Periodic Properties of Force Constants of Small Transition-Metal and Lanthanide Clusters

Neutral silver cluster isomers Agn (n=2 to 12) were studied by Kohn–Sham density functional theory. There is a strong even-odd oscillation in cluster stability due to spin subshell closing. Nearest-neighbor interatomic distances do not evolve continuously from the diatomic (2.53 A) to the bulk (2.89 A). After adding an empirical correction to the calculated values, we estimate that they are always near 2.68 A for 3⩽n⩽6, and near 2.74 A for 7⩽n⩽12. We find several low-energy isomers at all cluster sizes larger than seven atoms with one exception: Ag10 has a D2d twinned pentagonal bipyramid isomer predicted to be 0.20 eV more stable than any other isomer. The ellipsoidal jellium model predicts rather well the shapes of stable silver clusters. Other models (extended Huckel, empirical potential) fail to reproduce the energy ordering of cluster isomers. The structural attributes of low-energy silver cluster isomers Agn (n⩾7) are, in decreasing order of importance: a high mean coordination; a shape that conforms to the ellipsoidal jellium model; and uniformity in atomic coordinations.

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Theoretical study of the structure of silver clusters

Mise au point. Donnees sur le phenomene de diffusion Raman augmentee en surface (SERS) et sur sa relation avec les effets de «rugosite» et de resonance electromagnetique; sur l'augmentation classique; sur l'interaction polariton plasmon de surface-adsorbat comparee a l'interaction electron du metal-adsorbat; sur la relation entre l'effet de resonance electromagnetique et SERS; et enfin sur les interactions entre les electrons du metal et l'adsorbat dans la SERS

Surface-enhanced Raman scattering: “Classical” and “Chemical” origins

Gas phase spectroscopic studies of copper trimer are reported. A band system in the 5430–5225 A region has been assigned to Cu3 and has been studied by resonant two‐photon ionization spectroscopy, spectroscopy by depletion through predissociation, and by radiation repopulation of excited vibrational states in a cold supersonic molecular beam. The spectrum fits well to a 2E″←2E′ transition of a D3h molecule with both states undergoing Jahn–Teller distortions. In this interpretation, the excited state is well described by a weak Jahn–Teller effect, stabilizing the molecule at a configuration in which one Cu–Cu bond length differs from the others by 0.03 A, with a Jahn–Teller stabilization energy of 9 cm−1. The ground electronic state corresponds to a deep Jahn–Teller situation with the lowest vibronic levels trapped in the distorted configuration. The spectra indicate the tunneling splitting of the lowest vibrational levels in the ground state is 12±7 cm−1. These assignments are based on measured vibrationa...

H.U. Becker

Papers

Matrix Raman spectra of Ag2 and Ag3