and attachment to C60 has been studied as a function of electron energy using a crossed molecular beam—electron beam double focusing mass spectrometer system. It was possible to determine absolute ionization cross section functions (up to 80 eV) for the reactions C60 + e → C+60, C+58, C+56 and C+54, respectively. The huge kinetic shifts observed for the appearance of the various fragment ions are interpreted in terms of RRKM theory yielding a corresponding dissociation energy for C2 evaporation of (7.1±0.4) eV. The attachment cross section curve observed for the production of C−60 exhibits a strong zero energy resonance and evidence of equally strong autoscavenging processes up to energies of 14 eV.

Formation of the superhalogen ion SF−7 via electron attachment to SF6 clusters

Electron attachment to SF6 and SF6 clusters has been investigated in a molecular beam/electron impact ion source/mass spectrometer system. In accordance with recent theoretical predictions we were able to produce the superhalogen ion SF7− by electron attachment to SF6 clusters. SF7− has not been previously observed; its relative attachment cross section curve shows features more similar to that of F2− from SF6 than that of F− from SF6.

SF−6 production via excimer-mediated electron attachment to mixed rare gas/SF6 clusters

Abstract Electron attachment to mixed rare gas/SF 6 clusters — in contrast to pure SF 6 clusters — shows for the production of SF − 6 ions, besides the zero energy resonance, an additional resonance peak at higher electron energies in the attachment cross-section function. The process of SF − 6 production via this new resonance channel involves a multiple collision electron scavenging mechanism followed by an excimer-(R 2 ★ ) induced decay of an intermediate complex ion (R m ·R 2 ★ SF − 6 plus neutral products.