Photoelectron spectroscopic study of charge-transfer states in clusters

Excited charge transfer (CT) bands are observed in the photoelectron spectra of mass-selected clusters consisting of halide anions (Cl^-, Br^-, I^-) complexed with several different amine molecules. We attribute these bands to the detachment of electrons from the highest occupied molecular orbital of the amine ligands. We show that the vertical binding energies of the CT bands in the photoelectron spectra of the 1:1 complexes are consistent with the results obtained from a simple electrostatic model. In clusters consisting of one chloride anion and several amine ligands, the relative intensity of the CT bands scales linearly with the number of ligands and reaches saturation at a given cluster size. The saturation size is 4 for the triethylamine ligand and larger than 8 for the smaller trimethylamine ligand. This size dependence is consistent with the observation of CT bands generated via direct ionization of the nearest neighbors to the anion. We suggest that the size dependence of the observed CT bands could be used to evaluate the coordination number of the solvated anions.