Tamotsu Kondow

Reactive scattering of clusters and cluster ions from solid surfaces

Specific chemical reactions take place in a cluster when it impinges on a solid surface. These intracluster processes ranging from vibrational excitation to atomic rearrangements are called ‘cluster-impact’ processes, the features of which change specifically with the collision energy and the cluster size. The specificity of the cluster-impact processes arises from impulsive energy transmission to specific modes of the cluster followed by rapid energy redistribution among other degrees of freedom, including those of the surface. In this review, citing several representative collision systems (cluster + surface), we explain the features of a cluster-impact process by dividing the collision energy into several energy ranges, in each of which a characteristic feature is manifested; high vibrational excitation of fullerenes in the lowest energy range, mechanical bond splitting of I − and a four-centre reaction between N2 and O2 in a higher energy range, etc.

Unisized two-dimensional platinum clusters on silicon(111)-7×7 surface observed with scanning tunneling microscope

Uni-sized platinum clusters (size range of 5-40) on a silicon(111)-7 x 7 surface were prepared by depositing size-selected platinum cluster ions on the silicon surface at the collision energy of 1.5 eV per atom at room temperature. The surface thus prepared was observed by means of a scanning tunneling microscope (STM) at the temperature of 77 K under an ambient pressure less than 5 x 10(-9) Pa. The STM images observed at different cluster sizes revealed that (1) the clusters are flattened and stuck to the surface with a chemical-bond akin to platinum silicide, (2) every platinum atom occupies preferentially the most reactive sites distributed within a diameter of approximately 2 nm on the silicon surface at a cluster size up to 20, and above this size, the diameter of the cluster increases with the size, and (3) the sticking probability of an incoming cluster ion on the surface increases with the cluster size and reaches nearly unity at a size larger than 20.

Formation of small gold clusters in solution by laser excitation of interband transition

Gold nanoparticles with ∼10 nm in average diameter were prepared by laser ablation of a gold metal plate in an aqueous solution of sodium dodecyl sulfate (SDS) and were fragmented by excitation of an interband transition of gold nanoparticles under irradiation of an intense 355-nm pulsed laser. Fragmentation dynamics was investigated by comparing the fragmentation by excitation of a surface plasmon band of gold nanoparticles by a 532-nm laser. It is found that gold nanoparticles with 1.5-nm average diameter are produced together with small gold clusters by properly optimizing the surfactant concentration.

Electronic structures of size-selected single-layered platinum clusters on silicon(111)-7×7 surface at a single cluster level by tunneling spectroscopy

Tunneling spectra of size-selected single-layered platinum clusters (size range of 5-40) deposited on a silicon(111)-7x7 surface were measured individually at a temperature of 77 K by means of a scanning tunneling microscope (STM), and the local electronic densities of states of individual clusters were derived from their tunneling spectra measured by placing an STM tip on the clusters. In a bias-voltage (V(s)) range from -3 to 3 V, each tunneling spectrum exhibits several peaks assignable to electronic states associated with 5d states of a constituent platinum atom and an energy gap of 0.1-0.6 eV in the vicinity of V(s)=0. Even when platinum cluster ions having the same size were deposited on the silicon(111)-7x7 surface, the tunneling spectra and the energy gaps of the deposited clusters are not all the same but can be classified in shape into several different groups; this finding is consistent with the observation of the geometrical structures of platinum clusters on the silicon(111)-7x7 surface. The mean energy gap of approximately 0.4 eV drops to approximately 0.25 eV at the size of 20 and then decreases gradually as the size increases, consistent with our previous finding that the cluster diameter remains unchanged, but the number density of Pt atoms increases below the size of 20 while the diameter increases, but the density does not change above it. It is concluded that the mean energy gap tends to decrease gradually with the mean cluster diameter. The dependence of the mean energy gap on the mean Pt-Pt distance shows that the mean energy gap decreases sharply when the mean Pt-Pt distance exceeds that of a platinum metal (0.28 nm).

Photon-trap spectroscopy applied to molecules adsorbed on a solid surface: probing with a standing wave versus a propagating wave

We apply photon-trap spectroscopy, a generalized scheme of cavity ringdown spectroscopy, to infrared spectroscopy of molecular adsorbates on a solid substrate. The storage lifetime of light in a high-finesse Fabry-Perot cavity provides a high absorbance sensitivity for the substrate sample, which is placed exactly normal to the light beam in the cavity to minimize optical losses. Infrared spectra of the C-H stretching vibration of alkylsiloxane monolayer films on a silicon substrate are measured in three ways, namely by employing pulsed and continuous-wave lasers as well as by conventional Fourier transform infrared spectroscopy. The magnitude of optical absorption is shown to vary by the character of the interacting light used in the measurement, i.e., a standing wave versus a propagating wave.

Electronic and geometric structures of Co2Cn− and V2Cn−: Initial growth mechanisms of late and early 3d transition-metal carbide clusters

Photoelectron spectra of Co2Cn− (n=2, 3) and V2Cn− (n=2–4) were measured in the energy range below 3 eV. Analyses of these spectra by the density-functional theory deduced their electronic states and geometric structures. The growth mechanisms of the 3d transition-metal carbide clusters were discussed on the basis of the structural models obtained. The geometric structures of Co2Cn− exhibit a tendency that carbon atoms aggregate to form a Cn substructure. In contrast, V2Cn− consists of VC2 building blocks, which prelude the formation of a vanadium-carbide network. These features illustrate the differences in the carbide-formation processes of the late and the early 3d transition metals, that is, only the latter forms large metal-carbide networks such as metallocarbohedrens and metal carbide compounds.

Ejection mechanism of molecules and neutral clusters from liquid beam under irradiation of IR laser

Abstract Resorcinol molecules and those solvated with solvent water molecules were isolated in the gas phase from a liquid beam of an aqueous solution of resorcinol by resonant vibrational excitation of solvent water molecules under IR-laser irradiation. The spatial distribution of the ejected species at various delay times from the IR-laser irradiation indicates that two different isolation mechanisms operate: One dominates in a time range shorter than ∼1 μs (early-time domain), and the other in a time range longer than ∼1 μs (late-time domain). A time-dependent measurement of the liquid-beam profile by optical diffraction shows that the beam has a smooth surface in the early-time domain, whereas in the late-time domain the surface roughness overweighs the wavelength of the illumination laser.

Photon-trap spectroscopy of mass-selected ions in an ion trap: Optical absorption and magneto-optical effects

A novel experimental technique has been developed to observe a trace of optical absorption of free mass-selected ions. The technique combines a linear radio-frequency ion trap with a high-finesse optical cavity to perform cavity ring-down spectroscopy (photon-trap spectroscopy for generality), where the storage lifetime of photons in the cavity provides a sensitivity high enough to probe the trapped ions. Absorption spectra of the manganese ion Mn(+) are presented, showing hyperfine structures for the (7)P(2,3,4)<--(7)S(3) transitions in the ultraviolet range. Implementation of a solenoidal magnet allows us to observe the Zeeman splitting and the Faraday rotation as well.

Electronic states of the manganese dimer ion probed by photodissociation spectroscopy

The optical spectrum of the manganese dimer ion, Mn2+, was obtained by measurement of the photodissociation action spectrum in the photon-energy range from 1.9 through 5.6 eV. The spectrum was analyzed by calculating its electronic and geometric structures using density functional theory including nonlocal corrections. The simulation was in reasonable agreement with the experimental result, allowing the assignment of the electronic states involved in the optical transitions. The ground state was shown to be a 12Σg+ state. The excited electronic states corresponding to the transitions around 2.9, 4.0, and 5.3 eV were assigned to 12Σu+, 12Σu+ together with 12Πu, and 12Πu, respectively. The high-spin character indicates a ferromagnetic coupling of all the 3d electrons.

Continuous-wave cavity ringdown spectroscopy applied to solids: properties of a Fabry-Perot cavity containing a transparent substrate

Cavity ringdown spectroscopy, or photon-trap spectroscopy for generality, is shown to be applicable to a sample in the solid phase by theoretical and experimental studies. In the technique investigated, a solid in a substrate form having optically flat parallel surfaces is inserted exactly normal to a light beam in a high-finesse optical cavity; the light reflected at the substrate surface is coupled back to the cavity and thus the optical loss is minimized. Thereby the trapping lifetime of photons in the cavity is measured to obtain total optical loss including absorption by the solid sample. As the solid substrate behaves as an extra cavity splitting the original cavity, the trapped photons are susceptible to an interference effect inherent to the triply coupled cavity. To elucidate this effect, the coupling efficiency of the incident light and the trapping lifetime of photons dissipating exponentially were analyzed theoretically for a Fabry-Perot cavity containing a transparent substrate as a model. An experiment was performed on a silicon substrate transparent in the mid-infrared range with a cw optical parametric oscillator based on periodically poled lithium niobate. The optical loss caused by insertion of the substrate was measured to be 2.3 × 10^−4 per round trip, which meets a low-loss requirement of the photon-trap technique. The trapping lifetime of photons was found to depend on the location of the substrate as predicted by theory. By optimizing the experimental conditions, the present technique provides a high sensitivity to optical absorption associated with a trace amount of dopants in solids and adsorbates on surfaces.