On the Electronic and Atomic Structures of Small Au_N~- (N = 4 - 14) Clusters: A Photoelectron Spectroscopy and Density-Functional Study

摘要

We report a joint experimental and theoretical study of the electronic and atomic structures of small gold clusters with up to 14 atoms. Well-resolved photoelectron spectra were obtained for Au_N~- (N = 1 - 14) at several photon energies. Even-odd alternations were observed, where the even-sized clusters (except Au_(10)~-) exhibit an energy gap between the lowest binding energy peak and the rest of the spectrum, indicating that all the neutral even-sized clusters have closed shells. The Au_(10)~- spectrum reveals the existence of isomers, with the ground-state cluster exhibiting an extremely high electron binding energy. Evidence of multiple isomers was also observed in the spectra of N = 4, 8, 12, and 13. The structures of the gold cluster anions in the range N = 4 - 14 were investigated using first-principles simulations. A striking feature of the anionic clusters in this range is the occurrence of planar ground-state structures, which were predicted in earlier theoretical studies (Hakkinen, H.; et al. Phys. Rev. Lett. 2002, 89, 033401) and observed in ion-mobility experiments (Furche, F.; et al. J. Chem. Phys. 2002, 117, 6982) and the existence of close-lying isomers. The calculated electron detachment energies and density of states were compared with the measured data, which confirmed the ground-state structures of the anions. It is found that the main isomers observed experimentally indeed consist of planar clusters up to Au_(12)~-, whereas for Au_(13)~- and Au_(14)~- the theoretical results from three-dimensional isomers agree better with the experiment, providing further support for the 2D to 3D structural transition at Au_(12)~-, as concluded from previous ion mobility experiments. We also find that small neutral clusters exhibit a tendency to form two-dimensional structures up to a size of 13 atoms.