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.

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 − 2 and a four-centre reaction between N 2 and O 2 in a higher energy range, etc.

A combined experimental and theoretical investigation of Ag‐Pt sub‐nanometer clusters as heterogeneous catalysts in the CO→CO2 reaction (COox) is presented. Ag9Pt2 and Ag9Pt3 clusters are size‐selected in the gas phase, deposited on an ultrathin amorphous alumina support, and tested as catalysts experimentally under realistic conditions and by first‐principles simulations at realistic coverage. In situ GISAXS/TPRx demonstrates that the clusters do not sinter or deactivate even after prolonged exposure to reactants at high temperature, and present comparable, extremely high COox catalytic efficiency. Such high activity and stability are ascribed to a synergic role of Ag and Pt in ultranano‐aggregates, in which Pt anchors the clusters to the support and binds and activates two CO molecules, while Ag binds and activates O2, and Ag/Pt surface proximity disfavors poisoning by CO or oxidized species.

Studies of silicon cluster-metal atom compound formation in a supersonic molecular beamThe chemistry of sizedselected silicon clusters as studied by fourier transform mass spectrometry AIP Conf.

The absolute absorption cross section of C 60 in the gas phase ͑830-870 K͒ was measured as a function of the photon energy ͑3.5-11.4 eV͒ ͑absorption spectrum͒. Absorption peaks at 7. 87, 8.12, 8.29, 9.2 eV and a dip at 8.45 eV observed are assigned as Feshbach resonances in the photoexcitation involving superexcited states. The superexcited states responsible for the 7.87, 8.12, and 9.2 eV peaks are assigned to be core-excited Rydberg states converging to the second, the third and the fourth ionization limits of C 60 ͑8.89, 9.12, 10.82-11.59 eV͒, respectively. The 8.29 eV peak is considered to originate from vibrational excitation of a totally symmetric pentagonal pinch mode of the superexcited state responsible for the 8.12 eV peak. Further, a relative photoionization quantum yield was estimated from the absorption cross section measured and the relative photoionization cross section reported. The yield increases particularly in the vicinity of 8 eV in accordance with a high efficiency of autoionization of the superexcited states. Ionization efficiency is not high in the vicinity of the first ionization energy, probably because of rapid energy dissipation into its vibrational modes. The spectrum below the ionization energy resemble the absorption spectra of C 60 in its solutions.

Dynamical processes involved in the collision of aluminum cluster anions, Al−N (4≤N≤25), with a silicon surface were investigated. Intact and fragment cluster anions, Al−n (n≤N), were produced upon the collision. The surf02ace-tangent and surface-normal recoil velocity components of these product a0n0ions were determined. The tangential recoil velocities of the fragment cluster anions were considerably slow, ranging from 5% to 30% of the velocity of the incident parent cluster anion, while the normal velocities were conserved relatively well. These results are explained in such a manner that the fragment cluster anion is evaporated from the parent cluster anion while it interacts with the surface and loses its tangential momentum. The dynamics and the energetics derived from these results show further that the fragmentation process involves not only sequential evaporation of aluminum atoms but also simultaneous production of several small fragment clusters. Comparison of the present result with that of the collision-induced dissociation by a rare-gas atom 88lends a further support on this nonsequential fragmentation 1/1mechanism.

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Studied were cluster-size dependence of catalytic activity of CO oxidation driven by unisized platinum clusters, Pt N (N = 10, 30, and 60), directly bound to a silicon substrate surface. Temperature-programmed desorption measurements were repeated for a given Pt N /Si catalyst with systematic change of the reaction condition. The CO oxidation on the Pt N /Si catalyst is described in a manner similar to the bulk Pt(111) surface; the Langmuir−Hinshelwood mechanism by molecular oxygen activated by the catalyst at 120−140 K (α reaction) and the dissociatively adsorbed atomic oxygen in the temperature range of 130−350 K (β reaction). However, the Pt N /Si catalyst has the advantage of a lower-temperature activity compared with the bulk Pt(111) surface. Furthermore, the Pt 60 /Si catalyst has 1.5 times higher activity per Pt atom than Pt 30 /Si, while no catalytic activity for the Pt 10 /Si sample. These results are interpreted in relation to the geometric structure and the electron accumulation of the Pt clusters on the Si surface.