Photoelectron spectroscopy of sodium-coatedC60andC70cluster anions

A systematic study of photoelectron spectroscopy for ${\mathrm{C}}_{60}{\mathrm{Na}}_N^{-}$ $(N=0\mathrm{}–33)$ and ${\mathrm{C}}_{70}{\mathrm{Na}}_N^{-}$ $(N=0\mathrm{}–15)$ clusters is reported. These clusters are produced by a laser vaporization source and their photoelectron spectrum is measured using a magnetic-bottle time-of-flight spectrometer. For $N<\mathrm{}13,$ the adiabatic electron affinity and the vertical detachment energy display strong regular features with increasing N. A packing of the sodium atoms in stable ${\mathrm{Na}}_3$ trimers is inferred, with each atom lying above a pentagonal ring. For larger clusters $(N>~13),$ a very different trend is observed, signing the appearance of a metallic behavior for the sodium layer at the fullerene surface. The specific case of ${\mathrm{C}}_{70}$ is also discussed.