Single-Photon Multiple Detachment in Fullerene Negative Ions: Absolute Ionization Cross Sections and the Role of the Extra Electron

We have obtained experimental photo-double- and photo-triple-detachment cross sections for the fullerene negative ion using Advanced Light Source photons of 17–90 eV. The cross sections are 2 and 2.5 times larger than those for ${\mathrm{C}}_{60}$ and appear to be compressed and shifted in photon energy as compared to ${\mathrm{C}}_{60}$. Our analysis reveals that the additional electron in $\mathrm{C}_{60}{}^{-}$ primarily produces screening which is responsible for the modification of the spectrum. Both screening effects, the shift and the compression, can be quantitatively accounted for by a linear transformation of the energy axis. Applying the transformation allows us to map the neutral and negative ion cross sections onto each other, pointing out the close relationship of correlated few-electron dynamics in neutral and negatively charged extended systems. In contrast, dynamics of neutral and negatively charged atoms or small molecules are typically not closely related.

Figure 1

Measured absolute photoion yields from the removal of 2 (top) or 3 (bottom) electrons. Solid curves are present $\mathrm{C}_{60}{}^{-}$ photodetachment results. For comparison, photoionization results for ${\mathrm{C}}_{60}$ [17] are also plotted (broken curves, relative to the $\mathrm{C}_{60}{}^{-}$ ground state), magnified by a factor of 3 ($\mathrm{C}_{60}{}^{2+}$) and 4 ($\mathrm{C}_{60}{}^{3+}$) for presentation purposes. Vertical lines indicate positions of relevant state energies [17, 25, 26]. Dotted curves are threshold region best-fit results. Near-threshold data (dots, 1 standard deviation statistical errors bars) are shown in the inset, and also magnified by a factor of 3 with a 10 Mb shift, for clarity.

Figure 2

Scaled $\mathrm{C}_{60}{}^{-}$ photodetachment cross section ${\sigma }_{\beta }^{-}({\epsilon }^{\circ })$ [see Eq. (1)] (solid curve) compared to ${\mathrm{C}}_{60}$ photoionization from 17 (broken curve) for the removal of 2 (top) or 3 (bottom) electrons. (${\epsilon }^{\circ }$ is the excess energy for photo-double ionization or triple photoionization of ${\mathrm{C}}_{60}$.)