Angle-differential observation of plasmon electrons in the double-differential cross-section spectra of fast-ion-induced electron ejection from ${\text{C}}_{60}$

We report on the measurement of double-differential distribution of soft electron emission from ${\text{C}}_{60}$ fullerene, induced by a fast-moving Coulomb field of 76 MeV energy bare fluorine ions. A broad “plasmon-electron” peak, riding on the Coulomb-ionization continuum, is observed due to the deexcitation of the giant dipole plasmon resonance state in ${\text{C}}_{60}$. The angular distribution of the plasmon electrons goes through a dip around ${90}^{\circ }$, which is contrary to that observed in ion-atom collisions measured in situ, indicating the alignment of the induced dipole moment along the projectile beam direction. A model based on the photoelectron angular distribution which is modified due to the ion-induced postcollisional interaction provides an excellent agreement with the observed asymmetric distribution. The distribution smoothly changes from a dip at ${90}^{\circ }$ to a peak with the variation of ejected electron energy indicating transition from a collective plasmon behavior of the whole system to a single ion-atom interaction. The single-differential cross section was also derived, which preserves the signature of the collective excitation.

Figure 1

Energy distribution of absolute $e^{-}$-DDCS for ${\text{C}}_{60}$ (filled symbols), Ne (open symbols), and the CDW-EIS model for the single C atom (solid line). The Ne data and CDW-EIS(C) calculations have been normalized to the ${\text{C}}_{60}$ DDCS at 40 eV. Insets highlight the plasmon-peak region. In (d), the inset in the upper corner shows the KLL peak.

Figure 2

Electron-SDCS spectrum along with the normalized CDW-EIS prediction (red solid line) for a C atom (normalized at one point). Inset: The same electron-SDCS spectrum in linear scale.

Figure 3

Angular distribution of $e^{-}$-DDCS for ${\text{C}}_{60}$ (filled circles) and Ne (open squares joined by dotted line). The CDW-EIS (solid line) and the B1 (dash-dotted line) calculations for the C atom are also shown. The data for the Ne and the CDW-EIS calculations are normalized to the ${\text{C}}_{60}$ data in each plot. The dash-double-dotted line through the ${\text{C}}_{60}$ data points for 9 eV electrons in (a) is to guide the eyes.

Figure 4

Measured $e^{-}$-DDCS for ${\text{C}}_{60}$ (circles) target, same as in Fig. 2, along with the calculated distributions: $W_{pe}$ (dash-dotted line), $W_{PCI}$ (dotted line: scale on right axis), and total distribution $W_{\mathrm{tot}}$ (thick solid line).