Dynamics of Clusters Initiated by Photon and Surface Impact

Clusters of atoms/molecules show dynamics characteristic of the method of excitation. Two contrasted processes are discussed:  (1) electronic excitation via single-photon absorption and (2) impulsive excitation of nuclear motions by surface impact. Process 1 is exemplified by photodissociation dynamics of size-selected metal cluster ions. The electronic energy is converted most likely to vibrational energy of internal modes; dissociation follows via statistical mechanism to produce energetically favored fragments. Exceptionally, a silver cluster ion, Ag4+, is shown to undergo nonstatistical dissociation along the potential-energy surface of the excited state. Energy partitioning to translational and vibrational modes of fragments is analyzed as well as bond dissociation energies. Furthermore, the spectrum of the photodissociation yield provides electronic and geometrical structures of a cluster with the aid of ab initio calculations; manganese, MnN+, and chromium, CrN+, cluster ions are discussed, where the importance of magnetic interactions is manifested. On the other hand, momentum transfer upon surface impact plays a role in process 2. An impulsive mechanical force triggers extraordinary chemical processes distinct from those initiated by atomic collision as well as photoexcitation. Experiments on aluminum, AlN-, silicon, SiN-, and solvated, I2-(CO2)N, cluster anions provide evidence for reactions proceeding under extremely high temperatures, such as pickup of surface atoms, annealing of products, and mechanical splitting of chemical bonds. In addition, a model experiment to visualize and time-resolve the cluster impact process is performed by using a micrometer-sized liquid droplet. Multiphoton absorption initiates superheating of the droplet surface followed by a shock wave and disintegration into a number of small fragments (shattering). These studies further reveal how the nature of chemical bonds influences the dynamics of clusters

Ion trajectory simulation of linear multipole ion traps for analysis of spatial ion distribution

We explore the spatial distribution of ions in a linear multipole radio-frequency ion trap by ion-trajectory simulation using the SIMION software. As a typical example, a two-dimensional column density is characterised for Ag2+ ions stored in several types of the trap. Taking into account the effects of the space charge and ion–buffer He collisions, we are able to reproduce a ring profile of an ion distribution experimentally observed for a linear octopole trap at a high density of ions close to the space-charge limit. This simulation demonstrates also that the profile of the ion distribution is dependent on the number of ions stored and the temperature of a buffer He gas. As for a quadrupole trap, an ion distribution is shown to be located around the central axis of the trap as observed by experiment. An influence of the electrode structure is also discussed for the quadrupole trap.</p

Adsorption and Subsequent Reaction of a Water Molecule on Silicate and Silica Cluster Anions

We present reactions of size-selected free silicate, Mg_<i>l</i>SiO_<i>m</i>^–, and silica, Si_<i>n</i>O_<i>m</i>^–, cluster anions with a H₂O molecule focusing on H₂O adsorption. It was found that H₂O adsorption to Mg_<i>l</i>SiO_<i>m</i>^– with <i>l</i> = 2 and 3 (<i>m</i> = 4–6) is always followed by molecular oxygen release, whereas reactivity of the clusters with <i>l</i> = 1 (<i>m</i> = 3–5) was found to be much lower. On the contrary, in the reaction of Si_<i>n</i>O_<i>m</i>^– (<i>n</i> = 3–8, 2<i>n</i> – 1 ≤ <i>m</i> ≤ 2<i>n</i> + 2), a H₂O adduct is observed as a major reaction product. Larger and oxygen-rich clusters tend to exhibit higher reactivity; the rate constants of the adsorption reaction are 2 orders of magnitude larger than those of CO adsorption previously reported. DFT calculations revealed that H₂O is dissociatively adsorbed on Si_<i>n</i>O_<i>m</i>^– to form two SiO₃(OH) tetrahedra. The site selectivity of H₂O adsorption is governed by the location of the singly occupied molecular orbital (SOMO) on Si_<i>n</i>O_<i>m</i>^–. The present findings give molecular-level insights into H₂O adsorption on silica and silicate species in the interstellar environment

Reaction Sites of CO on Size-Selected Silicon Oxide Cluster Anions: A Model Study of Chemistry in the Interstellar Environment

We present reactions of size-selected free silicon oxide cluster anions, Si_<i>n</i>O_<i>m</i>^– (<i>n</i> = 3–7, 2<i>n</i> – 1 ≤ <i>m</i> ≤ 2<i>n</i> + 2), with a CO gas. Adsorption of CO on Si_<i>n</i>O_<i>m</i>^– is observed as a major reaction channel. The rate constant of the adsorption reaction is high for the oxygen-rich clusters with <i>m</i> ≥ 2<i>n</i> + 1, whereas almost no reaction product is observed for <i>m</i> ≤ 2<i>n</i>. DFT calculations revealed that a pair of dangling O atoms on 4-fold-coordinated Si atoms plays a key role, which is the adsorption site of CO on Si_<i>n</i>O_<i>m</i>^–. Bond formation between CO and one of the dangling O atoms is associated with electron transfer from the CO molecule to the other dangling O atom. The present findings give molecular-level insights into adsorption of CO molecules on silicates in the interstellar environment

Electronic and Geometric Effects on Chemical Reactivity of 3d-Transition-Metal-Doped Silver Cluster Cations toward Oxygen Molecules

We report electronic and geometric structures of 3d-transition-metal-doped silver cluster cations, AgN–1M+ (M = Sc–Ni), studied by chemical reaction with oxygen molecules. The evaluated reaction rate coefficients for small sizes, N, are 2–6 orders of magnitude higher than those of undoped AgN+, whereas those for large N are comparable with those of AgN+. The low reactivity at large sizes is attributed to a geometric effect, that is, encapsulation of the dopant atom, which provides an active site located on the surface of the cluster in small sizes. In addition, a reactivity minimum is observed for AgN–1M+ with M = Sc, Ti, V, Fe, Co, and Ni at a specific size, where the cluster possesses 18 valence electrons including 3d electrons. With the aid of density functional theory calculations, the reactivity minimum is suggested to be due to an electronic effect, that is, formation of a closed electronic shell by the 18 valence electrons, implying delocalized 3d electrons. Ag13Cr+ and Ag12Mn+, possessing 18 valence electrons as well, are noted to be exceptions, where d electrons are supposed to be localized on the dopant atom because of the half-filled nature of Cr and Mn 3d orbital

Exploring s–d, s–f, and d–f Electron Interactions in Ag_<i>n</i>Ce⁺ and Ag_<i>n</i>Sm⁺ by Chemical Reaction toward O₂

We investigate gas-phase reactions of free AgnCe+ and AgnSm+ clusters with oxygen molecules to explore s–d, s–f, and d–f electron interactions in the finite size regime; a Ce atom has a 5d electron as well as a 4f electron, whereas a Sm atom has six 4f electrons without 5d electrons. In the reaction of AgnCe+ (n = 3–20), the Ce atom located on the cluster surface provides an active site except for n = 15 and 16, as inferred from the composition of the reaction products with oxygen bound to the Ce atom as well as from their relatively high reactivity. The extremely low reactivity for n = 15 and 16 is due to encapsulation of the Ce atom by Ag atoms. The minimum reactivity observed at n = 16 suggests that a closed electronic shell with 18 valence electrons is formed with a delocalized Ce 5d electron, while the localized Ce 4f electron does not contribute to the shell closure. As for AgnSm+ (n = 1–18), encapsulation of the Sm atom was observed for n ≥ 15. The lower reactivity at n = 17 than at n = 16 and 18 implies that an 18-valence-electron shell closure is formed with s electrons from Ag and Sm atoms; Sm 4f electrons are not involved in the shell closure as in the case of AgnCe+. The present results suggest that the 4f electrons tend to localize on the lanthanoid atom, whereas the 5d electron delocalizes to contribute to the electron shell closure

Photoelectron Imaging Signature for Selective Formation of Icosahedral Anionic Silver Cages Encapsulating Group 5 Elements: M@Ag₁₂^– (M = V, Nb, and Ta)

An assembly of 13 atoms can form highly symmetric architectures like those belonging to D3h, Oh, D5h, and Ih point groups. Here, using photoelectron imaging spectroscopy in combination with density functional theory (DFT) calculations, we present a simple yet convincing experimental signature for the selective formation of icosahedral cages of anionic silver clusters encapsulating a dopant atom of group 5 elements: M@Ag12– (M = V, Nb, and Ta). Their photoelectron images obtained at 4 eV closely resemble one another: only a single ring is observed, which is assignable to photodetachment signals from a 5-fold degenerate superatomic 1D electronic shell in the 1S21P61D10 configuration of valence electrons. The perfect degeneracy represents an unambiguous fingerprint of an icosahedral symmetry, which would otherwise be lifted in all of the other structural isomers. DFT calculations confirm that Ih forms are the most stable and that D5h, Oh, and D3h structures are not found even in metastable states

Photoelectron Imaging Signature for Selective Formation of Icosahedral Anionic Silver Cages Encapsulating Group 5 Elements: M@Ag₁₂^– (M = V, Nb, and Ta)

An assembly of 13 atoms can form highly symmetric architectures like those belonging to D3h, Oh, D5h, and Ih point groups. Here, using photoelectron imaging spectroscopy in combination with density functional theory (DFT) calculations, we present a simple yet convincing experimental signature for the selective formation of icosahedral cages of anionic silver clusters encapsulating a dopant atom of group 5 elements: M@Ag12– (M = V, Nb, and Ta). Their photoelectron images obtained at 4 eV closely resemble one another: only a single ring is observed, which is assignable to photodetachment signals from a 5-fold degenerate superatomic 1D electronic shell in the 1S21P61D10 configuration of valence electrons. The perfect degeneracy represents an unambiguous fingerprint of an icosahedral symmetry, which would otherwise be lifted in all of the other structural isomers. DFT calculations confirm that Ih forms are the most stable and that D5h, Oh, and D3h structures are not found even in metastable states

Size-Dependent Ligand Quenching of Ferromagnetism in Co₃(benzene)_<i>n</i> ⁺ Clusters Studied with X‑ray Magnetic Circular Dichroism Spectroscopy

Cobalt–benzene cluster ions of the form Co₃(bz)_<i>n</i> ⁺ (<i>n</i> = 0–3) were produced in the gas phase, mass-selected, and cooled in a cryogenic ion trap held at 3–4 K. To explore ligand effects on cluster magnetic moments, these species were investigated with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. XMCD spectra yield both the spin and orbital angular momenta of these clusters. Co₃ ⁺ has a spin magnetic moment of μ_S = 6 μ_B and an orbital magnetic moment of μ_L = 3 μ_B. Co₃(bz)⁺ and Co₃(bz)₂ ⁺ complexes were found to have spin and orbital magnetic moments identical to the values for ligand-free Co₃ ⁺. However, coordination of the third benzene to form Co₃(bz)₃ ⁺ completely quenches the high spin state of the system. Density functional theory calculations elucidate the spin states of the Co₃(bz)_<i>n</i> ⁺ species as a function of the number of attached benzene ligands, explaining the transition from septet to singlet for <i>n</i> = 0 → 3