Collision Dynamics of Argon Cluster Ions, Ar^+_n (n=3-23) : Molecular Dynamics Simulation Based on Diatomics-in-Molecules Method

The molecular dynamics method combined with a quantum mechanical calculation has been used to simulate the collision between an argon atom and an argon cluster ion, Ar^+_n(n=3-23), which contains a given amount of internal energy. Two pathways were observed; (i) Evaporation after collisional energy transfer to the internal degrees of freedom vs. (ii) fusion via complex formation. The total reaction cross sections were compared with those experimentally obtained. It is found that the branching fractions of the evaporation and the fusion depend critically on the internal energy and the impact parameter

Breathing Vibration of Ar Clusters Analyzed by Molecular Dynamics Calculation : Cluster-Shape Dependence of the Mode-Separation

The vibrational motion of Ar clusters (Ar_n, n=20 and 30) having isomers in a variety of shapes was simulated by use of the molecular dynamics method and the mode-separation of the breathing vibration from the quadruple spheroidal vibration was investigated. It was found that these modes of highly spherical isomers were almost fully separated from each other, while coupling between these modes were significant in non-spherical isomers. The relation between the cluster shape and the mode-separation was elucidated

Low-energy collisions of helium clusters with size-selected cobalt cluster ions

Collisions of helium clusters with size-selected cobalt cluster ions, Com+ (m ≤ 5), were studied experimentally by using a merging beam technique. The product ions, Com+Hen (cluster complexes), were mass-analyzed, and this result indicates that more than 20 helium atoms can be attached onto Com+ at the relative velocities of 103 m/s. The measured size distributions of the cluster complexes indicate that there are relatively stable complexes: Co2+Hen (n = 2, 4, 6, and 12), Co3+Hen (n = 3, 6), Co4+He4, and Co5+Hen (n = 3, 6, 8, and 10). These stabilities are explained in terms of their geometric structures. The yields of the cluster complexes were also measured as a function of the relative velocity (1 × 102−4 × 103 m/s), and this result demonstrates that the main interaction in the collision process changes with the increase of the collision energy from the electrostatic interaction, which includes the induced deformation of HeN, to the hard-sphere interaction

CO oxidation by copper cluster anions

Reactions of CO and O2 on size-selected copper cluster anions, Cun- (n = 4–11), have been investigated at the collision energy of 0.2 eV by use of a guided ion beam-tandem mass spectrometer. Oxygen-adsorbed copper anions, CunO2-, in particular Cu5O2- and Cu9O2-, show an evidence of the CO oxidation, that is, the formation of the monoxide CunO−. The density functional theory calculation reveals that the CO oxidation occurs more exothermically on Cu5O2- and Cu9O2- than the other clusters. This can be explained by the relatively small dissociation energy of their Cu–O bonds. In addition, the calculations on Cu5O2+/- indicate that the CO oxidation proceeds via a low-energy pathway for the anion owing to the structural rearrangement of the copper cluster compared to the cation

Stability of Aluminum-Doped Copper Cluster Cations and Their Reactivity toward NO and O₂

Aluminum-doped copper cluster cations, Cu_<i>n</i>Al⁺, were produced via an ion sputtering method and analyzed by mass spectrometry. The measured size distributions show that Cu₆Al⁺ and Cu₁₈Al⁺ are highly stable species, which can be understood in terms of the electronic subshell 1P and 2S closings, respectively. Furthermore, the reactions of size-selected Cu_<i>n</i>Al⁺ (<i>n</i> = 4–6 and 8–16) with NO and O₂ were studied at near thermal energies by using a tandem-type mass spectrometer. The doping of an Al atom improves the reactivity of the clusters toward NO in particular for <i>n</i> = 9, 11, 13, and 15, whereas it does not change the reactivity toward O₂ significantly. Consequently, it was found that Cu_<i>n</i>Al⁺ (<i>n</i> = 9, 11, 13 and 15) are more reactive toward NO than toward O₂. The high reactivity of Cu₉Al⁺ toward NO compared to that of Cu₁₀⁺ is explained in terms of the increase of the adsorption energy and the lowering of the barrier to dissociative adsorption, with the aid of calculations based on density functional theory. Moreover, the multiple-collision reactions of Cu_<i>n</i>Al⁺ (<i>n</i> = 9, 11, and 13) with NO result in the production of cluster dioxides, Cu_<i>n</i>–3AlO₂⁺, (i.e., release of N₂), which clearly indicates that NO decomposition proceeds on these clusters

Gas-Phase Reactions of Copper Oxide Cluster Cations with Ammonia: Selective Catalytic Oxidation to Nitrogen and Water Molecules

Reactions of copper oxide cluster cations, Cu_<i>n</i>O_<i>m</i>⁺ (<i>n</i> = 3–7; <i>m</i> ≤ 5), with ammonia, NH₃, are studied at near thermal energies using a guided ion beam tandem mass spectrometer. The single-collision reactions of specific clusters such as Cu₄O₂⁺, Cu₅O₃⁺, Cu₆O₃⁺, Cu₇O₃⁺, and Cu₇O₄⁺ give rise to the release of H₂O after NH₃ adsorption efficiently and result in the formation of Cu_<i>n</i>O_<i>m</i>–1NH⁺. These Cu_<i>n</i>O_<i>m</i>⁺ clusters commonly have Cu average oxidation numbers of 1.0–1.4. On the other hand, the formation of Cu_<i>n</i>O_<i>m</i>–1H₂⁺, i.e., the release of HNO, is dominantly observed for Cu₇O₅⁺ with a higher Cu oxidation number. Density functional theory calculations are performed for the reaction Cu₅O₃⁺ + NH₃ → Cu₅O₂NH⁺ + H₂O as a typical example of H₂O release. The calculations show that this reaction occurs almost thermoneutrally, consistent with the experimental observation. Further, our experimental studies indicate that the multiple-collision reactions of Cu₅O₃⁺ and Cu₇O₄⁺ with NH₃ lead to the production of Cu₅⁺ and Cu₇O⁺, respectively. This suggests that the desirable NH₃ oxidation to N₂ and H₂O proceeds on these clusters

NO Decomposition Activated by Preadsorption of O₂ onto Copper Cluster Anions

Reactions of anionic copper cluster dioxides, Cu_<i>n</i>O₂^– (<i>n</i> = 8, 10, and 12), with NO were studied in the gas phase by using a guided ion beam tandem mass spectrometer. A product ion, Cu_<i>n</i>–2O₄^–, is observed under multiple collision conditions together with the single-collision reaction products, Cu_<i>n</i>O₂NO^– and Cu_<i>n</i>–1O₂NO^–. The NO-pressure dependence studies show that Cu_<i>n</i>–2O₄^– is formed via adsorption of two NO molecules followed by releasing an N₂ molecule, which is clear evidence for NO decomposition. Density functional theory calculations were also performed for the reaction of Cu₈O₂^– with two NO molecules and confirm the production of Cu₆O₄^– under the experimental conditions used