Photoelectron spectroscopy of mass-selected metal cluster anions. I. Cu_n^-, n=1-10

Negative ion photoelectron spectra of Cu_n^- (n=1-10) are reported for the 0-2.4 eV region at an instrumental resolution of 10 meV. The cluster anions were prepared in a flowing afterglow ion source incorporating a cold cathode dc discharge. This very simple source provides a convenient, general method to prepare continuous beams of near-thermal metal cluster ions at intensities (up to 10^-11 A) sufficient for spectroscopic or chemical studies. Photoelectron spectra of the copper cluster anions yield measurements for vertical electron binding energies and adiabatic electron affinities as a function of cluster size. The overall trend observed is well described by the classical spherical drop electrostatic model. In addition, quantum effects are apparent in the higher electron affinities generally observed for clusters containing odd numbers of atoms. Excited electronic states in the photoelectron spectra show that the transition energy in the neutral molecule decreases rapidly with cluster size. Vibrational structure resolved in the Cu ₂^- spectrum yields measurements for the vibrational frequency (210±15 cm^-1), bond length (2.345±0.010 Å), dissociation energy (1.57±0.06 eV), and vibrational temperature (450±50 K) of the anion.