Published papers

The electronic properties of copper cyanide cluster anions [Cu-n(CN)(m)(-); n=1-6, m=1-6] were studied using photoelectron spectroscopy (PES) with a magnetic-bottle type electron spectrometer. Both the anions and the cations of the Cu-n(CN)(m) cluster were generated by laser vaporization of a molded copper cyanide rod in a He carrier gas. In the mass spectra, abundant clusters were produced at the composition of (n,m)=(n,n+1;n=1-6) and (n,n;n=4 and 5) for the anions, whereas more abundant clusters were observed at (n, n-1; n=1-9) for the cations. The stability of Cu-n(CN)(n+1)(-) and Cu-n(CN)(n-1)(+) clusters is attributed to their electronic structure, where ionic Cu+ and CN- are linked alternately in a linear geometry. The PES spectra of the Cu-n(CN)(m)(-) anions show that the (n,n+1) clusters exhibit an extremely large EA of above 4.5 eV, while the EA's of the less abundant (n,n) clusters increase monotonously with cluster size from 1.3 eV (n=1) to 3.12 eV (n=6), except for n=4 and 5. Together with theoretical calculations by the density functional theory (DFT), two different linear isomers have been found for (n,n) clusters, where CN takes a opposite direction toward Cu. For Cu-4(CN)(4)(-) and Cu-5(CN)(5)(-), moreover, the PES spectra show two components of distinctly different peak shape, suggesting that a ring isomer should coexist with the linear ones. (C) 2000 American Institute of Physics. [S0021- 9606(00)00429-3].

Optical properties of the deposited germanium-oxide (Ge-O) prepared from the laser ablation of GeO2 were investigated. When the GenOm clusters, having the compositions mainly at n = m, were deposited onto a substrate as a precursor, strong visible photoluminescence peaked at 500 nm was observed under the excitation of 325 nm laser light. X-ray photoelectron spectroscopy for the deposited Ge-O on the substrate consistently shows the substantial component of the oxidation state of Ge2+, which shows the contributions from 1:1 composition of Ge2+-O2-. Correspondingly, the electronic structures of the germanium-oxide cluster were studied by using photoelectron spectroscopy in gas phase. Our developed method of the halogen atom doping enables us to determine the highest occupied molecular orbital-the lowest unoccupied molecular orbital gap of the corresponding neutral GenOn clusters experimentally, and it was found that the gap of the GenOn (n = 2-5) clusters reasonably corresponds to the energy of the visible light. These results imply that the GenOn clusters can be ascribed to the origin of the visible photoluminescence.

In the gas phase, novel network structures were found in organometallic clusters between metal atoms and C-60. Lanthanide organometallic clusters, Ln(n)(C-60) consisting of lanthanide atoms (Ln; Ln = Eu and Ho) and C60 molecules were produced predominantly at compositions of (n, m)=(n, n+3) [n= 1 -4). The cluster having (1, 4) composition is proposed to form a tetrahedral structure, in which a Ln atom is surrounded by four C60 molecules. Together with the results of sequential chemisorption of Cl atoms on cluster cations, photoelectron spectroscopy of cluster anions leads us to conclude that Lnn(C-60)(m) clusters are charge transfer complexes expressed as Ln(n)(x+)(C-60)(m)(x-), and that Eu and Ho atoms exist as Eu2+ and Ho3+ ions. Furthermore, the electronic and geometric properties of gas-phase exohedral NanC60 and AunC60 cluster are investigated. For NanC60, the high stability of the n=3, 6, 9, and 12 anions was observed, which suggests a "wet" structure having these atoms packed into stable trimers on the surface. In contrast, the electron affinity of AunC60 clearly shows even-odd alternation with the number of Au atoms, which suggests a "dry" structure where An atoms are segregated from each other.

Scandium-arene complex anions, Sc1(arene)m - (m = 1, 2; arene = benzene and toluene) were generated by the gas phase reaction of laser vaporized atoms with aromatic molecules. Sc1(toluene)2 - was identified as a sandwich complex anion in the gas phase. The electronic structures of the complex anions were observed by photoelectron spectroscopy.

Substituted Al clusters with 8 atom, which is trivalent but smaller than Al atom, or with covalent C atom were investigated using photoelectron spectroscopy (PES). In Aln-mBm- (n greater than or equal to 5 for m = 1, n greater than or equal to 10 for m = 2), three valence electrons of each B atom as well as Al atom contribute to electron shell structures, while the C atom strongly binds two free electrons from Al-n(-) in AlnC- (n = 5-30). The substitution with B atom allowed us to determine that s-p mixing starts at n = 6 in pure Al clusters. Perturbed electron shells and the geometry of Al12B1- Al11B2-. and Al12C- are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.

The analysis of the PES spectra was performed on gas-phase Au- and Na-coated fullerenes. As a result, the geometric structure of the exterior Au and Na atoms on the C60 and specific case of sodium among the alkali elements were taken into account.

The photoelectron spectra of small gas-phase YnOm- cluters (n = 2-10, m = 1-3) are measured at photon energies of 3.49 and 4.66 eV. Cluster generation is performed with the aid of laser vaporization, whereas the kinetic energy of electrons is measured using a magnetic bottle photoelectron spectrometer. The evolution of the electronic structure of neutral yttrium oxide clusters is discussed in terms of both sequential oxygen addition to the metal cluster core and sequential metal atom increase. Electron affinities and vertical detachment energies are measured and compared to ionization potentials of the monoxide cluster series [Knickelbein, M. J. Chem. Phys. 1995, 102, 1]. A general trend in the observed photoelectron spectra is a shifting of threshold energies to higher binding energies with increasing cluster size n. At low electron binding energies, main spectral features are contributed from metal d-band orbitals whereas the O 2p, contribution becomes apparent with increasing oxygen content due to a higher degree of hybridization of yttrium s, d, and oxygen 2p orbitals. The differences of yttrium-oxygen dissociation energies are determined and compared.

The photoelectron spectra of small mass-selected aluminum-rich AlnCo- (n=8-17) and cobalt-rich ConAlm- clusters (n = 6,8, 10; m = 1,2) are measured at photon energies of 3.49 eV with the aid of a magnetic bottle photoelectron spectrometer. The electronic structures of the bimetallic clusters are compared with those of pure Al-n(-) and Co-n(-) clusters, which are measured under the same conditions. The threshold behavior (electron affinities and vertical detachment energies) is analyzed, and the application of the electronic shell model reveals a similar shell structure of Co-doped Al clusters with a single heteroatom. Additional heteroatom doping induces shell perturbations. Compared to pure Al,, clusters, single-atom doped clusters show a hybridization of Al s, p and Co d orbitals resulting in a merging of spectral features at low binding energies. The evolution of the electronic structure is compared with reported ionization potentials [Menezes and Knickelbein, Chem. Phys. Lett. 183, 357 (1991); Z. Phys. D 26, 322 (1993)]. The hydrogen chemisorption behavior of neutral aluminum cobalt clusters [Nonose et al., Chem. Phys. Lett. 164, 427 (1989)] shows a clear influence of Co d states at low binding energies in the case of Al,,Co, whereas for ConAlm clusters (n >m) the geometric effect becomes more dominant. (C) 2001 American Institute of Physics.

A supersonic cluster beam containing isomers of Al(C6H6) complexes was generated by laser evaporation, and non-destructively selected using a 2 m long electrostatic hexapole. The focusing curve shows clear evidence that there are two kinds of Al(C6H6) isomers which are slightly different from each other in geometry; one is an asymmetric 1,2-complex and the other nearly C-6v symmetric 1,4-complex. The electric dipole moments of the two isomers are found to be 1.5 +/- 0.1 and 1.4 +/- 0.1 D, respectively. We carried out computation using density functional theory in order to estimate their structures. We found that the 1,2-complex is more stable than the 1,4-complex. The present work confirms that the electrostatic hexapole technique is useful for non-destructive selection of geometrical isomers in a beam.

Photodetachment photoelectron spectroscopy experiments on NbnAl- clusters have been carried out in a size range n = 4 to 12 at 3.49 eV photon energy. The evolution of the electronic structure is discussed as a function of cluster size and composition. Electron affinities (EA) and vertical detachment energies are determined. One central point of this study is the direct comparison of the electronic structure of pure and Al-doped Nb-n(-) clusters in the threshold binding energy region with regard to hybridization effects. Direct correlation between electronic structure and H-2 chemisorption. rate constants reported by Nonose et al. [Chem. Phys. Lett. 164 (1989) 427], is found for n = 5 to 8 and n > 9. (C) 2001 Elsevier Science B.V. All rights reserved.

Gas-phase anion complexes are produced containing one or more vanadium or titanium atoms bound to one or more molecules of the polycyclic aromatic hydrocarbon (PAH) coronene (C24H12). These species are prepared with coincident laser vaporization of a metal rod and a pressed pellet of coronene in a pulsed-nozzle cluster source. The anion complexes produced are mass-selected with a time-of-flight spectrometer and photoexcited with various Nd:YAG laser harmonics. A, magnetic bottle instrument is employed for time-of-flight photoelectron spectroscopy measurements on these complexes. Electron affinities (EA's) are observed to be greater for the organometallic complexes than they are for the isolated metal clusters or the isolated coronene molecule. EA's increase with the number of metal atoms in the complex. The spectra observed for V-coronene complexes are similar in appearance to those seen previously for V-benzene and V-C-60 suggesting that the metal binds as a pi complex in a symmetric ring site. V-(coronene)(2) complexes exhibit different spectra under different conditions attributed to the formation of sandwich and external-metal isomers.

The geometric and the electronic structures of terbium-silicon anions, TbSin-(6 less than or equal to n less than or equal to 16) were investigated by using photoelectron spectroscopy (PES) and a chemical-probe method. The clusters were produced by a double-rod laser vaporization technique. From trends observed in the electron affinities (EAs), the TbSin- clusters were categorized into three groups of (I) 6 less than or equal to n less than or equal to 9, (II) n = 10, 11, and (III) n greater than or equal to 12. Together with adsorption reactivity toward H2O it is concluded that a Tb atom is encapsulated inside a Si-n cage at n 2: 10; Tb@Si-n.

Behavior of silicon (Si) and germanium (Ge) atoms (or clusters) on the surface of a C-60 fullerene has been revealed by using time-of-flight mass spectroscopy and photoelectron spectroscopy. The mixed clusters of C(60)Sin and C60Gem were generated by two-lasers vaporization of a molded C-60 rod and a Si (or a Ge) rod in He carrier gas. The size distributions of anionic and cationic C60Sin/C60Gem clusters monotonically decreased with the number of Si/Ge atoms, and were limited only up to n = 4 and m = 3. The photoelectron spectra of C60Sin- and C60Gem- are similar to that of pure C-60(-) at n, m = 3, 4, whereas the spectra at n, m = 1, 2 are different from that of C-60. These results show that Si/Ge atoms assemble together into Si/Ge clusters on the C-60 cage with an increase in the number of Si/Ge atoms.

Photoelectron spectroscopy of size-selected ConOm- clusters (n = 4-20; m = 0-2) is performed in a pulsed molecular beam experimental at photon energies of 3.49 and 4.66 eV. Cobalt oxide cluster anions are generated in a laser vaporization source and the photoelectron spectra are measured with a magnetic bottle photoelectron spectrometer. The size-dependent evolution of the electronic structure of the oxide clusters is discussed in terms of cluster composition. Threshold binding energy features are mainly contributed from Co d-derived orbitals throughout the complete series, indicating that the incoming oxygen atoms cause a minor influence on the electronic structures. Although for n > 13 EAs and VDEs of Con and ConOm clusters overlap within the experimental resolution, at smaller cluster sizes, the values increase in the order of Con < ConO < ConO2. For Co4O3- and Co5O3-, the spectra indicate the presence of isomers with low EAs peroxo and ozonide isomers. To compare the different channels of oxygen chemisorption, relative dissociation energies are determined via thermochemical cycles.

Palladium-doped gold clusters. AunPd- (n = 1-4). were investigated using anion photoelectron spectroscopy at 4.66 eV photon energy. Electron affinities (EAs) and vertical detachment energies (VDEs) are determined, and the electronic structures of Pd-doped Au-n clusters are compared to those of pure Au-n clusters. A peak shape analysis reveals electronic and geometric similarity between Au-n(-) and Aun-1 Pd- clusters and it is found that (1) an electron promotion occurs from 4d to 5s orbital in the Pd atom, and that (2) the bond of Au-Pd is formed through sigma orbital between 6s of Au and 5s of Pd. (C) 2002 Published by Elsevier Science B.V.

Photoionization efficiency (PIE) curves were measured for sandwich vanadium-benzene clusters, V-n-(benzene)(n+1) (n = 1-5), which were produced by the reaction of laser-vaporized vanadium atoms with benzene vapor. A two-rod laser vaporization source was used for enhancing the production of the larger clusters. The second lowest ionization energies of the sandwich clusters were probed experimentally for the first time. The PIE curves of the sandwich clusters clearly present evidence that the clusters take a quasi-band electronic structure which results from a one-dimensional sandwich structure.