Reaction of size-selected iron-oxide cluster cations with methane: a model study of rapid methane loss in Mars' atmosphere.

We report gas-phase reactions of free iron-oxide clusters, FenOm+, and their Ar adducts with methane in the context of chemical processes in Mars' atmosphere. Methane activation was observed to produce FenOmCH2+/FenOmCD2+ and FenOmC+, where the reactivity exhibited size and composition dependence. For example, the rate coefficients of methane activation for Fe3O+ and Fe4O+ were estimated to be 1 × 10-13 and 3 × 10-13 cm3 s-1, respectively. Based on these reaction rate coefficients, the presence of iron-oxide clusters/particles with a density as low as 107 cm-3 in Mars' atmosphere would explain the rapid loss of methane observed recently by the Curiosity rover.

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.

Photodestruction Action Spectroscopy of Silver Cluster Anions, AgN- (N = 3-19), with a Linear Ion Trap: Observation of Bound Excited States above the Photodetachment Threshold.

Electron detachment thresholds of metal cluster anions, MN-, are a few electron volts. The excess electron is therefore detached by visible or ultraviolet light, which also creates low-lying bound electronic states, MN-*; i.e., MN-* energetically overlaps with the continuum, MN + e-. Here, we perform action spectroscopy of photodestruction, leading either to photodetachment or to photofragmentation, for size-selected silver cluster anions, AgN- (N = 3-19), to unveil such bound electronic states embedded in the continuum. The experiment takes advantage of a linear ion trap that enables us to measure photodestruction spectra with high quality at well-defined temperatures, where bound excited states, AgN-*, are clearly identified above their vertical detachment energies. Structural optimization of AgN- (N = 3-19) is conducted by using density functional theory (DFT), which is followed by calculations of vertical excitation energies by time-dependent DFT to assign the observed bound states. Spectral evolution observed as a function of cluster size is also discussed, where the optimized geometries are found to be closely related to the observed spectral profiles. A plasmonic band consisting of nearly degenerate individual excitations is observed for N = 19.

Photoelectron imaging of size-selected metal cluster anions in a quasi-continuous mode.

We present a novel high-repetition-rate photoelectron imaging (PEI) apparatus for exploring electronic structures of metal cluster anions. A continuous beam of mass-selected metal cluster anions, generated by a magnetron-sputtering cluster-ion source coupled with a quadrupole mass filter, is chopped into sub-megahertz ion bunches using a high-voltage pulser. The quasi-continuous anion beam is introduced into a PEI spectrometer, where the anions are photodetached using a 404 nm (3.07 eV) continuous-wave laser diode. As a demonstration, we acquire photoelectron images for size-selected Ag cluster anions, AgN - (N = 3, 7, 14), and show that each image can be obtained in a short accumulation time (50 s) with a kinetic energy resolution (ΔE/E) of 4% at E = 1.77 eV. The quasi-continuous PEI technique enables high-count-rate, space-charge-free acquisition of photoelectron spectra and angular distributions not only from size-selected metal cluster anions but also from anions prepared by other continuous ion sources, such as electrospray ionization.

Exploring s-d, s-f, and d-f Electron Interactions in AgnCe+ and AgnSm+ 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.

Correction: Electron counting in cationic and anionic silver clusters doped with a 3d transition-metal atom: endo- vs. exohedral geometry.

Correction for 'Electron counting in cationic and anionic silver clusters doped with a 3d transition-metal atom: endo- vs. exohedral geometry' by Kento Minamikawa et al., Phys. Chem. Chem. Phys., 2022, DOI: 10.1039/d1cp04197e.

Electron counting in cationic and anionic silver clusters doped with a 3d transition-metal atom: endo- vs. exohedral geometry.

Electron counting is a concept that often governs properties of molecules, clusters, and complexes. Here we explore silver clusters doped with a transition-metal atom, where it has been an issue whether or not 3d electrons delocalize to participate in electron counting. The experiment is performed on AgNM+/- (M = Sc-Ni) clusters to examine their stability through chemical reactivity, enabling systematic control of the number of valence electrons by the cluster size, the charge state, and the transition-metal element across the periodic table. It is revealed for 18-valence-electron clusters that 3d electrons participate in electron counting to show exceptional stability only when the transition-metal atom is endohedrally doped, except for Cr and Mn doping that forces 3d electrons to localize. We thus present new entries for superatomic metal clusters as well as a geometric factor that regulates the behavior of 3d electrons in the nanoscale regime.

Mn12 -Acetate Complexes Studied as Single Molecules.

The phenomenon of single molecule magnet (SMM) behavior of mixed valent Mn12 coordination clusters of general formula [MnIII 8 MnIV 4 O12 (RCOO)16 (H2 O)4 ] had been exemplified by bulk samples of the archetypal [MnIII 8 MnIV 4 O12 (CH3 COO)16 (H2 O)4 ] (4) molecule, and the molecular origin of the observed magnetic behavior has found support from extensive studies on the Mn12 system within crystalline material or on molecules attached to a variety of surfaces. Here we report the magnetic signature of the isolated cationic species [Mn12 O12 (CH3 COO)15 (CH3 CN)]+ (1) by gas phase X-ray Magnetic Circular Dichroism (XMCD) spectroscopy, and we find it closely resembling that of the corresponding bulk samples. Furthermore, we report broken symmetry DFT calculations of spin densities and single ion tensors of the isolated, optimized complexes [Mn12 O12 (CH3 COO)15 (CH3 CN)]+ (1), [Mn12 O12 (CH3 COO)16 ] (2), [Mn12 O12 (CH3 COO)16 (H2 O)4 ] (3), and the complex in bulk geometry [MnIII 8 MnIV 4 O12 (CH3 COO)16 (H2 O)4 ] (5). The found magnetic fingerprints - experiment and theory alike - are of a remarkable robustness: The MnIV 4 core bears almost no magnetic anisotropy while the surrounding MnIII 8 ring is highly anisotropic. These signatures are truly intrinsic properties of the Mn12 core scaffold within all of these complexes and largely void of the environment. This likely holds irrespective of bulk packing effects.

Reaction of nitric oxide molecules on transition-metal-doped silver cluster cations: size- and dopant-dependent reaction pathways.

We report size- and dopant-dependent reaction pathways as well as reactivity of gas-phase free AgnM+ (M = Sc-Ni) clusters interacting with NO. The reactivity of AgnM+, except for M = Cr and Mn, exhibits a minimum at a specific size, where the cluster cation possesses 18 or 20 valence electrons consisting of Ag 5s and dopant's 3d and 4s. The product ions range from NO adducts, AgnM(NO)m+, and oxygen adducts, AgnMOm+, to NO2 adducts, AgnM(NO2)m+. At small sizes, AgnMOm+ are the major products for M = Sc-V, whereas AgnM(NO)m+ dominate the products for M = Cr-Ni in striking contrast. In both cases, these reaction products are reminiscent of those from an atomic transition metal. However, the reaction pathways are different at least for M = Sc and Ti; kinetics measurements reveal that the present oxygen adducts are formed via NO adducts, while, for example, Ti+ is known to produce TiO+ directly by reaction with a single NO molecule. At larger sizes, on the other hand, AgnM(NO2)m+ are dominantly produced regardless of the dopant element because the dopant atom is encapsulated by the Ag host; the NO2 formation on the cluster is similar to that reported for undoped Agn+.

Preadsorption Effect of Carbon Monoxide on Reactivity of Cobalt Cluster Cations toward Hydrogen.

We report gas-phase reactions of free Con(CO)m+ (n = 3-11, m = 0-2) with H2, expecting a catalytic reaction of coadsorbed CO and H2 on Con+. Preadsorption of CO molecules is found to promote H2 adsorption, in particular, on Con(CO)+ (n = 5, 8-10). Density functional theory (DFT) calculations reveal that the reactivity is governed by the molecular-orbital energy of Con+, which is tuned by preadsorbed CO molecules. Collision-induced-dissociation experiments performed on ConCOH2+ (n = 8-10) imply that at least some of the CO and H2 molecules are bound together on Con+.

Charge-state analysis of small barium-oxide clusters by x-ray absorption spectroscopy.

Small barium-oxide clusters [Formula: see text] are studied by mass spectrometry and x-ray absorption spectroscopy (XAS) to discuss stability of the clusters and oxidation state of constituent atoms. It is found that clusters with bulk composition, n = m, are stable, which can accommodate one or two excess oxygen atoms additionally as manifested by n = m + 1 and m + 2 species in the mass spectrum. XAS spectra of [Formula: see text] and [Formula: see text] reveal that the oxidation state of barium atoms stays at +2 (the bulk BaO value) even after binding excess oxygen, whereas spectral features originating from oxygen exhibit composition dependence. The present finding suggests that stoichiometric small barium-oxide clusters bind less-negatively-charged oxygen atoms without change in the charge state of barium.

Freezing of micrometer-sized liquid droplets of pure water evaporatively cooled in a vacuum.

Freezing processes are reported for pure-water droplets generated in a vacuum in the size range of 49-71 µm in diameter. The process is characterized for each size by measurement of a freezing curve, where the fraction of frozen droplets is evaluated as a function of time. The 49 µm droplet was found to freeze at a time between 7.0 and 7.9 ms after being generated at room temperature, where the fraction of frozen droplets increased from 5% to 95%; the freezing time was thus distributed statistically within 1 ms. The freezing time was retarded by about 3 ms as the size increases from 49 to 71 µm, while the rise time of the freezing curve was almost unchanged. Numerical simulation of a cooling curve, i.e., the temperature of a droplet as a function of time, revealed that the droplets in the present size range are frozen at almost the same temperature between 233 and 236 K. The freezing curves measured in the experiment were well reproduced by numerical simulation based on the simulated cooling curves combined with the temperature dependence of the volume-based homogeneous ice nucleation rates of pure water reported previously. It was also found that a droplet is disintegrated into a few fragments upon freezing, which suggests formation of a frozen shell in the outer region of a droplet.

The role of electronegativity on the extent of nitridation of group 5 metals as revealed by reactions of tantalum cluster cations with ammonia molecules.

Reactions of the free tantalum cation, Ta+, and tantalum cluster cations, Tan+ (n = 2-10), with ammonia are presented. The reaction of the monomer cation, Ta+, with two molecules of NH3 leads to the formation of TaN2H2+ along with release of two H2 molecules. The dehydrogenation occurs until the formal oxidation number of the tantalum atom reaches +5. On the other hand, all the tantalum cluster cations, Tan+, react with two molecules of NH3 and form TanN2+ with the release of three H2 molecules. Further exposure to ammonia showed that TanNmH+ and TanNm+ are produced through successive reactions; a pure nitride and three H2 molecules are formed for every other NH3 molecule. The nitridation occurred until the formal oxidation number of the tantalum atoms reaches +5 as in the case of TaN2H2+ in contrast to other group 5 elements, i.e., vanadium and niobium, which have been reported to produce nitrides with lower oxidation states. The present results on small gas-phase metal-nitride clusters show correlation with their bulk properties: tantalum is known to form bulk nitrides in the oxidation states of either +5 (Ta3N5) or +3 (TaN), whereas vanadium and niobium form nitrides in the oxidation state of +3 (VN and NbN). Along with DFT calculations, these findings reveal that nitridation is driven by the electron-donating ability of group 5 elements, i.e., electronegativity of the metal plays a key role in determining the composition of the metal nitrides.

Size-Dependent Ligand Quenching of Ferromagnetism in Co3(benzene)n+ Clusters Studied with X-ray Magnetic Circular Dichroism Spectroscopy.

Cobalt-benzene cluster ions of the form Co3(bz)n+ (n = 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 µS = 6 µB and an orbital magnetic moment of µL = 3 µB. 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)n+ species as a function of the number of attached benzene ligands, explaining the transition from septet to singlet for n = 0 → 3.

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, SinOm(-) (n = 3-7, 2n - 1 ≤ m ≤ 2n + 2), with a CO gas. Adsorption of CO on SinOm(-) is observed as a major reaction channel. The rate constant of the adsorption reaction is high for the oxygen-rich clusters with m ≥ 2n + 1, whereas almost no reaction product is observed for m ≤ 2n. 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 SinOm(-). 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.

Maximum spin polarization in chromium dimer cations as demonstrated by X-ray magnetic circular dichroism spectroscopy.

X-ray magnetic circular dichroism spectroscopy has been used to characterize the electronic structure and magnetic moment of Cr2 (+) . Our results indicate that the removal of a single electron from the 4sσg bonding orbital of Cr2 drastically changes the preferred coupling of the 3d electronic spins. While the neutral molecule has a zero-spin ground state with a very short bond length, the molecular cation exhibits a ferromagnetically coupled ground state with the highest possible spin of S=11/2, and almost twice the bond length of the neutral molecule. This spin configuration can be interpreted as a result of indirect exchange coupling between the 3d electrons of the two atoms that is mediated by the single 4s electron through a strong intraatomic 3d-4s exchange interaction. Our finding allows an estimate of the relative energies of two states that are often discussed as ground-state candidates, the ferromagnetically coupled (12) Σ and the low-spin (2) Σ state.

Water-induced adsorption of carbon monoxide and oxygen on the gold dimer cation.

It is demonstrated, using tandem mass spectrometry and ion trap, that preadsorption of a H2O molecule on the gold dimer cation, Au2(+), enhances adsorption of CO and O2 molecules, which is otherwise inert toward these molecules. The rate of adsorption of CO on Au2(H2O)(+) was found to be higher by 2 orders of magnitude relative to bare Au2(+). The enhancement of the CO adsorption rate is due to the presence of a reaction channel, which cleaves the Au-Au bond, leading to the formation of Au(H2O)(CO)(+). Such an observation can be attributed to weakening of the Au-Au bond upon adsorption of a water molecule. Further, it was also observed that preadsorption of H2O leads to dramatic enhancement of O2 adsorption on the Au2(+) ion, which can be attributed to the changes in the electron density following water adsorption.

Probing structures of small gold cluster cations with dinitrogen.

Small gold cluster cations, Au(n)(+), adsorb N(2) molecules effectively under multiple collision conditions. The saturation number for N(2) adsorption on a gold cluster cation depends on the number of dangling gold atoms and the triangular apexes. The saturation numbers were interpreted to arrive at the geometrical structures of gold cluster cations. The gold cluster cations of sizes 3 and 6 have single isomers, whilst cluster cations with 4, 5, and 7 atoms have two isomers. For the gold cluster cation with six atoms, a higher-energy and lower-symmetry structure was observed. The gold cluster cations with eight or more atoms do not adsorb N(2), which can be attributed to a two-dimensional to three-dimensional structural transition at n = 8.

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.

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.