Angle-differential observation of plasmon electrons in the double-differential cross-section spectra of fast-ion-induced electron ejection fromC60

Abstract

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 ${90}^{\ensuremath{\circ}}$, 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 ${90}^{\ensuremath{\circ}}$ 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.