Formation of ${\mathrm{C}}_{60}^{2-}$ Dianions in Collisions between ${\mathrm{C}}_{60}^{-}$ and Na Atoms

We have observed the formation of ${\mathrm{C}}_{60}^{2-}$ and ${\mathrm{C}}_{\mathrm{70}}^{\mathrm{2}\mathrm{-}}$ in collisions between ${\mathrm{C}}_{\mathrm{60}}^{\mathrm{-}}/{\mathrm{C}}_{\mathrm{70}}^{\mathrm{-}}$ and Na atoms. Cross sections for the electron transfer to the monoanion are determined to be $36\pm 9$ and $57\pm 14{\AA }^2$ for ${\mathrm{C}}_{60}^{-}$ and ${\mathrm{C}}_{\mathrm{70}}^{\mathrm{-}}$, respectively. A simple model investigation suggests that the electron is transferred from a Na atom to a low-lying electronic state of the fullerene to form a dianion. The method leads to pico-ampere energetic beams of ${\mathrm{C}}_{60}$ dianions that can be used for spectroscopy and lifetime studies.

Figure 1

Illustration of experimental setup.

Figure 2

Mass spectra of negative ions obtained after collisions with Na atoms, (a) ${\mathrm{C}}_{\mathrm{60}}^{\mathrm{-}}+\mathrm{Na}$, (b) ${\mathrm{C}}_{70}^{-}+\mathrm{Na}$.

Figure 3

The resonance binding energy for ${\mathrm{C}}_{\mathrm{60}}^{\mathrm{2}\mathrm{-}}$ after electron transfer at a distance of $R$, for Na (solid line) and Xe (dashed line). Dotted lines indicate the positions of critical distances. The horizontal dotted line indicates the second electron binding energy of ${\mathrm{C}}_{60}$.