Photoelectron spectroscopy of mass-selected metal-water cluster negative ions: Cu^-(H₂O)_n and Na^-(H₂O)_n

The electronic structures of hydrated metal-atom clusters have been investigated by negative-ion photoelectron spectroscopy. We have obtained the photoelectron spectra of Cu^-(H₂O)_n with n=0-4 and Na^-(H₂O)_n with n=0-12. For the former clusters, we also detected the electron detachment from the CuOH^-(H₂O)_n-1 which coexists with Cu^-(H₂O)_n. The observed bands for both Cu^-(H₂O)_n and Na^-(H₂O)_n were all assigned to the transitions to the states originating in those of the metal atoms, which are shifted as a result of hydration. This result implies that the ground states of the neutral clusters still have a one-center character at the size range examined. In contrast, for the Na-water clusters, increasing character of the Rydberg-type ion-pair state in the negative-ion state is suggested from the vertical-detachment-energy dependence on the solvent number.