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

Neutral ${\mathrm{C}}_{60}$ 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 for ${\mathrm{C}}_{60}^{+}$, ${\mathrm{C}}_{60}^{2+}$, and ${\mathrm{C}}_{60}^{3+}$ ions in the 17–75 eV energy range show an additional resonance near 40 eV. Time-dependent density functional calculations confirm the collective nature of this feature, which is characterized as a dipole-excited volume plasmon made possible by the special fullerene geometry.

Figure 1

Absolute cross-section measurement of single photoionization of ${\mathrm{C}}_{60}$ (open circles). The thin solid line results from the fit to the measured data of a linear background (not shown) plus the two separately displayed Lorentzian curves. These represent the surface and the volume plasmon resonances near 22 and 38 eV, respectively, that correspond to collective excitations of the ${\mathrm{C}}_{60}^{+}$ valence electrons.

Figure 2

Absolute cross sections for single photoionization of ${\mathrm{C}}_{60}^{+}$, ${\mathrm{C}}_{60}^{2+}$, and ${\mathrm{C}}_{60}^{3+}$. The single-photoionization cross sections of neutral ${\mathrm{C}}_{60}$ measured by Hertel et al. [6], Yoo et al. [7], and Reinköster et al. [14] are also shown for comparison. The present cross sections for ${\mathrm{C}}_{60}$ ions are on an independently absolute scale. The only previous absolute photoabsorption cross-section measurements for fullerenes were performed by Smith [26] and Yasumatsu et al. [27] for neutral ${\mathrm{C}}_{60}$ at photon energies below 11 eV. Berkowitz [28] normalized the photoionization data of Hertel et al. and Yoo et al. to these measurements. Reinköster et al. used the total ${\mathrm{C}}_{60}$ photoionization cross section reported by Berkowitz for 40.8 eV photons to scale their relative data.

Figure 3

Single-photoionization cross section of ${\mathrm{C}}_{60}^{+}$ from experiment (circles) and from TDLDA (solid line) and LDA (dashed line) calculations. The theoretical data are convoluted with a Gaussian of width 0.1 eV, and the TDLDA result is scaled to match the giant resonance near 22 eV (see text). The inset shows the contributions of $\sigma$ and $\pi$ electrons to the second giant resonance