Giant plasmon excitation in free<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mrow><mml:mn>60</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mrow><mml:mn>70</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>molecules studied by photoionization

Single-photon ionization of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ has been studied using synchrotron radiation in an energy range from the ionization potential (7.54\ifmmode\pm\else\textpm\fi{}0.04 eV for ${\mathrm{C}}_{60}$ 7.3\ifmmode\pm\else\textpm\fi{}0.2 eV for ${\mathrm{C}}_{70}$) up to 35 eV. The photoion yield is dominated by a strong resonance at about 20 eV with a FWHM of \ensuremath{\sim}10 eV. We interpret this as being due to autoionization via the giant plasmon resonance recently predicted in the RPA approximation. ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ offer the unique possibility for a quantitative comparison of this collective excitation in the free molecule with that observed in the solid phase by electron-energy-loss spectroscopy at about 28 eV.

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