Hisato Yasumatsu

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.

Energy redistribution in cluster–surface collision: I2− (CO2)n onto silicon surface

Fragmentation of I2−(CO2)n (n=1−30) by its collision on a silicon surface was investigated by measuring the fragment anions and their translational energy parallel to the surface (surface–parallel translational energy) in a tandem time‐of‐flight mass spectrometer equipped with a collision chamber evacuated down to ∼10−8 Pa. At the collision energy (per I2−) of 50 eV and the incident angle of 26° with respect to the surface normal, the distributions of the surface–parallel translational energies of the fragment anions from a given parent cluster anion were found to obey the one‐dimensional Maxwell–Boltzmann distribution with the same translational temperature, Ts∥ The results show that the cluster anion and its neighboring surface atoms reach quasiequilibrium before the fragment anions leave the surface. A general increasing trend of Ts∥ (6000–12 000 K) with n is interpreted as an increasing extent of cluster–impact heating with n, while the reduction of Ts∥ in the 13≤n≤∼19 range is attributable to efficie...

Splitting a chemical bond with a molecular wedge via cluster-surface collisions

A cluster anion, I2−(CO2)n (n=0−30), was allowed to collide onto a silicon surface at collision energies (per I2−) of 1−80 eV in an ultrahigh vacuum surface-collision chamber equipped with a tandem time-of-flight (TOF) mass spectrometer. The product anions show that the core ion, I2−, dissociates by the collision of I2−(CO2)n on the silicon surface. The branching fraction for the I2− dissociation (fdis) was determined as functions of the collision energy and the number of the CO2 molecules, n. The marked n-dependence of fdis at a collision energy (per I2−) higher than 30 eV was explained in terms of a wedge effect in which a CO2 molecule in the vicinity of the mid point of the I2− bond splits the I2− bond as if a piece of wood is split by a hammer thrust against a wedge vs a cage effect, in which the I2− dissociation is suppressed by geminate recombination between the dissociating I and I− pair in a complete solvation shell. The wedge and cage effects in the I2− dissociation were also verified by use of t...

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).

Preparation of intense multi-element metal cluster ions with single composition

A source of composition-selected multi-element cluster ions has been developed toward investigation of chemical reactivity of the clusters supported on a solid surface. The cluster ions are produced in a gas aggregation cell equipped with several magnetron sputtering devices, and their composition is selected by a quadrupole mass filter. The translational and internal kinetic energies of the single-composition cluster ions are reduced by collision with cold helium to achieve cluster-impact deposition onto the surface at a low collision energy. It has been succeeded to obtain single-composition silver-copper bimetal cluster ions more intense than several tens pA. A typical translational energy width is 0.5 eV per cluster.

Mechanism of wedge effect in splitting of chemical bond by impact of X2−(CO2)n onto silicon surface (X=Br, I)

Dissociation of Br2− (into Br and Br−) in a cluster anion, Br2−(CO2)n, by impact of Br2−(CO2)n on a silicon surface was investigated as a function of the number of CO2 molecules, n, at a collision energy per Br2− of 30–50 eV. The branching fraction of the Br2− dissociation used as a propensity of the Br2− dissociation rate showed a marked n-dependence similar to that observed in the collisional dissociation of I2−(CO2)n on a silicon surface. The result is explained in terms of wedge effect in which a CO2 molecule at a midpoint of Br2− splits the Br2− bond as a wedge vs. cage effect in which the bond splitting of Br2− in the CO2 solvent cage is suppressed. The agreement of the n-dependence between the Br2−(CO2)n and I2−(CO2)n collisions lends a further support for the validity of the bond splitting by the wedge action. A molecular dynamics simulation reproduces the wedge effect in the Br2−(CO2)n collision, as well. The cage effect appearing in the vicinity of n=12 corresponds to the completion of the first...

Fragmentation dynamics of silicon cluster anions in collision with a silicon surface: contrast to aluminum cluster anions

Abstract The collision of a size-selected silicon cluster anon, Si N − (8 ⩽ N ⩽ 12), with a silicon surface was investigated by using a tandem time-of-flight mass spectrometer equipped with an ultrahigh-vacuum collision chamber. The size-selected parent cluster anion, Si N − , was fragmented dominantly into Si N − ( n ≈ N /2) in the collision energy range of several electron volts per atom. The measured recoil velocity of the fragment anion from the surface revealed that about 40% of the collision energy is directly converted to the translation kinetic energies of the anion and its counter neutral product scattered from the surface. This conversion efficiency was much higher than that observed for Al N − , aluminum cluster anions, reported previously. This marked contrast indicates that Al N − interacts for a longer time with the surface and transmits its collision energy more efficiently to the surface than Si N − .

SOLVATION EFFECTS ON COLLISIONAL PROCESSES OF SIZE-SELECTED

Collisional processes of with a silicon surface were investigated. Size-selected (n=0–30) were allowed to collide with a silicon surface at collision energies per of 1–150 eV at almost normal incidence angles. All the anions scattered from the surface were I−, I−(CO2), I−(CO2)2, , , and . The molecular dynamics simulation revealed that the dissociation of occurs through vibrational and rotational excitation. The dissociation was found to be either retarded or accelerated by the presence of the solvent CO2 molecules. The solvation effects on the dissociation of were explained in terms of energy transfer between and CO2 solvent molecules and a complex formed on the surface at the moment of the collision.

{\rm{I}}_2^- ({\rm{CO}}_2)_n

Abstract The charge survival yield for a cluster anion, I 2 − (CO 2 ) n ( n = 0–30) during its collision onto a silicon surface covered with silicon oxide layers of ∼2 nm in thickness was measured as a function of the number of the CO 2 molecules, n , and the collision energy (per I 2 − in the range of 1–80 eV. A monotonic increase in the charge survival yield with n was observed. This means that the efficiency of the charge transfer from the core ion, I 2 − , to the surface is reduced with the increase of n . It was concluded that the CO 2 molecules suppress the charge transfer by behaving as an electrostatic ‘stabilizer’ of the core ion and as a ‘spacer’ between the core ion and the surface.