Published papers

In the gas phase, novel network structures were found in organometallic clusters between metal atoms produced by laser ablation and organic ligand molecules. For 3d metal-benzene, MnBzm, two kinds of structures of multiple sandwich and rice-ball were formed, depending on the metal elements. Early transition metals (ME) of Sc, Ti, and V form the multiple-decker sandwich structure of (n, m) = (n, n + 1) in which metal atoms and benzene are alternately piled up, while late transition metals (ML) of Fe, Co, and Ni form the rice-ball structure in which central metal clusters were fully covered by benzene molecules. The ionization energy of ME-Bz drops significantly with increasing layers, which can be explained by delocalization of d electrons along the molecular axis. M-C60 binary clusters were also generated by a two-laser vaporization method; ME-C60 clusters efficiently form a chain or a ring structure consisting of a dumbbell unit, in which metal atoms and C60 are alternately connected. For ML-C60 clusters, however, the metal atom is tricapped by C60 and a face-centered tetrahedron structure is formed at (n, m) = (4, 4). A similar multiple-decker sandwich structure is formed also between lanthanide metal atoms (Ln) and an organic ligand of cyclooctatetraene (COT). The Ln-COT cluster is a charge transferred cluster consisting of positively charged Ln and negatively charged COT. Their electronic structure is fairly ionic and is localized around each metal atom. These novel structures of organometallic clusters should inspire new thoughts in material science because it is hoped that the regular arrangement of metal ions can introduce useful properties such as electroconductivity and magnetism.

Vanadium (V)benzene cluster ions, produced by laser ablation with reaction toward benzene vapor, were size-selected and deposited into a low-temperature Ar matrix. Infrared spectrum of V-1(benzene)(2) in the Ar matrix was measured after one-hour deposition with the deposition energy of 20 eV. The spectrum was in agreement with both the reported spectrum and our theoretical calculations, showing that (1) V-1(benzene)(2), prepared in the gas-phase reaction, takes a sandwich structure and that (2) the ions were soft-landed onto the Ar matrix and were neutralized by charge transfer from a metal substrate without fragmentation. (C) 2001 Elsevier Science B.V. All rights reserved.

The electronic properties of gold-silver binary cluster anions (AunAgm-; 2 less than or equal ton+m less than or equal to4) were studied by using photoelectron spectroscopy (PES) with a magnetic-bottle-type electron spectrometer. The AunAgm- cluster anions were generated by a laser vaporization of a gold-silver alloy rod. For triatomic AunAg3-n clusters, monotonous increases of electron affinity (EA) were observed by the replacement of Ag atom by Au atom. In contrast, the change in EA of tetratomic AunAg4-n clusters was irregular; for n=0-2 their EAs are almost the same around 1.5 eV, whereas for n=3,4 they increase to around 2.7 eV. This irregularity in EA of AunAg4-n can be attributed to the contribution of an ionic bonding. For Au1Ag3- and Au2Ag2-, moreover, the PES spectra show two components whose intensity ratio depends on cluster source conditions, showing that two isomers should coexist at these two compositions. (C) 2001 American Institute of Physics.

Vanadium (V)-C-60 mixed clusters were produced by two laser vaporization methods. In the mass spectra of cationic V-n(C-60)(m)(+), the abundant clusters were produced at the composition of (n,m>=(1,1), (1,2), (2,3), (3-4,4), and (5,5). This pattern is explicable in terms of a multiple dumbbell structure; V atom and C-60 are alternatively piled up. The ionization energy and the reactivity of the dumbbell clusters toward O-2 and CO show V-1(C-60)(2) takes a structure of V-1(eta(6) -C-60)(2). (C) 1997 American Institute of Physics.

Sodium doped silicon clusters (SinNam; 1 less than or equal to n less than or equal to 14, 1 less than or equal to m less than or equal to 5) produced by two types of laser vaporization were studied. The adsorption of Na atoms on the Si-n clusters leads the substantial lowering of the ionization energy, E-i, of SinNam clusters. Their reactivity toward NO molecules was measured with a fast flow reactor, sind the anticorrelation between the E(i)s and the reactivity was clearly observed: species having low E-i exhibit high reactivity and vice versa. Moreover, the clear parallelism between the E(i)s of SinNa and the EAs of Si-n is found. This is consistent with the fact that the structure of SinNa clusters keeps the frame of the corresponding Si-n cluster unchanged and that the electronic structure of SinNa is similar to that of the corresponding negative ion Si-n(-). In addition to the experimental studies, the geometries, adsorption energies, and vertical ionization energies of SinNa (n=1-7) were investigated with ab initio MO calculations including electron correlation; The Moller-Plesset perturbation theory was used and the configuration interaction (CI) calculation was carried out, particularly for a diatomic molecule, SiNa. (C) 1997 American Institute of Physics.

Electronic properties of silicon-sodium cluster anions (SinNam-; 4 less than or equal to n less than or equal to 11, 1 less than or equal to m less than or equal to 3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The electron affinities (EAs) of SinNam were determined from the threshold energies in the photoelectron spectra of SinNam-. Besides, the geometries and energies of SinNa- (n=1-7) were investigated with ab initio MO (molecular orbital) calculations; Moller-Plesset perturbation and coupled cluster methods were used to take into account the electron correlation. By an Na adsorption on Si-n, the EA decreases for n=4-6, but increases for n greater than or equal to 7. The calculations for SinNa- show that the Si-n framework for n=4-6 accepts at least two excess electrons. For Si7Na-, however, the charge transfer from the Na atom scarcely occurs to avoid distorting the close-packed Si-7 framework. Moreover, the ionization energy (E-i) and EA of SinNam show several patterns with the number of the Na atoms, which reflect the charge transfer mechanism in the cluster reflecting the nature of the electron-accepting orbitals of the corresponding Si-n and the adsorption types of the Na atoms. (C) 1997 American Institute of Physics. [S0021-9606(97)03347-3].

Cobalt (Co)-C-60 mixed clusters were produced by the combination of laser vaporization and molecular beam methods. Cationic Co-n(C-60)(m)(+) clusters were produced predominantly with the compositions (n, m) = (0-1, 1), (1, 2), (1, 3), (2-4, 4), and (5, 5). Both mass spectrometry and a chemical probe method have revealed that compositions of (1, 3) and (4, 4) correspond to a tricapped planar structure and a tricapped trigonal pyramid structure, respectively, in which each Co atom is surrounded by three C-60.

Geometries and energies of SinF- and SinF (n=1-7) were investigated with ab initio MO calculations; the Møller-Plesset perturbation and coupled cluster methods were used to take into account the electron correlation. The F atom is bound to the apex atom in the Sin frame, and the electronic structures of the Sin part in SinF- and SinF are similar to those of the corresponding Sin and Si+n, respectively. The calculated electron affinities (EAs) of SinF are in agreement with the experimental values. The size dependence of EAs of SinF is similar to that of the ionization energies of Sin at n≥5, whereas it is different at n≤4. In the HOMO of SinF (n≤4), the weak antibonding nature between F and Si greatly makes the SinF- anions unstable, resulting in small EA. The vibrationally resolved photoelectron spectra (PES) of SinF- (n=1-3) were measured, and were completely reproduced theoretically.

Transition metal (Sc, Ti, V, and CT)-C-60 binary clusters were produced in the gas phase by a two-laser vaporization method. In the mass spectra of cationic M-n(C-60)(m)(+), for M = SC, Ti, and V, clusters having the composition of (n, m) (1, 1)(+), (1, 2)(+), (2, 3)(+), (3, 4)(+), (4, 4)(+), and (5, 5)(+) were predominantly produced. The peaks having (n, n + 1)+ compositions are explicable in terms of multiple dumbbell structures, in which metal atoms and C-60 are alternately stacked. Clusters with composition (4, 4)(+) and (5, 5)(+) are presumed to be ring structures. For Cr-n(C-60)(m)(+), however, a Cr atom is surrounded by three C-60, forming a tricapped structure, and the structure acts as a unit for larger stable clusters of (2, 4)(+), (3, 4)(+), and (4, 4)(+). Ionization energy measurements for neutral M-n(C-60)(m) and a chemical probe method using CO, O-2, and C(2)H(4 )as reactants gave information concerning the geometrical and electronic structures.

The electronic structures of germanium cluster (Gen; n=4-32) were studied by using photoelectron spectroscopy (PES) for Gen -, GenF-, and GenCl-. The halogen atom doping method enables us to determine the HOMO-LUMO gap of the corresponding neutral Gen cluster to be 0.8-1.0 eV around n=30. This result implies that the Gen cluster itself never emits visible emissions.

Organometallic lanthanide complexes of Ln(n)(C8H8)(m) (Ln = lanthanide metals of Ce, Nd, Eu, Ho, and Yb; C8H8 = 1,3,5,7-cyclooctatetraene) were produced by a combination of laser vaporization and molecular beam methods. The complexes having (n, m) = (n, n + 1) for it 1-5 were prominently produced as magic numbers in the mass spectra. Using mass spectrometry, photoionization spectroscopy, and photoelectron spectroscopy, it is concluded that these magic-numbered complexes form multiple decker sandwich structures in which Ln atoms and C8H8 molecules are alternately piled up. The obtained spectroscopic data show that these sandwich complexes are formed through ionic bonds and are charge transfer complexes, where the Ln atoms are multiply-charged cations of Ln(k+) (k = 2 and 3) and C8H8 molecules are multiply charged anions.

A systematic study of photoelectron spectroscopy for C60Na-N (N=0–33) and C70Na-N (N=0–15) clusters is reported. These clusters are produced by a laser vaporization source and their photoelectron spectrum is measured using a magnetic-bottle time-of-flight spectrometer. For N < 13, the adiabatic electron affinity and the vertical detachment energy display strong regular features with increasing N. A packing of the sodium atoms in stable Na3 trimers is inferred, with each atom lying above a pentagonal ring. For larger clusters (N> 13), a very different trend is observed, signing the appearance of a metallic behavior for the sodium layer at the fullerene surface. The specific case of C70 is also discussed. © 1999 The American Physical Society.

We have recorded photoelectron spectra of Wn- (n=20-90) using a magnetic bottle type photoelectron spectrometer. Each photoelectron spectrum consists of a single, broad peak, shifted successively towards a higher electron binding energy (EBE) with increasing n. We estimated the threshold EBEs from the spectra. The threshold EBE versus n-1/3 plot showed a straight line and was extrapolated to 4.50 eV which is consistent with the W bulk work function, 4.55 eV. We also estimated the bond dissociation energies of Wn- cluster anions using a simple energetic model.

Neutral metal-benzene complexes, M-n(benzene)(m) (M = Sc to Cu), are produced for all of the 3d transition metals in the gas phase by using the laser vaporization method. These species are characterized by mass spectrometry, photoionization spectroscopy, and chemical probe experiments. Depending on the metal, there are two types of structures for M-n(benzene)(m): multiple-decker sandwich structures and metal clusters fully covered with benzene molecules (rice-ball structures). The former sandwich structure is characteristic of the complexes for early transition metals (Sc-V), whereas the latter is formed for late transition metals (Fe-Ni). Electronic structures of M-1(benzene)(x) (x = 1, 2) complexes are investigated through systematic measurements of ionization energies (E-i's).

In gas phase, Na-atom adducts were produced for organometallic lanthanide complexes of Ln(n)(C8H8)(n+1) [Ln = europium (Eu) and holmium (Ho); C8H8 = 1,3,5,7-cyclooctatetraene (COT)] by a combination of two-laser vaporization and a molecular beam method. The number of attached Na atoms can be reasonably explained by charge distributions of the complex, where Eu and Ho atoms exist as Eu2+ and Ho3+, respectively, and C8H8 molecules as COT2-, accepting two excess electrons. The behavior of the Na adducts chemically demonstrates that the organometallic lanthanide complex is formed through ionic bonding implying that it is possible to be isolated as its Na salt. (C) 1999 Elsevier Science B.V. All rights reserved.

Anions of coronene (C24H12) and its clusters are produced by laser ablation of a pressed pellet sample in a double rod, pulsed nozzle source. Coronene anions are produced efficiently only when transition metal vapor is present in the same plasma from co-vaporization of a metal sample. Anion monomers and clusters of coronene are size-selected and studied with photoelectron spectroscopy in a magnetic bottle spectrometer. Spectra at 532, 355 and 266 nm probe electron affinities and neutral electronic states. The electron affinity of coronene is 0.47±0.09 eV and that of coronene dimer is 0.67±0.09 eV.

Organometallic clusters of lanthanide (Ln = Nd, Er, Eu, and Yb) and 1,3,5,7-cyclooctatetraene (C8H8) were produced by a combination of laser vaporization and molecular beam methods. In the mass spectra of [Lnn,(C8H8)m], compositions of m = n + 1 were magic numbers. From mass spectrometry, photo-ionization spectroscopy, and photoelectron spectroscopy, we have concluded that these magic-numbered clusters take multiple-decker sandwich structures in which Ln atoms and C8H8 molecules are alternatedly piled up. These sandwich clusters are formed through ionic bonds composed of multiply charged cations [Lnk+ (k = 2 and 3)] and multiply charged anions [C8Hh-8 (h = 1, 1.5, and 2)].

The 7-azaindole (7-AzI) cluster anions and their H2O adducts were generated by electron attachment to the 7-AzI/7-AzI-H2O clusters, and their electronic properties studied with mass spectrometry and photoelectron spectroscopy. In the mass spectra of the 7-AzI cluster anions, (7-AzI)-n, monomer anions were missing due to negative electron affinity (EA), while clusters (n ≥ 2) have positive EA, and those having even n were more abundant than those at odd n. EA increased with cluster size from 0.88 eV at n = 2, and an even-odd alternation in the EA was seen. The even-odd alternation showed that the 7-AzI dimer acts as a unit for cluster formation. In the photoelectron spectrum of (7-AzI)-2 at 266 nm, the second peak was observed 2.4 eV above the first peak; this is attributed to the triplet state of the neutral (7-AzI)2. Depending on the detachment energy, a movable peak, with the detachment of 420-300 nm (2.9-4.1 eV), was additionally observed between the first and the second peaks; this was thought to be caused by a delayed photoemission via an electronic excited state of the (7-AzI)-2 anion.

Novel lanthanide organometallic clusters, Lnn(C60)m, consisting of lanthanide atoms (Ln; Ln = Eu and Ho) and C60molecules were produced in gas phase by a two-laser vaporization method. The Lnn(C60)mclusters were produced predominantly at compositions of (n, m) = (n, n + 3) [n = 1-4]. These stoichiometries showed no chemisorption reactivity toward CCl4and O2reactants. The cluster having (1,4) composition is proposed to form a tetrahedral structure, in which a Ln atom is surrounded by four C60molecules. In larger clusters with (n, n + 3) composition, each Ln atom is found in a local (1,4) group. Together with the results of sequential chemisorption of Cl atoms on cluster cations, photoelectron spectroscopy of cluster anions leads us to conclude that Ln1(C60)mclusters are charge-transfer complexes expressed as Ln1x+(C60)mx-, and that Eu and Ho atoms exist as Eu2+and Ho3+ions, respectively.

Electronic properties of tin (Sn) and lead (Pb) cluster anions were studied using photoelectron spectroscopy (PES) with a magnetic bottle-type electron spectrometer, where the feasibility of a halogen-doping method we recently developed was explored. The binary cluster anions containing a chlorine atom (Cl) were produced by a laser vaporization of a target rod in a He carrier gas mixed with a small amount of CCl4gas. Comparing the PES of SnnCl-and those of Sn-n, it was found that the doped Cl atom in SnnCl-deprives each Sn-ncluster of the excess electron without any serious rearrangement of the Snnframework, which is similar to our previous work on SinF-and GenF-. The halogen-doping method enables us to reveal the HOMO-LUMO gap of the corresponding neutral cluster of the Snnclusters, showing that the Snnclusters are semiconductor clusters, especially below n=30, similar to the Sinand the Genclusters. For the Pbnclusters, however, the halogen-doping method was inapplicable, which implies that the Pbnclusters are different from the other group-14 clusters, probably exhibiting a metallic nature. © 2000 Elsevier Science B.V. All rights reserved.