Published papers

Photoelectron spectra of AlnSm- cluster anions (0 less than or equal to n, m less than or equal to 10) were measured at photon energies of 3.49 and 4.66 eV, by using a magnetic bottle electron spectrometer. The AlnSm- clusters were produced by laser vaporization of an aluminum-sulfur mixed rod in He carrier gas. The abundant AlnSm- clusters are three types of AlS-(Al2S3)(k), AlS2-(Al2S3)(k), and (Al2S3)(k)(-), where an unit of Al2S3 aggregates with ion cores of AlS-, AlS2-, and Al2S3-, respectively. Al3S3- is exceptionally stable with its geometrical structure.

Iron-sulfur cluster anions (FenSm -, n=1-6, m=1-6) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle-type time-of-flight electron spectrometer. The FenSm -. cluster anions were formed in a laser vaporization cluster source. It was found that the stable cluster ions are the ones with compositions of n=m and n=m±1. The electron affinities were measured from the onsets of the PES spectra. Three low-lying electronic states were observed for FeS. The PES spectra of FenS- (n=1-4) series show a unique similarity, indicating that the Fe atom addition to FeS- has little effect on the electronic property of FeS. The PES spectra of FenS2 - series show a similarity among the cluster anions with n=2-4, showing that Fe2S2 - is the structural framework of these clusters. For FenS3 - series, Fe3S3 - is proposed to be the structural framework. The electronic properties and geometrical structures of the clusters are discussed.

Electronic properties of silicon-fluorine cluster anions (SinFm-; n=1-11, HZ = 1-3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The binary cluster anions were generated by a laser vaporization of a silicon rod in an He carrier gas mixed with a small amount of SiF4 or F2 gas. The highly abundant clusters are SiFm- (m=3 and 5) and SinF- (n=6, 7, and 10) in their mass spectra. In the photoelectron spectra of SiFm- (m = 1-5), the clusters having odd m have higher electron affinity (EA) than those having even in, indicating that the even/odd alternation in EA is attributed to their electronic structures of a closed/open valence shell. Comparison between photoelectron spectra of SinF- and Sin- (n=4-11) gives the insight that the doped F atom can remove one electron from the corresponding Sin- cluster without any serious rearrangement of Sin- framework, because only the first peak of Sin-, corresponding singly occupied molecular orbital (SOMO), disappears and other successive spectral features are unchanged with the F atom doping. In some clusters, furthermore, the vibrational structures could be resolved to determine a vibrational frequency and to presume the geometry with ab initio molecular orbital calculations.

Photoelectron spectra of CnSim- cluster anions (3≤ n, m ≤6) were measured at the photon energy of 3.49 eV, by using a magnetic-bottle electron spectrometer having ∼60-meV resolution. The CnSim- clusters were produced by laser vaporization of a carbon-silicon mixture rod in a He carrier gas. The spectra of the C1Sim-1- clusters are similar to those of pure Si-m clusters in the peak positions and their envelopes, which is attributed to the similarity in electronic structure of Si and C atoms, leading to a similar geometry. In contrast, the similarity in the photoelectron spectra is not observed between C-n and Cn-1Si1- clusters, which is attributed to a change in their geometry; from chain to ring.

Photoelectron spectra of MnS-1 (M = Al, Cu, Ag, and Au) 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. The electron affinities of AlS-, CuS-, AgS-, and AuS- are 2.60(3), 2.10(3), 2.09(3), and 2.44(3) eV, respectively. The large EA of AlS- is attributed to large stability by six bonding electrons, and a new electronic state of A′2Π can be found 0.4 eV above the AlS ground state, which is in agreement with a theoretical calculation. In the photoelectron spectrum of AuS-, two peaks due to a splitting by the L-S coupling could be observed, and the amount of the splitting is 0.18(2) eV. The results of aluminum clusters containing sulfur atom will also be presented.

Photoelectron spectra of AlnSm- and SinNa-m were measured using magnetic-bottle-type photoelectron spectrometer. Sulfur atoms were found to perturb the structural factor of Al clusters and several magic number clusters such as AlS- and Al3S3- were newly observed. Sodium atoms perturb the electronic factor of Si clusters with small disturbance of the geometries of the genuine clusters.

The geometries and energies of silicon-sodium binary clusters (SinNa, n = 2-7) were investigated with the inclusion of electron-correlation effects using the second-order Møller-Plesset perturbation theory (MP2) with the 6-31G basis set. In addition, the fourth-order (MP4) energies were evaluated to determine the ground-state isomers. Si4Na cluster was calculated with the polarized 6-31G* basis set. For all of the SinNa clusters, the most stable isomers have bridge-site type structures. Apart from the bridge-site type Si6Na, the structure of the most stable isomer of SinNa keeps the frame of the corresponding Sin cluster unchanged, and the electronic structure of SinNa is similar to that of the corresponding negative ion Si- n. The silicon framework of the bridge-site type of Si6Na is distorted from the compressed octahedron to the face-capped trigonal bipyramid.

Metal-molecule clusters, Vn(benzene)m, Con(benzene)m, and their ions, were synthesized by the reaction of laser-vaporized vanadium atoms with benzene vapor. Both neutral and cationic Vn(benzene)m clusters exhibit magic number behavior at m = n + 1 (n = 1-5), which was explicable in terms of multiple-decker sandwich structure.

シリコンに他元素を混ぜた２成分クラスターを生成させ、その負イオンの光電子分光によって電子状態変化を観測し、異原子ドープの影響を論じた。

気相中のレーザー蒸発によって生成させた有機金属クラスターが遷移金属の種類によって多様な形態をとることを総合的に論じた。

シリコンに炭素またはナトリウムをドープして得られる２成分クラスターの構造を理論計算によって明らかにし、実験値の対比から異原子効果を論じた。

Binary clusters containing silicon atoms such as SinNam, SinCm and SinF were produced in a molecular beam using laser vaporization. The reactivity of SinNam toward NO adsorption was found to have an anti-correlation with the ionization energy of SinNam. The results of ab-initio calculation indicate that on doping with a sodium atom, the electron charge migrates from the sodium atom to the silicon cluster without disturbing seriously the original framework of the silicon cluster. Photoelectron spectroscopy of silicon-sodium cluster anions was conducted using a magnetic bottle type photoelectron spectrometer. The determined electron affinity of SinNa was also found to have clear parallelism with the ionization energy of Si-n clusters. This also confirms that the parentage of the singly occupied molecular orbital (SOMO) of SinNa is the lowest unoccupied molecular orbital (LUMO) of Si-n. In the case of SinF, the electron migrates from the silicon cluster to the fluorine atom, which is reflected clearly in the photoelectron spectra of SinF- clusters. The similarity of the photoelectron spectra of Sin-1C- and Si-n(-) was attributed to the similarity of their geometric structure (3 less than or equal to n less than or equal to 6). In contrast to this, Cn-1Si- and C-n(-) exhibit different photoelectron spectra, indicating the structural difference.

Electronic properties of silicon-fluorine and germanium-fluorine cluster anions (SinFm-; n = 1-9, m = 1-3, GenFm-; n = 1-9, m = 1-3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The binary cluster anions were generated by a laser vaporization of a silicon/germanium rod in an He carrier gas mixed with a small amount of SiF4 or F-2 gas. Comparison between photoelectron spectra of SinF-/GenF- and Si-n(-)/Ge-n(-) (n = 4-9) gives the insight that the doped F atom can remove one electron from the corresponding Si-n(-)/Ge-n(-) cluster without any serious rearrangement of Si-n/Ge-n framework, because only the first peak of Si-n(-)/Ge-n(-), corresponding singly occupied molecular orbital (SOMO), disappears and other successive spectral features are unchanged with the F atom doping.

Metal (M)-sulfur cluster anions (M = Ag, Fe and Mn) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle type time-of-flight electron spectrometer. The MnS-m cluster anions were formed in a laser vaporization cluster source. For Ag-S, the largest coordination number of Ag atoms (nmax) is generally expressed as nmax=2m-1 in each series of the number of S atoms (m). For Fe-S and Mn-S, it was found that the stable cluster ions are the ones with compositions of n = m and n = m±1. Their electron affinities were measured from the onset of the PES spectrum. For Ag-S, the EAs of Ag1Sm are small and around 1 eV, whereas those of AgnSm (n ≥ 2) become large above 2 eV. The features in the mass distribution and PES suggest that Ag2S unit is preferentially formed with increasing the number of Ag atoms. For Fe-S and Mn-S, the PES spectra of FenS-m/MnnS-m show a unique similarity at n ≥ m, indicating that the Fe/Mn atom addition to FenS-n/MnnS-n has little effect on the electronic property of FenSn/MnnSn. The PES spectra imply that the FenSn cluster is the structural framework of these clusters, as similarly as the determined structure of the FenSn cluster in nitrogenase enzyme.

In the gas phase, we have successfully synthesized organometallic clusters, Mn(benzene)m (M = 3d transition metal atoms), by using a laser vaporization method. The measurements of mass spectra and ionization energies (Ei) have revealed that the organometallic clusters can take two types of structures; layered sandwich structures (m = n + 1) and metal clusters saturatedly covered with benzenes. For early transition metals of Sc, Ti, and V, only the multiple decker sandwich structure clusters were preferentially produced, in which benzene and metal atoms are alternately piled up. For late transition metals of Co and Ni, the metal clusters saturatedly surrounded by benzenes were also produced as well as the sandwich clusters. Furthermore, the Eis of M1(benzene)2 (M = Sc-Ni) were systematically measured and their electronic properties will be discussed.

The geometric structures of 7-azaindole-water complexes, (7-AzI)-(H2O)n (n = 1-3), and 7-AzI dimer were investigated by laser-induced fluorescence (LIF) spectroscopy with high resolution (∼0.01 cm-1). For the 7-AzI-(H2O)n complexes (n = 1-3), the LIF spectra show partially resolved rotational structure, which has been analyzed in combination with theoretical calculations. This analysis yields the rotational constants and characterizes the structures. In 7-AzI-(H2O)1, the H2O molecule is located in the molecular plane, forming a six-membered ring with two hydrogen bonds. Analyses for 7-AzI-(H2O)2 and -(H2O)3 show, furthermore, that both the second and the third H2O are located in the molecular plane of 7-AzI, forming a network ring of hydrogen bonds. For the 7-AzI dimer, the LIF spectrum shows an unresolved rotational envelope. The rotational contour has been analyzed, and we discuss the dimer's likely structure.

Electronic properties of germanium-fluorine cluster anions (GenFm-; n = 1-11, m = 1-3) were studied using photo-electron spectroscopy (PES) with a magnetic-bottle type electron spectrometer. Compared between the PES of GenF- and those of the Ge-n(-), it was found that the doped F atom in GenF- deprives each Ge-n(-) cluster of the excess electron without any serious rearrangement of the Ge-n framework. The F doping method enables us to estimate the HOMO-LUMO gap of the corresponding neutral Ge-n clusters (n = 4-11). For a diatomic GeF- cluster, furthermore, the vibrational structures could be resolved to determine its vibrational frequency.

Vanadium-arene complex anions, V1(arene)-m (m =1,2; arene = benzene, fluorobenzene, and toluene) were synthesized by the gas phase reaction of laser vaporized metal atoms with aromatic compound vapors. According to the 18-electron rule, it was anticipated that bis(benzene)vanadium should be stabilized by an electron attachment. However, it was experimentally concluded that bis(benzene)vanadium has negative electron affinity. The photoelectron spectra for the observed complex anions were measured, and the electronic configurations of metal-arene complexes and their structures are discussed.

Vanadium-arene complexes, Vn(arene)m (arene = benzene (C6H6), toluene (C6H5CH3), and fluorobenzene (C6H5F)), were synthesized by the reaction of laser-vaporized vanadium atoms with arene vapor. Both the analysis of the mass spectra and the measurements of their ionization energies indicate that both Vn(C6H5-CH3)m and Vn(C6H5F)m, clusters take a multiple-decker sandwich structure similar to the structure of Vn-(C6H6)m clusters. The relative reactivity between benzene and toluene toward vanadium was also determined; toluene is 4 times as reactive as benzene for V1(C6H5CH3)1/V 1(C6H6)1 production and 2 times for V1(C6H5-CH3)2/V 1(C6H6)2 production because of reactivity enhancement of the electron-donating ring substituent. In the case of the Vn(C6H5F)m cluster, it is found that the electron-withdrawing ring substituent of fluorobenzene results in much less reactivity than is observed for benzene and toluene.

Transition-metal benzene clusters, Mn(benzene)m (M = Ti, V, and Cr), were synthesized by the reaction of laser-vaporized metal atoms with benzene vapor. All the clusters exhibit magic number behavior at m = n + 1, which is rationalized by the structure of a multiple-decker sandwich, but V atoms can efficiently take the sandwich structure (up to n = 5) in particular. This metal specificity of the V atoms and their growth mechanism were examined by quantum chemical calculations, the full valence configurational interaction (FVCI) method with configuration-averaged SCF Orbitals. The calculation results imply that (1) total spin conservation in growth process plays an important role and (2) the production in the sandwich clusters particularly favors a process through lower spin states. The combination between experimental and theoretical investigations leads us to a better comprehension of both the bonding scheme in the sandwich clusters and the growth mechanism, and accordingly, a more efficient production method is proposed generally for the transition-metal sandwich complexes.