Collision of Li+ and Na+ with C60 : Insertion, fragmentation, and thermionic emission

Interactions of ${\mathrm{Li}}^{+}$ and ${\mathrm{Na}}^{+}$ with ${\mathrm{C}}_{60}$ molecules have been studied over the collision energy range from 0 to 150 eV. We observe insertion of the alkali-metal ions to form the endohedral [${\mathrm{LiC}}_{60}$${]}^{+}$ and [${\mathrm{NaC}}_{60}$${]}^{+}$ species, with energy thresholds of 6 and 20 eV, respectively. At higher collision energies, the excited [${\mathrm{NaC}}_{60}$${]}^{+}$ appears to relax mainly by loss of ${\mathrm{C}}_{2}$ units from the fullerene cage, yielding [${\mathrm{NaC}}_{60\mathrm{\ensuremath{-}}2\mathit{n}}$${]}^{+}$. For [${\mathrm{LiC}}_{60}$${]}^{+}$, escape of the ${\mathrm{Li}}^{+}$ competes effectively with ${\mathrm{C}}_{2}$ loss. For both ${\mathrm{Li}}^{+}$ and ${\mathrm{Na}}^{+}$, the signal for pure carbon fragment ions ${\mathrm{C}}_{60\mathrm{\ensuremath{-}}2\mathit{n}}^{+}$ increases sharply at \ensuremath{\sim}30 eV, which is attributed to thermionic emission.