Photoelectron spectroscopy of sodium-coated C₆₀ and C₇₀ cluster anions

A systematic study of photoelectron spectroscopy for C₆₀Na^-_N (N=0–33) and C₇₀Na^-_N (N=0–15) 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 < 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 Na₃ 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 C70 is also discussed.