Highly Charged Clusters of Fullerenes: Charge Mobility and Appearance Sizes

Clusters of fullerenes $({\mathrm{C}}_{60},{\mathrm{C}}_{70}{)}_n$ are produced in a gas aggregation source and are multiply ionized in collisions with highly charged ${\mathrm{X}\mathrm{e}}^{20+,30+}$ ions. Their stabilities and decay processes are analyzed with high-resolution time-of-flight mass spectrometry. Fullerene clusters in charge states up to $q=5$ have been observed and appearance sizes are found to be as small as $n_{\mathrm{a}\mathrm{p}\mathrm{p}}=5$, 10, 21, and 33 for $q=2$, 3, 4, and 5, respectively. The analysis of the multicoincident fragmentation spectra indicates a high charge mobility. This is in contrast to charge localization effects which have been reported for ${\mathrm{A}\mathrm{r}}_n^{q+}$ rare gas clusters. Clusters of fullerenes are found to be conducting when multiply charged.

Figure 1

Time-of-flight mass spectrum of fullerenes and fullerene clusters, ionized in collisions with ${\mathrm{X}\mathrm{e}}^{20+}$ projectiles at 400 keV. Arrows indicate positions of odd numbered doubly charged clusters ($T_{\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{n}}=525°\mathrm{C}$).

Figure 2

Detailed view of a time-of-flight mass spectrum, showing contributions from multiply charged clusters. The notation ($k,l$) marks a cluster $({\mathrm{C}}_{60}{)}_k({\mathrm{C}}_{70}{)}_l$. $T_{\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{n}}=560°\mathrm{C}$.

Figure 3

Time-of-flight mass spectra: (a) ${\mathrm{X}\mathrm{e}}^{25+}+{\mathrm{C}}_{60}$, one stop; (b) ${\mathrm{X}\mathrm{e}}^{30+}+({\mathrm{C}}_{60}{)}_n$, one stop; (c) ${\mathrm{X}\mathrm{e}}^{30+}+({\mathrm{C}}_{60}{)}_n$, two stops; (d) ${\mathrm{X}\mathrm{e}}^{30+}+({\mathrm{C}}_{60}{)}_n$, nine stops; (e) ${\mathrm{A}\mathrm{r}}^{+}+{\mathrm{C}}_{60}$, 2 keV, one stop.

Figure 4

Correlation spectra for two stops obtained in collisions between ${\mathrm{X}\mathrm{e}}^{30+}$ projectiles (600 keV) and $({\mathrm{C}}_{60}{)}_n$ clusters (most likely $n=2$).