Ionization ofC70andC60molecules by slow highly charged ions: A comparison

Abstract

We have studied collisions between slow highly charged ions and pure ${\mathrm{C}}_{70}$ and ${\mathrm{C}}_{60}$ molecular targets, and report on measurements of target ionization and fragmentation in electron transfer processes. The intensity distributions in the fragmentation spectra for ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ are rather similar, indicating that similar roles are played by evaporation (neutral ${\mathrm{C}}_{2}$ emission) and fission processes (charged particle emission) in the two systems. For ${\mathrm{C}}_{70}$, intact molecular ions are formed in charge states up to $10+$, while the maximum charge state for ${\mathrm{C}}_{60}$ molecules is $9+$ following collisions with ${\mathrm{Xe}}^{23+}$ at $69\phantom{\rule{0.3em}{0ex}}\text{keV}$. The kinetic-energy releases measured for asymmetric fission of ${\mathrm{C}}_{70}$ ions $(\mathrm{C}_{70}{}^{r+}\ensuremath{\rightarrow}\mathrm{C}_{68}{}^{(r\ensuremath{-}1)}+\mathrm{C}_{2}{}^{+})$ are mostly close to the corresponding ones for ${\mathrm{C}}_{60}$, while both sets of results are significantly lower than those reported for electron-impact ionization studies of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$. Kinetic-energy releases and fission barrier heights are estimated for ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ ions using an electrostatic model. An increased intensity of the higher charge states of ${\mathrm{C}}_{70}$ ions is observed compared to the ${\mathrm{C}}_{60}$ case, which most likely is due to a larger number of degrees of freedom on which the internal excitation energy may be distributed.