PLASMA SPECTROSCOPY PROPOSED FOR C-60 AND C-70

The fullerenes C60 (quasispherical though actually icosahedral) and C70 (quasispheroidal) are very like hollow shells (graphiteroles) made from a single hexagonal layer from a graphite crystal. They should support multipolar (l greater-than-or-equal-to 1) plasma oscillations, closely related to the plasmons seen in graphite. We estimate their frequencies using the so-called hydrodynamic model, with (provisionally) just one parameter calibrated on graphite. We predict pi plasmons in the range between 6 and 8 eV, and sigma plasmons near and above 25 eV. The best, if not the only way to observe them is by electron energy-loss spectroscopy. Only the dipole (l = 1) excitation is allowed optically; in the solid, its frequency should be shifted according to the Lorentz-Lorenz formula. If, improbably, the l = 1 pi plasmon is observable in solution, its frequency should be unprecedentedly sensitive to the refractive index of the solvent. On C70, the multipoles are split and shifted relative to C60, but only by surprisingly little.