Published papers

We present a negative ion photoelectron spectroscopic study on the solvation energetics of an excess electron in the size-selected aromatic hydrocarbon nanoclusters, (benzene)(n)(-) (n=53-124) and (toluene)(n)(-) (n=33-139). The formation and stability of these negatively charged clusters were investigated using mass spectrometry with two different sources: (1) low-energy (approximate to0.3 eV) electron attachment and (2) high-energy electron impact. The results reveal that very large coordination numbers (nsimilar to25) are necessary for the formation of stable benzene and toluene cluster anions. This suggests that the second solvation layer (effect) is essential for stable binding of the excess electron in these clusters. The energetics of the cluster anions were also explored by negative ion photoelectron spectroscopy. The photoelectron spectra obtained were related to bulk parameters, e.g., solvent reorganization energy and conduction band minimum, via an analysis of vertical detachment energies versus n(-1/3). The bulk solvent reorganization energies thus estimated have revealed the excess electron trapping levels in bulk benzene and toluene. The applicability of the generalized cluster size equation is examined in these nondipolar aromatic hydrocarbon systems for the first time. (C) 2002 American Institute of Physics.

Terbium (Tb)-phthalocyanine (pc) sandwich clusters, Tbn(pc)n+1 (n = 1-4), and their Na adducts, TbNak(pc)2 (k = 1-4) were produced by dual laser vaporization of a Tb rod and the molded rod of metal-free or disodium phthalocyanine (H2pc/Na2pc) in gas phase. The minimum number of Na atoms attached to the cationic/neutral/anionic Tb1(pc)2 clusters is 2, 1, and 0, respectively. Together with the ionization energies of the Na adducts of Tb(Na)k(pc)2 (k = 1 and 2), it is revealed that the Tb-pc clusters are formed through fairly ionic bondings, where each component exists as Tb3+, Na+, and pc2-, respectively.

We have studied geometric and electronic structures of metal (M) atom doped silicon (Si) clusters, MSin (M = Ti, Hf, Mo and W), using mass spectrometry, a chemical-probe method and photoelectron spectroscopy. In the mass spectra for all of the mixed cluster anions, MSin-, both MSi15- and MSi16- were abundantly produced compared to neighbors. Together with the result of the adsorption reactivity and photoelectron spectroscopy, it has been revealed that one metal atom can be encapsulated inside a Sin cage at n ≥ 15.

The photoelectron spectra of bimetallic, mass-selected NbnCo- clusters (n = 3-15) measured at 4.66 eV photon energy are reported. The electronic structure changes of the binary clusters are discussed in comparison with the electronic structures of pure Nb-n(-) clusters as a function of size evolution. Spectroscopic parameters like electron affinities (EAs) and vertical detachment energies (VDEs) are determined. The addition of one Co atom to the Nb-n cluster core significantly perturbs the electronic structures of pure Nb-n clusters at small cluster sizes. Electronic state merging can be explained by hybridization of Co 3d and 4s as well as Nb-n 4d-derived valence orbitals. Tentative geometric structure postulations are carried out revealing the influence of the incoming Co atom toward the intact Nb-n cluster core. In strong contrast to pure Nb-n clusters, a highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap closing of NbnCo at n = 6 indicates a bulk-like evolution of electronic structures induced by charge transfer of Co 3d electrons. (C) 2003 Elsevier B.V. All rights reserved.

The coexistence of solvated electrons and solvent valence anions in negatively charged acetonitrile clusters, (CH3CN)-n (n=10-100) was discussed. The distinctly different nature of the anions were found to be revealed by hole-burning-type photoelectron spectroscopy. The analysis showed that the coexistence was due to the closely lying nature of their anionic state at just the number of solvent molecules sufficient to almost to complete the first solvation layer.

The fabrication of ferromagnetic nanocluster rods by magnetic trapping was discussed. The morphology of the produced nanomaterials was analyzed by electron microscopy and the composition by electron energy-loss spectroscopy. It was found that one dimensional structured soft magnets, up to ∼ 150 nm in length were formed consisting of magnetic clusters a few nanometers in diameter. The mechanism and driving behind their growth process and their magnetic evolution was also proposed.

The magnetic trapping of gas phase CocorcCoOshell nanoparticles of approximate to160 and 5-10 nm in diameter showed unique patterns of two-dimensional circular or oval distribution and one-dimensional aggregation, respectively, suggesting that granular nano-wire can be formed when shell to core ratio is less than approximate to0.38 at 0.5 T.

We have studied the electronic structures of several gas phase exohedral lanthanide (Ln)-C-60 clusters, Ln(n)C(60) (Ln=Pr, Ho, Tb, Tm, Eu, and Yb) with n=1-4, by photoionization spectroscopy of the neutrals and photoelectron spectroscopy of their anions. Both of the spectroscopic analyses reveal that most of the Ln atoms preferably take +3 oxidation states, while Eu atoms alone assume +2 oxidation states, and that C-60 accepts up to twelve donated electrons in Ln(n)C(60). An additional photoionization examination of the oxygen atom mixing into the Ln(n)C(60) clusters demonstrated that each oxygen atom reduces two electrons from C-60. This result implies that the number of accepted electrons in C-60 can be varied by a suitable choice of the number of Ln atoms and O atoms. (C) 2004 American Institute of Physics.

A molecular beam of multilayer vanadium-benzene organometallic complexes V n(C 6H 6) m was produced by a laser vaporization synthesis method. The magnetic moments of the complexes were measured by a molecular beam magnetic deflection technique, and were found to increase with the number of vanadium atoms in the cluster, showing that the unpaired electrons, which occupy the nonbonding d σ orbitals localized on the metal atoms, couple ferromagnetically. These sandwich species represent a new class of one-dimensional molecular magnets in which the transition metal atoms are formally zerovalent.

The coexistence of two types of anion sates in nanometer-scale polyacene cluster anions, was analyzed. Nanoclusters of naphthalene and anthracene having single excess electron were synthesized in the gas-phase. The photoelectron spectra of size-selected cluster anions containing 2 to 100 molecules exhibited rigid 'crystal-like' cluster anion in which surrounding neutral molecules reorganize around the charge core. The results show that two anion states appear to correlate to negative polaronic states formed in corresponding crystals.

Negative ion photoelectron spectroscopy was employed to investigate the electronic structure of the acridine molecular anion and its monohydrated anion in the gas phase. Their adiabatic electron affinities were measured to be 0.896±0.010 and 1.18±0.05 eV, and the low-lying electronic excited states in both neutral acridine and in its monohydrate were revealed. The photoelectron spectra clearly exhibit the presence of low-lying singlet and triplet states having a (π, π*) configuration in an uncomplexed acridine molecule. Comparison of the photoelectron spectrum of acridine with that of anthracene shows that photodetachment processes into the excited states of (n, π*) configuration have little intensity, implying a relatively large intramolecular structural relaxation in the (n, π*) states.

Electron-beam-induced surface quasimelting of Co granular nanowires (GNWs) of 6-10 nm in diameter and ~30 nm in length has been studied using a high-resolution electron microscope. From the time evolution of electron microscope images as a function of the irradiation time, it has been revealed that the morphological change in the Co GNW is quite different from that in a Co nanocluster (NC) in terms of structures and phase transitions. For example, the allotropic Β→α transition, inhibited in the Co NC, can proceed in the Co GNW through the lattice softening of fcc (111) facets without dimensional collapse.

The change of the 7-azaindole-water cluster structure upon electronic excitation was determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of five different vibronic bands. A total of 105 emission band intensities were fitted, together with the changes of rotational constants of one isotopomer. These rotational constants have been obtained from a fit to the rovibronic contour of the cluster. The geometry change upon electronic excitation to the π π* state can be described by a strong and asymmetric shortening of the hydrogen bonds and a deformation of both the pyridine and the pyrrole rings of 7-azaindole. The resulting geometry changes are interpreted on the basis of ab initio calculations.

A molecular beam of europium-cyclooctatetraene sandwich nanowires Eu-n(COT)(m) was produced by a laser vaporization synthesis method. The formation mechanism of the nanowires was quantitatively revealed by photoelectron and photoionization spectroscopies of the Eu-COT species, together with supporting theoretical calculations. From these results. it is confirmed that growth processes extending the length of Eu-COT nanowires involve a series of elementary reactions in which efficient charge transfer occur at the terminal reaction sites. In every elementary step, the reaction proceeds between one reactant having low ionization energy and the other reactant having high electron affinity, probably via a "harpoon" mechanism.

The ionization energies of Eu-n(COT)(n) (n = 1-4) were found to decrease asymptotically with the cluster size. The low-spin state X and the high-spin state a were characterized with DFT calculations; their adiabatic energy difference was found to decrease with the cluster size. The calculated ionization energies of the X states reproduced the experimental size dependence. Those of the a states also agreed except for n = 1. These features were explained by the significant variation of the electrostatic potential at ionization sites. The state-specific permanent dipole moments were calculated to clarify the difference in the electronic states. (C) 2005 Elsevier B.V. All rights reserved.

We have measured the photoelectron spectra of the multiple-decker 1:1 sandwich clusters of Eu-n(COT)(n)(-) (n = 1-4; COT = 1,3,5,7-cyclooctatetraene), synthesized in the gas phase, and studied theoretically the bonding scheme, charge distribution, valence orbital energies, and photodetachment energies. We calculated the ground electronic state X- and the first excited electronic state A(-), both of which have strong ionic bonding and a characteristic charge distribution. Moreover, we found that the valence orbital energies of Eu (6s) and COT (L-delta) depend strongly on cluster size and their positions in the clusters. With the calculated vertical detachment energies for these valence orbitals, we assigned the peaks in the experimental photoelectron spectra and analyzed the origin of their interesting behavior by employing simple point charge models. From these analyses, it became clear that the characteristic behavior of the spectra is due to the strong ionic bonding and the charge distribution. In addition, using the point charge models, we estimated the vertical detachment energies of the one-dimensional polymer [Eu(COT)](infinity)(-).

Metal-encapsulated silicon cage clusters are a new class of clusters and are opening up new avenues for silicon-based nanoscale materials. We present experimental evidence for a highly stable cluster corresponding to M@Si16 (M = Sc, Ti, and V). Mass spectrometry and anion photoelectron spectroscopy show that the cluster features an electronically closed TiSi16 neutral core which undergoes a change in the number of valence electrons involving (i) substitution of neighboring metals with Sc and V, or (ii) addition of a halogen atom to the TiSi16 anion, and that VSi16F is predicted to form an ionically bound superatom complex. Copyright © 2005 American Chemical Society.

Stern-Gerlach type magnetic deflection measurements were performed for two types of multiple sandwich clusters: vanadium-benzene Vn(C 6H6)n+1 and terbium-cyclooctatetraene Tb n(C8H8)n+1. Beams of V n(C6H6)n+1 clusters (n = 1-4) showed symmetric broadening induced by the inhomogeneous field, indicating free spin behavior similar to that displayed by isolated paramagnetic atoms. By contrast, beams of Tbn(C8H8)n+1 clusters displayed one-sided deflection, indicating that fast spin relaxation occurs within the clusters. The difference in the magnetic deflection behavior exhibited by these two systems is explained by their electronic structures, specifically the bonding characteristics between metal atoms and ligand molecules.

V(benzene)(2) sandwich cluster cations produced in the gas phase were size-selectively deposited onto a self-assembled monolayer of n-hexadecanethiols (HDT-SAM) chemisorbed on a Au(111) surface as well as onto a bare Au(111) surface. The thermal chemistry of the neutralized clusters on each substrate was studied with temperature programmed desorption (TPD). From the analyses of the threshold in the TPD, the desorption activation energies of the clusters deposited were determined to be 64.4 +/- 12.8 kJ/mol for the Au(111) and 130 +/- 10 kJ/mol for the HDT-SAM. The remarkably large desorption activation energy from the SAM suggests that the deposited clusters are incorporated into the SAM matrix and firmly trapped inside the alkyl chains of the SAM.