Photoelectron spectroscopy of sodium-coated C-60 and C-70 cluster anions

A systematic study of photoelectron spectroscopy for C60NaN- (N = 0-33) and C70NaN- (N = 0-15) clusters is reported. These clusters are produced by a laser vaporization source and their photoelectron spectrum is measured usin g 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 Na-3 t rimers is inferred, with each atom lying above a pentagonal ring. For large r clusters (N greater than or equal to 13), a very different trend is obser ved, signing the appearance of a metallic behavior for the sodium layer at the fullerene surface. The specific case of C-70 is also discussed. [S0163- 1829(99)16531-7].