Published papers

Cobalt-sodium bimetallic clusters (ConNam, n = 3-48) were produced by a two independent laser-vaporization method. Ionization potentials of the ConNam clusters were measured up to m = 3 using a tunable ultraviolet laser combined with a time-of-flight (TOF) mass spectrometer. In general, the ionization potentials decrease monotonically with the number of sodium atoms, and the ionization potentials of Co nNam+1 decrease by 0.2-0.8 eV compared to those of the corresponding ConNam cluster. However, the amount of IP decrement by the Na doping is relatively large at n ≤ 17 whereas it is small and constant at n ≥ 18. This feature can be explained by a geometric rearrangement at small n, the Na doping induces a large geometric rearrangement of the cluster, but at large n, the geometric change is small. Reactivity of ConNam cluster toward H2 was also measured and the effect of the Na doping was studied. The reactivity also suggests the geometric change induced by the Na doping. Moreover, we examined the anticorrelation between IP and reactivity of the ConNam clusters, and no anticorrelation between them could be revealed. © 1992 American Institute of Physics.

Rotational alignment of the product BaO in the crossed-beam reaction Ba+SO2 → BaO+SO was studied as a function of collision energy. The collision energy was varied from 3.0 to 8.0 kcal/mol by changing the stagnation condition of the supersonic SO2 beam. The rotational alignment was probed by means of the polarized laser-induced fluorescence (PLIF) of the product. The distribution of the rotational vector was isotropic at the collision energy of 3.0 kcal/mol, but anisotropic at a higher collision energy, with the most probable alignment being perpendicular to the relative velocity vector. These results indicate that the reaction proceeds via a long-lived complex at a low collision energy, whereas at a higher collision energy, the lifetime of the complex is short enough to couple the rotational angular momentum to the total angular momentum. © 1992 American Institute of Physics.

Negative cluster ions of sodium oxide, NanO- m (m= 1 and 2) , were generated by laser vaporization of a sodium rod, and their mass distribution was measured. In the mass spectra, a number of intense peaks compared to neighbors were observed, in NanO -1 the "magic numbers" were observed at n=9,21,27,35,41, and 59, and in Na.05, they were observed at n= 11, 17,23, 29, 37, 43, and 61. Compared with the magic numbers predicted by the jellium shell model, two and four sodium atoms are increased for the magic numbers by adding one and two oxygen atom (s) , respectively. Therefore, it is concluded that (1) no valence electrons of oxygen atoms contribute to form the electronic shells of clusters, and that (2) two sodium atoms in the NanO-m clusters are bound by one oxygen atom, forming in Na2O unit as a spectator.

ナトリウムと他の金属元素を合金化させたクラスターを生成させ、その質量分析、イオン化エネルギー、反応性から、電子構造と幾何構造を明らかにした。

ボロン、アルミニウム、ガリウム、インジウムの第３族金属の合成クラスターを生成させ、その１３量体の正十二面体構造の安定性を質量分布の魔法数から考察した。

Indium-sodium (In(n)Na(m)) bimetallic clusters were produced by two independent laser vaporization methods. Ionization potentials (IPs) of the In(n)Na(m) clusters were measured up to m = 2 using a tunable ultraviolet laser combined with a time-of-flight (TOF) mass spectrometer. At small sizes (n = 3-15), the ionization potentials decrease by 0.1-0.8 eV with the addition of Na atom(s), whereas the IPs of larger In(n) (15 less-than-or-equal-to n less-than-or-equal-to 27) clusters do not decrease with Na addition. Moreover, IPS of In7Na1 and In13Na1 clusters are higher than those of In7 and In13, and the IP increments can be explained by electronic shell closings of the 1p (8e) and 2p shell (40 e), where in atoms in the clusters are monovalent and trivalent, respectively. The electronic shell structure was also examined by a magic number in mass distributions of In(n)Na(m)- cluster anions; the In12Na3- cluster can be observed as magic numbers, corresponding to the 2p shell closing. In contrast, no electronic shell structure is observed in pure In, clusters around n = 13. These results indicate that Na atom addition can induce s/p hybridization in the In(n) clusters. We also present mass distributions of aluminum-sodium cluster anions, Al(n)Na(m)-, whose feature can be understood by the electronic shell model.

Sodium doped binary clusters of indium and gold were produced by a laser vaporization method using two independent lasers. Photoionization efficiency spectra of these binary clusters were measured. The ionization potentials thus determined indicate the occurrence of the electron shell structure in these binary clusters.

Sodium doped silicon clusters (SinNam, 3≤n≤11, 1≤m≤4) were produced by two independent laser vaporization methods and their ionization potentials were measured by scanning the wavelength of the UV dye laser. The IPs of most SinNam clusters decrease monotonously with the number of Na atoms, but IPs of Si7Nam and Si10Nam clusters show an apparent even-odd alternation odd numbers of Na atoms efficiently decrease the IP but even numbers of Na atoms never significantly decrease the IPs. In addition, the reactivity of SinNam clusters for NO molecules was investigated with a fast flow reactor, and an anti-correlation between IP and the reactivity was clearly observed clusters having high IP show low reactivity and vice versa. © 1993 Springer-Verlag.

Ionization potentials of gold-sodium (Au(n)Na(m)) bimetallic clusters were measured by a tunable ultraviolet laser combined with a time-of-flight mass spectrometer. The ionization potentials of Au(n)Na(m) clusters generally decrease with the number of Na atoms, but discontinuities in the ionization potentials of Au(n)Na(m) clusters were observed when the total number of valence electrons in the clusters fills an electronic shell. This result indicates that valence electrons of both Au atoms and Na atoms are delocalized in the clusters, forming the electronic shell.

Photoelectron spectra of hexafluorobenzene (C6F6) cluster anions and its Au complex anion were measured in the gas phase at the photon energy of 3.49 eV, using a magnetic bottle electron spectrometer having almost-equal-to 80 meV resolution. The spectra show unresolved broad features: The vertical detachment energies (VDEs) were determined from the maxima of the spectra. The VDE of (C6F6)n- gradually increases by 0.1-0.2 eV with the cluster size (n = 1-8) from 1.56 (3) to 2.54 (6) eV, and the VDE of (Au-C6F6)- anion is 1.99 (5) eV.

Photoelectron spectra of iron-carbonyl cluster anions (Fen(CO)m-) were measured in the gas phase at the photon energy of 4.66 eV, using a magnetic bottle electron spectrometer. The iron-carbonyl cluster anions were produced by slow electron attachment of less than 1 eV to a supersonic expansion of Fe(CO)5 seeded in a He carrier gas. The photoelectron spectra of the Fen(CO)5n-1- anions show unresolved broad features, and the threshold energy (ET) and the vertical detachment energy (VDE) were determined: The ET of Fen(CO)5n-1- gradually increases by 0.1-0.3 eV with the cluster size. The study suggests that the iron carbonyl cluster anions produced are van der Waals clusters, in which Fe(CO)4- works as an ion core.

Two electronic transitions are observed for a new heteronuclear dimer, AlNa, using two-color two-photon ionization spectrocopy (2R2PI). The combination of the analysis of vibronic structure and ab initio calculation enables us to assign the two transitions as D 1Π←X 1Σ+ and C 1Σ+←X 1Σ+; the origins of the D and C states are located at 16427 and 15422 cm-1, and the vibrational frequencies being 151 cm-1 in the D state, 115 cm-1 in the C state, and 186 cm-1 in the X state, respectively.

The geometries, ionization energies and adsorption energies of silicon-sodium binary clusters (Si(n)Na, n = 1-7 ) were studied with ab initio MO calculations. All the calculations were carried out at the restricted Hartree-Fock level of approximation for both closed and open shell systems. The structure of the most stable isomer of the Si(n)Na clusters keeps the frame of the corresponding Si cluster unchanged. The electronic structure of Si(n)Na is similar to that of the corresponding negative ions Si(n)-. The ionization energies of Si(n)Na were evaluated with the DELTASCF method, and they become smaller than that of the corresponding Si(n) as was reported experimentally.

Aluminum-cesium bimetallic clusters (AlnCsm) were produced by the combination of laser vaporization and a high-temperature pulsed nozzle. To investigate their electronic structures, ionization energies (E(i)s) of the AlnCsm clusters were measured using a tunable ultraviolet laser combined with a time-of-flight mass spectrometer. In general, their Ep decrease by 0.4-1.0 eV with increasing number of cesium atoms, but discontinuous changes in E(i)s can be also observed, which are attributed to electronic shell structures; completeness of electron shell by the addition of a Cs atom induces high E(i), and an excess electron from the Cs atom results in large E(i) decrement. The electronic stability of the AlnCsm clusters can be also examined by their abundance.

Photoelectron spectra of A1S(-) radical were measured using a magnetic bottle electron spectrometer. Electron affinity of AIS was determined to be 2.60(3) eV, and a new electronic excited state, A'(2) Pi, was found 0.4 eV above the A1S ground state, which is consistent with theoretical calculations.

Photoelectron spectra of Sin,Cm- cluster anions (1≤n≤7 and 1≤m≤5) were measured at the photon energies of 3.49 eV, by using a magnetic bottle electron spectrometer. The Si nCm- clusters were produced either by a laser vaporization of a silicon-carbon mixed rod or by two laser vaporizations of carbon and silicon rods in a He carrier gas. The spectra of the Si nC1- (3≤n≤7) clusters are similar to those of pure Sin+1- clusters in the peak positions and their envelopes, which is attributed to the isovalent electronic structure of Si and C atoms as well as to the similar geometrical structure. In contrast, the similarity in the photoelectron spectra is not observed between Cm+1 and Si1Cm (2≤m≤5) clusters, which is attributed to a change in their geometry from chain to ring. These experimental conclusions are discussed with the results of our theoretical calculations. © 1995 American Institute of Physics.

Cobalt-benzene clusters, Con(benzene)m, were synthesized by the reaction of laser-vaporized Co atoms with benzene vapor. The clusters exhibit magic-number behavior not only at m = n + 1 (n = 1-3) but also at (n-m) = (3-3), (4-4), (5-4), (6-4), (7-4), (8-5), and (9-6). Observed magic-number behavior and ionization energies are consistently explained by assuming the structure of the clusters to be multiple-decker sandwiches for n-m (m = n + 1, n = 1 - 3) and to be an n atom metal cluster coated by m benzene molecules for n-m (n ≥ 4). In this paper, we will present the properties of Con(benzene)m clusters from the viewpoints of mass distribution, reactivity, and ionization energies. The structure and formation mechanism of the Con(benzene)m clusters will also be discussed. © 1995 American Chemical Society.

Metal-molecule clusters Vn(benzene)m were synthesized by the reaction of laser-vaporized vanadium atoms with benzene vapor. The clusters exhibit magic number behavior at m = n - 1, n, and n + 1 (n = 1-5), which is explicable in terms of the multiple decker sandwich structure for these stable clusters. The ionization energies of these sandwich clusters decrease significantly with increasing cluster size. © 1995 American Chemical Society.

The ionization energies (E(i)) for bimetallic clusters of cobalt and vanadium (ConVm) were determined by photoionization spectroscopy. A comparison of E(i) values to the hydrogen chemisorption reactivity reveals no correlation, indicating the importance of inhomogeneity of bimetallic clusters.