PHOTOELECTRON-SPECTROSCOPY OF METAL CLUSTER ANIONS - CUN-, AGN-, AND AUN-

Negative ion photoelectron spectra of Cun-, Ag n- (n = 1-10), and Aun- (n = 1-5) are presented for electron binding energies up to 3.35 eV at an instrumental resolution of 6-9 meV. The metal cluster anions are prepared in a flowing afterglow ion source with a cold cathode dc discharge. In the spectra of Cu 2-, Ag2-, and Au2 -, the M2 X 1Σg +←M2- X 2Σ u+ transitions are vibrationally resolved. We analyze these spectra to yield the adiabatic electron affinities, vibrational frequencies, bond length changes, and dissociation energies. The a 3Σu+ triplet states of Cu2 and Ag2 are also observed. Using experimental and theoretical data, we assign the major features in the Cu3- and Ag 3- spectra to the transition from the linear ground state of the anion (M3- 1Σg +) to an excited linear state of the neutral (M3 2Σu+). The Au3- spectrum is attributed to a two-photon process, photodissociation followed by photodetachment of the Au- or Au2- fragment. For larger clusters, we measure the threshold and vertical detachment energies as a function of size. Trends in the electron affinities and excited state energy levels as a function of cluster size and composition are discussed in terms of simple models. © 1990 American Institute of Physics.