Photoexcitation of a Volume Plasmon in C$_{60}$ Ions*

Neutral C$_{60}$ is well known to exhibit a giant resonance in its photon absorption spectrum near 20 eV. This is associated with a surface plasmon, where delocalized electrons oscillate as a whole relative to the ionic cage. Absolute photoionization cross-section measurements made at the Advanced Light Source for C$_{60}^{+}$, C$_{60}^{2+}$ and C$_{60}^{3+}$ ions in the 17-75 eV energy range show an additional broad resonance near 40 eV. Time-dependent density functional theory calculations confirm the collective nature of this feature, which is characterized as a dipole-excited volume plasmon made possible by the special fullerene geometry of a charged spherical shell.