Published papers

Aluminum-sodium bimetallic clusters (AlnNam, n = 2-26) were produced by two independent laser vaporization methods. To investigate the electronic structures, ionization potentials of the Al nNam clusters were measured up to m = 3 using a tunable ultraviolet laser combined with time-of-flight (TOF) mass spectrometer. In general, the ionization potentials monotonically decrease with the number of sodium atoms, and the ionization potentials of AlnNa1 generally decrease by 0.2-0.6 eV compared to those of the corresponding Al n cluster. In contrast, the ionization potentials of Al 13Na1 and Al23Na1 are higher than (or equal to) those of Al13 and Al23. The anomalous change in the ionization potentials of Al13Na1 and Al 23Na1 can be explained by the electronic shell model: By the addition of one sodium atom to the bare aluminum clusters, the total number of their valence electrons strictly satisfies the shell closing of 2p and 3s shells, respectively, and therefore, the clusters can be stabilized electronically. © 1991 American Institute of Physics.

Aluminum-boron negative cluster ions (AlnBm-, n = 5-30, m = 1-3) were generated by two independent laser-vaporization sources. The mass distribution of AlnB1- exhibits one peak at n=12 and one step between n = 22 and 23 as magic numbers. whereas that of AlnB2- exhibits only one step between n = 21 and 22. The loss of the magic number of Al11B2- can be explained by the geometric structures: Al12B1- is stabilized by both an electronic shell closing of the 2p shell and a close-packing structure of icosahedron. In contrast, Al11B2- can satisfy the electronic shell closing but cannot take the stable geometric structure because the diameters of Al and B atoms differ largely. The structure of Al12B1- can be an icosahedron with a B atom located at the center.

シリコンに炭素またはナトリウムをドープしたクラスターを生成させ、その光電子スペクトル測定から、構造と電子状態を考察した。

Photoelectron spectra of AlnS1(1≤n≤9) cluster anions were measured at the photon energies of 3.49 and 4.66 eV, using a magnetic bottle electron spectrometer having ∼60 meV resolution. AlS-has an electron affinity of as large as 2.6 eV, attributed to large stability by six bonding electrons. A new electronic state, A′2Π, was found 0.4 eV above the AlS ground state, which is in agreement with theoretical calculations. AlnS1-cluster anions are produced efficiently up to n=5, but the production of larger cluster sizes is found difficult. The mass distribution of AlnS1-cluster anions and electron binding energy can be explained by a geometric structure obtained by theoretical calculation. © 1995 American Institute of Physics.

The geometries and energies of SinC and SinC- (n=2-5) were investigated with ab initio MO calculations including electron correlation. The Moller-Plesset perturbation theory was used for n=3-5. The configuration interaction (CI) calculations were carried out for n=2 and 3. The most stable isomers of the neutral SinC clusters have the structure in which an Si atom having the largest sp hybridization in the corresponding Si-n+1 cluster is replaced by a C atom. For their anions, the structures in which an excess electron is distributed avoiding a C atom become stable. Moreover, the adiabatic electron affinities of SinC and vertical detachment energies of anionic SinC- were computed. They reproduced the trends of photoelectron spectra of SinC- reported previously. (C) 1996 American Institute of Physics.

The geometries and energies of SiCn and SiCn- (n=2-5) were investigated with ab initio calculations including electron correlation effects with the MP2/6-31G* method, followed by MP4 and CCSD(T) single-point calculations to determine the most stable isomers. The adiabatic electron affinities (AEA) were evaluated with the Delta MP4 method. Because of the complexity of the electronic structure of SiC2, its AEA was calculated at the CCSD(T)/aug cc-pVTZ//CCSD(T)/aug cc-pVDZ level of approximation. For the neutral SiCn clusters, the isomer having a triplet ground state has large EA, whereas the isomer having a singlet ground state has small EA. This is attributed to the bonding character between Si and C atoms in the orbital occupied by the extra electron. The calculated EA was used to assign the photoelectron spectra of SiCn- reported previously. Furthermore, the effects of Renner-Teller splitting for the linear isomers of anions are discussed. (C) 1997 American Institute of Physics. [S0021-9606(97)02247-2].

Negatively charged benzene clusters, (benzene)(n)(-), were produced by injecting low- and high-energy electrons into an intense supersonic jet expansion. Threshold size of n=53 was observed by slow-electron attachment, while the smaller (benzene)(n)(-) with 2 less than or equal ton less than or equal to 52 were also observed through the fragmentation of larger (benzene)(n)(-) by high-energy electron attachment. Photoelectron spectroscopy for (benzene)(n)(-) with n=53-124 has revealed a bulklike electron solvated state in (benzene)(n greater than or equal to 53)(-) through the vertical detachment energies (VDEs) versus n(-1/3) relationship. (C) 2001 American Institute of Physics.

The anion photoelectron (PE) spectra of small mass-selected vanadium oxide clusters VnOm- (n = 4-15; m = 0-2) are measured at a fixed photon energy of 4.66 eV with the aid of a magnetic bottle photoelectron spectrometer. Cluster anions are generated in a pulsed laser vaporization cluster source. The electronic structure of VnOm- clusters is investigated as a function of size n and composition m with special regard to the increasing oxidation state. The addition of one or two oxygen atoms to the vanadium cluster core induces a change of the electronic structure in the near-threshold binding energy region below 2 eV. Main spectral features are contributed from the transition metal d-derived orbitals, whereas the oxygen 2p contribution induces a hybridization between vanadium and oxygen frontier orbitals in the entire series of the investigated clusters n = 4-15. Generally, electron affinities and vertical detachment energies increase with increasing cluster size revealing size-dependent discontinuities. Furthermore, relative dissociation energies for different oxygen-loss channels from the parent VnOm- clusters are determined from thermochemical cycles, and first insights into the stability patterns of neutral and negatively charged vanadium oxide clusters are provided. (C) 2002 American Institute of Physics.

The electronic structures of gas-phase exohedral holmium (Ho)-C-60 clusters, HonC60 with n=1-5, were studied by photoionization spectroscopy of the neutrals and photoelectron spectroscopy of their anions. Both spectroscopic analyses reveal that each Ho atom donates its three valence electrons to the unoccupied molecular orbitals of C-60. The electron filling through the charge transfer allows us to observe the unoccupied levels of C-60 due to high electron acceptability. (C) 2002 American Institute of Physics.

The electronic properties of silicon clusters containing a transition or lanthanide metal atom from group 3, 4, or 5, MSin, (M=Sc, Ti, V, Y, Zr, Nb, Lu, Tb, Ho, Hf, and Ta) were investigated by anion photoelectron spectroscopy at 213 nm. In the case of the group 3 elements Sc, Y, Lu, Tb, and Ho, the threshold energy of electron detachment exhibits local maxima at n=10 and 16, while in case of the group 4 elements Ti, Zr, and Hf, the threshold energy exhibits a local minimum at n=16, associated with the presence of a small bump in the spectrum. These electronic characteristics of MSin are closely related to a cooperative effect between their geometric and electronic structures, which is discussed, together with the results of experiments that probe their geometric stability via their reactivity to H2O adsorption, and with theoretical calculations.

Pyrene molecular anion and its homogeneous clusters, (pyrene)(n)(-) (n = 2-20), have been studied by anion photoelectron spectroscopy. The adiabatic electron affinity (EA(a)) of pyrene was directly determined to be 0.406 +/- 0.010 eV. This EA(a) value agrees with that calculated using the method of density functional theory, showing that the excess electron occupies the lowest unoccupied molecular orbital of pyrene molecule. Photoelectron spectra of its cluster anions suggest that the excess electron in the pyrene dimer anion is delocalized over the two pyrene moieties, whereas in larger clusters the excess electron is predominantly localized on a single pyrene molecule. (C) 2004 Elsevier B.V. All rights reserved.

Laser-induced fluorescence from nascent N2 +(X 2Σg +) ions produced by electron impact on a N2 supersonic beam was observed. An analysis of the B 2Σu +-X 2Σ g + (0,0) band shows that the rotational state distributions cannot be represented by a single Boltzmann function, higher N″ levels being overpopulated. Experimental and analytical efforts were made to minimize the influence of cascading and relaxation on the rotational distributions. The rotational energy of N2 + (X) thus estimated increases with decreasing electron energy from 2.26±0.16 meV at 300 eV to 4.24±0.27 meV at 25 eV. This trend is explained qualitatively in terms of angular momentum transfer through multipole electron-molecule interactions. © 1987 American Institute of Physics.

The D1-D0 electronic spectrum of jet-cooled cyanocyclopentadienyl (C5H4CN) radical, produced by excimer laser photolysis of 1,3-cyclopentadiene-1-carbonitrile (C5H5CN), was measured by laser-induced fluorescence (LIF). The LIF spectrum was identical to that of radicals produced by photolysis of phenyl isocyanate and o-chloroaniline, which had been assigned to phenyl nitrene. This result shows that the nascent radical product in the photolysis of o-chloroaniline and phenyl isocyanate is isomerized to C5H5CN and the final product of C5H4CN radical is derived via the second photolysis. The rotational contour of the 0-0 band of the C54CN radical, at 27143 cm-1 (368.3 nm), is also presented. The electronic transition is A-type and π electron transition, and the symmetry of both the ground and excited electronic state is 2B1 (of the C2v point group).

The photodissociation dynamics of A approximately state ammonia molecules has been further investigated for deuterium-substituted derivatives NH2D, NHD2, and ND3 measuring the homogeneous line widths on action spectra. The line widths of the 20(1) vibronic band were determined to be 19, 12, and 1.3 cm-1 fwhm for NH2D, NHD2, and ND3, respectively. In consideration of our previously reported value of 26 cm-1 for NH3, the spectral line width decrease in proportion to the number of D atoms. This result shows that the dissociation rate in the A approximately state is additive as for the number of H atoms, and it is explained by the fact that the zero-point energies on the potential surface of A approximately state are equally shifted among deuterium-substituted ammonias.

The mass distributions of aluminum negative cluster ions and their binary cluster ions mixed with a carbon atom, generated by laser vaporization, were measured. In the range from n=5 to 70, the mass distribution of pure aluminum negative cluster ions, Al-n, exhibits one peak at n = 13 and one step between n=23 and 24. The mass distribution of the series of AlnC-, however, exhibits one peak at n=7 and one step between n=24 and 25. The absence of a peak in the series of AlnC- around n=13 can be explained by the electronic structure in relation to the electronic shell model, and both the peak and the step in the mass distribution of AlnC- clusters can be explained by their geometric structure.

Reactivity of positively charged cobalt cluster ions (Con+, n=2-22), produce by laser vaporization, with various gas samples (CH4, N2, H2, C2H4, and C2H2) were systematically investigated by using a fast-flow reactor. The reactivity of Con+ with the various gas samples is qualitatively consistent with the adsorption rate of the gas to cobalt metal surfaces. Con+ highly reacts with C2H2 as characterized by the adsorption rate to metal surfaces, and it indicates no size dependence. In contrast, the reactions of Con+ with the other gas samples indicate a similar cluster size dependence; at n=4, 5, and 10-15, Con+ highly reacts. The difference can be explained by the amount of the activation energy for chemisorption reaction. Compared with neutral cobalt clusters, the size dependence is almost similar except for Co4+ and Co5+. The reactivity enhancement of Co4+ and Co5+ indicates that the cobalt cluster ions are presumed to have an active site for chemisorption at n=4 and 5, induced by the influence of positive charge.

Bimetallic clusters containing manganese atoms were generated by applying a laser-vaporization method. The bimetallic clusters composed of Ta and Mn, TanMnm, were produced and could be mixed in any ratio, whereas those composed of Co and Mn, ConMnm, could only be produced with more than four cobalt atoms; Co4 plays an important role in the production of ConMnm clusters. The difference of mass distribution between TanMnm and ConMnm can be qualitatively explained by an analogy of their phase diagrams in the bulk.

The reaction of bimetallic ConVm+ cluster ions, generated by laser vaporization, toward hydrogen was investigated by using a fast-flow reactor. Compared with pure Con+ cluster ions, the reactivity of Con-1V+ cluster ions exhibits a drastic decrease at n ≥ 13 similar to that of neutral ConVm clusters. The result can be explained in terms of their geometrical structure; at n ≥ 13, the V atom is located at the center of the cluster and is surrounded by Co atoms. © 1991 American Chemical Society.

S1-S0 electronic spectra of jet-cooled p-dicyanobenzene (p-DCNB), its H2O complex, and the dimer ((p-DCNB)2) were recorded using laser-induced fluorescence and two-color multiphoton ionization methods. Several vibrational bands in S1 of the monomer were assigned to the corresponding vibrational modes in the ground S0 state. Geometric structures of p-DCNB-H2O and (p-DCNB)2 were determined by the analyses of rotational envelopes of the vibronic bands of these complexes. © 1992 American Chemical Society.