Photoelectron spectroscopy revealed that a 20-atom gold cluster has an extremely large energy gap, which is even greater than that of C60, and an electron affinity comparable with that of C60. This observation suggests that the Au20 cluster should be highly stable and chemically inert. Using relativistic density functional calculations, we found that Au20 possesses a tetrahedral structure, which is a fragment of the face-centered cubic lattice of bulk gold with a small structural relaxation. Au20 is thus a unique molecule with atomic packing similar to that of bulk gold but with very different properties.

http://cluster.physik.uni-freiburg.de/lehre/SS11/ueb11/Li2003b.pdf

Au20: A Tetrahedral Cluster

Experimental studies of the superconductive properties of fullerides are briefly reviewed. Theoretical calculations of the electron-phonon coupling, in particular for the intramolecular phonons, are discussed extensively. The calculations are compared with coupling constants deduced from a number of different experimental techniques. It is discussed why ${\mathrm{A}}_{3}$${\mathrm{C}}_{60}$ are not Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates of the Coulomb pseudopotential ${\mathrm{\ensuremath{\mu}}}^{\mathrm{*}}$, describing the effect of the Coulomb repulsion on the superconductivity, as well as possible electronic mechanisms for the superconductivity, are reviewed. The calculation of various properties within the Migdal-Eliashberg theory and attempts to go beyond this theory are described.

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Superconductivity in fullerides

Fullerenes are graphitic cage structures incorporating exactly twelve pentagons. The smallest possible fullerene is thus C20, which consists solely of pentagons. But the extreme curvature and reactivity of this structure have led to doubts about its existence and stability. Although theoretical calculations have identified, besides this cage, a bowl and a monocyclic ring isomer as low-energy members of the C20 cluster family, only ring isomers of C20 have been observed so far. Here we show that the cage-structured fullerene C20 can be produced from its perhydrogenated form (dodecahedrane C20H20) by replacing the hydrogen atoms with relatively weakly bound bromine atoms, followed by gas-phase debromination. For comparison we have also produced the bowl isomer of C20 using the same procedure. We characterize the generated C20 clusters using mass-selective anion photoelectron spectroscopy; the observed electron affinities and vibrational structures of these two C20 isomers differ significantly from each other, as well as from those of the known monocyclic isomer. We expect that these unique C20 species will serve as a benchmark test for further theoretical studies.

Gas-phase production and photoelectron spectroscopy of the smallest fullerene, C20

The development of femtosecond time-resolved methods for the study of gas-phase molecular dynamics is founded upon the seminal studies of Zewail and co-workers, as recognized in 1999 by the Nobel Prize in Chemistry.1 This methodology has been applied to chemical reactions ranging in complexity from bond-breaking in diatomic molecules to dynamics in larger organic and biological molecules, and has led to breakthroughs in our understanding of fundamental chemical processes. Photoexcited polyatomic molecules and anions often exhibit quite complex dynamics involving the redistribution of both charge and energy.2-6 These processes are the primary steps in the photochemistry of many polyatomic systems,7 are important in photobiological processes such as vision and photosynthesis,8 and underlie many concepts in molecular electronics.9 Femtosecond time-resolved methods involve a pump-probe configuration in which an ultrafast pump pulse initiates a reaction or, more generally, creates a nonstationary state or wave packet, the evolution of which is monitored as a function of time by means of a suitable probe pulse. Time-resolved or wave packet methods offer a view complementary to the usual spectroscopic approach and often yield a physically intuitive picture. Wave packets can behave as zeroth-order or even classical-like states and are therefore very helpful in discerning underlying dynamics. The information obtained from these experiments is very much dependent on the nature of the final state chosen in a given probe scheme. Transient absorption and nonlinear wave mixing are often the methods of choice in condensed-phase experiments because of their generality. In studies of molecules and clusters in the gas phase, the most popular methods, laser-induced fluorescence and resonant multiphoton ionization, usually require the probe laser to be resonant with an electronic transition in the species being monitored. However, as a chemical reaction initiated by the pump pulse evolves toward products, one expects that both the electronic and vibrational structures of the species under observation will change. Hence, these probe methods can be * To whom corresondence should be addressed. A.S.: telephone (613) 993-7388, fax (613) 991-3437, E-mail albert.stolow@nrc.ca. D.M.N.: telephone (510) 642-3505, fax (510) 642-3635, E-mail dan@radon.cchem.berkeley.edu. 1719 Chem. Rev. 2004, 104, 1719−1757

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Femtosecond time-resolved photoelectron spectroscopy.

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 AuN- (N = 1−14) at several photon energies. Even−odd alternations were observed, where the even-sized clusters (except Au10-) 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 Au10- 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. Let...

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

A high resolution experimental photoemission spectrum of ${\mathrm{C}}_{60}^{\phantom{\rule{0ex}{0ex}}\ensuremath{-}}$ is compared with calculations including the Jahn-Teller effect and multiple phonon satellites. The spectra show discrete loss features due to the excitation of phonons. From the intensity of these features, electron-phonon coupling constants are derived, which support the electron-phonon mechanism for superconductivity. The systematic deviations from previously calculated coupling constants are discussed in detail.

/pdf/photoemission-spectra-of-c60-electron-phonon-coupling-jahn-1q08hke4yg.pdf

Photoemission spectra of C60- : electron-phonon coupling, Jahn-Teller Effect, and superconductivity in the fullerides

/pdf/stable-configurations-of-carbon-clusters-chains-rings-and-4yeccfm95n.pdf

Stable configurations of carbon clusters: Chains, rings, and fullerenes.

Photoelectron spectra of Ag−n clusters with n=1–21 recorded at different photon energies (hν=4.025, 4.66, 5.0, and 6.424 eV) are presented. Various features in the spectra of Ag−2–Ag−9 can be assigned to electronic transitions predicted from quantum chemical ab initio calculations. While this comparison with the quantum chemical calculations yields a detailed and quantitative understanding of the electronic structure of each individual cluster, a discussion in terms of the shell model is able to explain trends and dominant patterns in the entire series of spectra up to Ag−21.

/pdf/electronic-shells-or-molecular-orbitals-photoelectron-3gqr9dfks2.pdf

Electronic shells or molecular orbitals: Photoelectron spectra of Ag−n clusters

Photoelectron spectra of Au−n with n=2–4 are reported. Due to the relatively high photon energy used in our experiment (hν=6.424 eV) and the energy resolution of about 50 meV, various transitions into excited states of the neutral clusters are resolved. It is demonstrated that photoelectron spectra can serve as a map of the electronic states of a cluster, while the high resolution of the resonant two‐photon ionization (R2PI) method gains information about the symmetry of the states. The comparison with similar data of Ag−n clusters indicates the influence of relativistic effects and the large spin–orbit splitting for Au.

/pdf/a-comparison-of-photoelectron-spectroscopy-and-two-photon-1vhs8ixa8a.pdf

A comparison of photoelectron spectroscopy and two-photon ionization spectroscopy : excited states of Au2, Au3, and Au4

Photoelectron spectroscopy measurements and density functional theory calculations are combined to determine structures of ${\mathrm{Nb}}_{n}^{\phantom{\rule{0ex}{0ex}}\ensuremath{-}}$ ( $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3--8$) clusters. A detailed comparison between observed and calculated electronic binding energies shows that the clusters have low-symmetry compact 3D structures and the lowest possible total spin, except for the three- and five-atom clusters which are in triplet states. We find evidence for the coexistence of two isomers of ${\mathrm{Nb}}_{8}^{\phantom{\rule{0ex}{0ex}}\ensuremath{-}}$ under some experimental conditions. This approach shows great promise for structural characterization of small clusters.

/pdf/photoelectron-spectra-and-geometric-structures-of-small-5cng3e4gdx.pdf

P. S. Bechthold

Papers

Photoemission spectra of C60- : electron-phonon coupling, Jahn-Teller Effect, and superconductivity in the fullerides

Stable configurations of carbon clusters: Chains, rings, and fullerenes.

Electronic shells or molecular orbitals: Photoelectron spectra of Ag−n clusters

A comparison of photoelectron spectroscopy and two-photon ionization spectroscopy : excited states of Au2, Au3, and Au4

Photoelectron Spectra and Geometric Structures of Small Niobium Cluster Anions.