An integrated instrument of DUV-IR photoionization mass spectrometry and spectroscopy for neutral clusters

Tailoring Titanium Carbide Clusters for New Materials: from Met-Cars to Carbon-Doped Superatoms

Tailoring materials with prescribed properties and regular structures is a critical and challenging research topic. Early transition metals were found to form supermagic M8C12 metallocarbohedrenes (Met-Cars); however, stable metal carbides are not limited to this common stoichiometry. Utilizing self-developed deep-ultraviolet laser ionization mass spectrometry, here, we report a strategy to generate new titanium carbides by reacting pure Tin clusters with acetylene. Interestingly, two products corresponding to Ti17C2 and Ti19C10 exhibit superior abundances in addition to the Ti8C12 Met-Cars. Using global-minimum search, the structures of Ti17C2 and Ti19C10 are determined to be an ellipsoidal D4d and a rod-shaped D5h geometry, respectively, both with carbon-capped Ti4C moieties and superatomic features. We illustrate the electronic structures and bonding nature in these carbon-doped superatoms concerning their enhanced stability and local aromaticity, shedding light on a new class of metal-carbide nanomaterials with atomic precision.

On the nature of Con±/0 clusters reacting with water and oxygen

Bulk cobalt does not react with water at room temperature, but cobalt nanometals could yield corrosion at ambient conditions. Insights into the cobalt cluster reactions with water and oxygen enable us to better understand the interface reactivity of such nanometals. Here we report a comprehensive study on the gas-phase reactions of Con±/0 clusters with water and oxygen. All these Con±/0 clusters were found to react with oxygen, but only anionic cobalt clusters give rise to water dissociation whereas the cationic and neutral ones are limited to water adsorption. We elucidate the influences of charge states, bonding modes and dehydrogenation mechanism of water on typical cobalt clusters. It is unveiled that the additional electron of anionic Con- clusters is not beneficial to H2O adsorption, but allows for thermodynamics- and kinetics-favourable H atom transfer and dehydrogenation reactions. Apart from the charge effect, size effect and spin effect play a subtle role in the reaction process. The synergy of multiple metal sites in Con- clusters reduces the energy barrier of the rate-limiting step enabling hydrogen release. This finding of water dissociation on cobalt clusters put forward new connotations on the activity series of metals, providing new insights into the corrosion mechanism of cobalt nanometals.

To What Extent Do Iodomethane and Bromomethane Undergo Analogous Reactions?

Iodomethane and bromomethane (CH3I/CH3Br) are common chemicals, but their chemistry on nanometals is not fully understood. Here, we analyze the reactivity of Rhn+ (n = 3-30) clusters with halomethanes and unveil the spin effect and concentration dependence in the C-H and C-X bond activation. It is found that the reactions under halomethane-rich conditions differ from those under metal-rich conditions. Both CH3I and CH3Br undergo similar dehydrogenation on the Rhn+ clusters in the presence of small quantity reactants; however, different reactions are observed in the presence of sufficient CH3I/CH3Br, showing dominant Rh(CH3Br)x+ (x = 1-4) products but a series of RhnCxHyIz+ species (x = 1-4, y = 1-12, and z = 1-5) pertaining to H2, HI, or CH4 removal. Density functional theory calculations reveal that the dehydrogenation and demethanation of CH3Br are relatively less exothermic and will be deactivated by sufficient gas collisions if helium cooling takes away energy immediately; instead, the successive adsorption of CH3Br gives rise to a series of Rh(CH3Br)x+ species with accidental C-Br bond dissociation.

Unusually High-Spin Fe12C12- Metallo-Carbohedrene Clusters

Ferromagnets constructed from nanometals of atomic precision are important for innovative advances in information storage, energy conversion, and spintronic microdevices. Considerable success has been achieved in designing molecular magnets, which, however, are challenging in preparation and may suffer from drawbacks on the incompatibility of high stability and strong ferromagnetism. Utilizing a state-of-the-art self-developed mass spectrometer and a homemade laser vaporization source, we have achieved a highly efficient preparation of pure iron clusters, and here, we report the finding of a strongly ferromagnetic metal-carbon cluster, Fe12C12-, simply by reacting the Fen- clusters with acetylene in proper conditions. The unique stability of this ferromagnetic Fe12C12- cluster is rooted in a plumb-bob structure pertaining to Jahn-Teller distortion. We classify Fe12C12- as a new member of metallo-carbohedrenes and elucidate its structural stability mechanism as well as its soft-landing deposition and magnetization measurements, providing promise for the exploration of potential applications.

The s-p Nonhybrid Nature Causes Adaptive Superatomic States of Bismuth Clusters

We report a joint experimental and theoretical study on the stability and reactivity of Bin + (n=5-33) clusters. The alternating odd-even effect on the reaction rates of Bin + clusters with NO is observed, and Bi7 + finds the most inertness. First-principles calculation results reveal that the lowest energy structures of Bi6-9 + exhibit quasi-spherical geometry pertaining to the jellium shell model; however, the Bin + (n≥10) clusters adopt assembly structures. The prominent stability of Bi7 + is associated with its highly symmetric structure and superatomic states with a magic number of 34e closed shell. For the first time, we demonstrate that the unique s-p nonhybrid feature in bismuth rationalizes the stability of Bi6-9 + clusters within the jellium model, by filling the 6s electrons into the superatomic orbitals (forming "s-band"). Interestingly, the stability of 18e "s-band" coincides with the compact structure for Bin + at n≤9 but assembly structures for n≥10, showing an accommodation of the s electrons to the geometric structure. The atomic p-orbitals also allow to form superatomic orbitals at higher energy levels, contributing to the preferable structures of tridentate binding units. We illustrate the s-p nonhybrid nature accommodates the structure and superatomic states of bismuth clusters.

Rh19-: A high-spin super-octahedron cluster

Probing atomic clusters with magic numbers is of supreme importance but challenging in cluster science. Pronounced stability of a metal cluster often arises from coincident geometric and electronic shell closures. However, transition metal clusters do not simply abide by this constraint. Here, we report the finding of a magic-number cluster Rh19- with prominent inertness in the sufficient gas-collision reactions. Photoelectron spectroscopy experiments and global-minimum structure search have determined the geometry of Rh19- to be a regular Oh‑[Rh@Rh12@Rh6]- with unusual high-spin electronic configuration. The distinct stability of such a strongly magnetic cluster Rh19- consisting of a nonmagnetic element is fully unveiled on the basis of its unique bonding nature and superatomic states. The 1-nanometer-sized Oh-Rh19- cluster corresponds to a fragment of the face-centered cubic lattice of bulk rhodium but with altered magnetism and electronic property. This cluster features exceptional electron-spin state isomers confirmed in photoelectron spectra and suggests potential applications in atomically precise manufacturing involving spintronics and quantum computing.

An integrated instrument of a tandem quadrupole mass spectrometer for cluster reaction and soft-landing deposition

We have developed an integrated instrument system of a multiple-ion laminar flow tube (MIFT) reactor combined with a tandem quadrupole mass spectrometer (TQMS) and soft-landing deposition (SD) apparatus. A customized water-cooling magnetron sputtering (MagS) source is designed, by which we are able to attain a highly efficient preparation of metal clusters of 1-30 atoms with tunable size distributions. Following the MagS source, a laminar flow tube reactor is designed, allowing for sufficient gas-collision reactions of the as-prepared metal clusters, which is advantageous for probing magic clusters and minimizing wall effects when probing the reaction dynamics of such clusters. The customized TQMS analyzer involves a conical octupole, two linear octupoles, a quadruple ion deflector, and a 19 mm quadruple mass analyzer, allowing to decrease the pressure stepwise (from ∼5 to ∼10^-9 Torr), thus ensuring high sensitivity and high resolution of the mass spectrometry analysis. In addition, we have designed a dual SD apparatus for the mass-selected deposition of clusters and their reaction products. For the whole system, abbreviated as MagS-MIFT-TQMS-SD, we have performed a detailed ions-fly simulation and quantitatively estimated the ions transfer efficiency under vacuum conditions determined by real experiments. Taking these advantages, well-resolved Pb_n ⁺, Ag_n ⁺, and Nb_n ⁺ clusters have been produced, allowing for meticulous studies of cluster reactions under sufficient gas-phase collisions free of electric field trapping. Also, we have tested the efficiency of the dual SD.

What Determines the Drastic Reactivity of Nbn+ Clusters with Nitric Oxide under Thermalized Conditions?

We report an in-depth study of the adsorption and reaction of NO with cationic Nb_n⁺ (n = 1-20) clusters under thermalized conditions in a laminar flow tube reactor in tandem with a customized triple quadrupole mass spectrometer (FT-TQMS). It is found that the small-sized Nb_n⁺ clusters (2 ≤ n ≤ 7) readily react with NO giving rise to dominant fragmentation products pertaining to the loss of a stable diatomic molecule NbO or NbN. In contrast, the reaction products of larger-sized clusters (n ≥ 10) proceed through diverse channels, including NO adsorption, N₂/N₂O release, and even NO₂ formation. These experimental observations provided the incentive for us to dig deep into the reaction mechanism with the help of DFT calculations. In contrast to the NO-donation coordination in transition metal complexes, here the cationic Nb_n⁺ clusters exhibit dominant electronic donation in initiating the reactions with NO molecules. We fully demonstrated the reaction rate constants, compared the reaction energy diagram of typical Nb_n⁺ clusters, and unveiled the distinct interaction mechanism of niobium clusters available for NO activation and conversion.

Plasma-Assisted Dinitrogen Activation on Small Cobalt Clusters: Co4 N9 + with Enhanced Stability

We have performed a study on the accommodation of nitrogen doping toward superatomic states of transition metal clusters. By reacting cobalt clusters with N₂ in the presence of plasma radiation, a large number of odd-nitrogen clusters were observed, typically Co₃ N_2m-1 ⁺ (m=1-5) and Co₄ N_2m-1 ⁺ (m=1-6) series, showing N≡N bond cleavage in the mild plasma atmosphere. Interestingly, the Co₃ N₇ ⁺ , Co₄ N₉ ⁺ , and Co₅ N₉ ⁺ clusters exhibit prominent mass abundances. First-principles calculation results elucidate the stability of the diverse cobalt nitride clusters and find unique stability of Co₄ N₉ ⁺ with a swallow-kite structure of which four coordinated N₂ molecules causes a significantly enlarged HOMO-LUMO gap, while the single N atom doping gives rise to superatomic states of 1S² 1P³ ||1D⁰ . We reveal an efficient dinitrogen activation strategy by reacting multiple N₂ molecules with cobalt clusters under a plasma atmosphere.

Cyclotrimerization of Acetylene on Clusters Con+/Fen+/Nin+(n = 1-16)

Cyclotrimerization of acetylene to benzene has attracted significant interest, but the role of geometric and electronic effects on catalytic chemistry remains unclear. To fully elucidate the mechanism of catalytic acetylene-to-benzene conversion, we have performed a gas-phase reaction study of the Fe_n⁺, Co_n⁺, and Ni_n⁺ (n = 1-16) clusters with acetylene utilizing a customized mass spectrometer. It is found that their reactions with acetylene are initiated by C₂H₂ molecular adsorption and allow for dominant dehydrogenation with the relatively low partial pressure of the acetylene gas. However, at high acetylene concentrations, the cyclotrimerization in M_n⁺ + 3C₂H₂ (M = Fe, Co, Ni) becomes the dominant reaction channel. We demonstrate theoretically the favorable thermodynamics and reaction dynamics leading to the formation of the M⁺(C₆H₆) products. The results are discussed in terms of a cluster-catalyzed multimolecule synergistic effect and the cation-π interactions.

Probing Cluster-π Interactions between Cun- and C2H2/C2H4 for Gas Separation

Copper-related materials are used for separation of ethylene and acetylene gases in chemistry; however, the precise mechanism regarding selectivity is elusive to be fully understood. Here, we have conducted a joint experimental and theoretical study of the Cu_n^- (n = 7-30) clusters in reacting with C₂H₄ and C₂H₂. It is found that all of the Cu_n^- clusters readily react with C₂H₂, giving rise to C₂H₂-addition products; however, Cu₁₈^- and Cu₁₉^- do not react with C₂H₄. We illustrate the superatomic stability of Cu₁₈^- and advocate its availability to separate C₂H₄ from C₂H₂. Further, we demonstrate the atomically precise mechanism regarding selectivity by fully unveiling the size-dependent cluster-π interactions.

Tetrahedral Pt10- Cluster with Unique Beta Aromaticity and Superatomic Feature in Mimicking Methane

Utilizing a customized metal cluster source in tandem with a flow tube reactor and a reflectron time-of-flight mass spectrometer, we have obtained well-resolved pure metal clusters Pt_n^- and observed their gas-phase reactions with a few small gas molecules. Interestingly, the remarkable inertness of Pt₁₀^- was repeatedly observed in different reactions. Meanwhile, we have determined the structure of Pt₁₀^- within a regular tetrahedron. Considering that Pt possesses 5d⁹6s¹ electron configuration, the tetrahedral Pt₁₀^- exhibits unexpected stability at neither a magic number of valence electrons nor a shell closure of geometric structure. Comprehensive theoretical calculations unveil the stability of Pt₁₀^- is significantly associated with the all-metal aromaticity. In addition to the classical total aromaticity, which is mainly due to 6s electrons, there is unique beta-aromaticity ascribed to spin-polarized beta 5d electrons pertaining to singly occupied multicenter bonds. Further, we demonstrate the superatomic feature of such a transition metal cluster Pt₁₀^-, as Pt₆@Pt₄^-, in mimicking methane.

Reactivity of Cobalt Clusters Con±/0 with Dinitrogen: Superatom Co6+ and Superatomic Complex Co5N6. J Phys Chem A 2021;125:2130-2138. [PMID: 33689326 DOI: 10.1021/acs.jpca.1c00483]

We report a joint experimental and theoretical study on the reactions of cobalt clusters (Co_n^±/0) with nitrogen using the customized reflection time-of-flight mass spectrometer combined with a 177.3 nm deep-ultraviolet laser. Comparing to the behaviors of neutral Co_n (n = 2-30) and anionic Co_n^- clusters (n = 7-53) which are relatively inert in reacting with nitrogen in the fast-flow tube, Co_n⁺ clusters readily react with nitrogen resulting in adducts of one or multiple N₂ except Co₆⁺ which stands firm in the reaction with nitrogen. Detailed quantum chemistry calculations, including the energetics, electron occupancy, and orbital analysis, well-explained the reasonable reactivity of Co_n⁺ clusters with nitrogen and unveiled the open-shell superatomic stability of Co₆⁺ within a highly symmetric (D_3d) structure. The D_3d Co₆⁺ bears an electron configuration of a half-filled superatomic 1P orbital (i.e., 1S²1P³||1D⁰), a large α-highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, symmetric multicenter bonds, and reasonable electron delocalization pertaining to metallic aromaticity. Topology analysis by atom-in-molecule illustrates the interactions between Co_n⁺ and N₂ corresponding to covalent bonds, but the Co-N interactions in cationic Co₂⁺N₂ and Co₆⁺N₂ clusters are apparently weaker than those in the other systems. In addition, we identify a superatomic complex Co₅N₆⁺ which exhibits similar frontier orbitals as the naked Co₅⁺ cluster, but the alpha HOMO-LUMO gap is nearly double-magnified, which is consistent with the high-abundance peak of Co₅N₆⁺ in the experimental observation. The enhanced stability of such a ligand-coordinated superatomic complex Co₅N₆⁺, along with the superatom Co₆⁺ with aromaticity, sheds light on special and general superatoms.

Vanadium Cluster Neutrals Reacting with Water: Superatomic Features and Hydrogen Evolution in a Fishing Mode

Hydrogen evolution reaction (HER) is known as the heart of various energy storage and conversation systems of renewable energy sources. Here we observe the cluster reactions of a light transition metal, vanadium, with water in a gas-phase flow tube reactor. While HER products of V₁ and V₂ were not observed, the effective HER of water on neutral V_n (n ≥ 3) clusters reveals reasonable and size-dependent reactivity of the vanadium clusters. Superatomic features and reaction dynamics of V₁₀, V₁₃, and V₁₆ are highlighted. Among the three typical superatoms, V₁₀ and V₁₆ exhibit an abnormal superatomic orbital energy level order, 1S|2S|1P|1D..., where the energy-reduced 2S orbital helps to accommodate the geometric structure and hence reinforce the cluster stability. In comparison, V₁₃ bears a less symmetrical structure and reacts readily with water, allowing for recombination of a hydroxyl atom with an adsorbed hydrogen atom, akin to a fishing-mode HER process. The joint experimental and theoretical study on neutral V_n clusters clarifies the availability of superatom chemistry for transition metals and appeals further development of cluster theory based on electronic cloud/orbital analysis instead of simply counting the valence electrons. Also, we provide insights into the HER mechanism of metal clusters and propose a strategy to design new materials for portable fuel cells of hydrogen energy.

An oxygen-passivated vanadium cluster [V@V10O15]− with metal–metal coordination produced by reacting Vn− with O2

An oxygen-passivated vanadium cluster [V@V₁₀O₁₅]⁻ is reported by reacting V_n⁻ with O₂, giving rise to superatom features of metal–metal coordination and 3D aromaticity.

Gas-phase preparation and the stability of superatomic Nb11O15. Phys Chem Chem Phys 2021;23:15766-15773. [PMID: 34286767 DOI: 10.1039/d1cp02128a]

We report a study of the reactions of pure metal clusters Nbn- with dioxygen in the gas phase. It is found that the presence of low-concentration dioxygen reactants results in oxygen-addition products, whereas sufficient high-concentration dioxygen enables oxygen-etching reactions giving rise to molecular niobium oxides. Interestingly, in the presence of a suitable gas flow rate of an intermediate dioxygen concentration, a highly selective product Nb11O15- shows up in the mass spectra. Utilizing density functional theory (DFT) calculations, we have discussed the reactivities of Nbn- (3 ≤ n ≤ 14) clusters with oxygen, and unveiled the reasonable stability of Nb11O15- pertaining to its unique geometric structure with a D5h Nb@Nb10 core fully protected by 15 bridge-oxygen atoms. The oxygen-passivated Nb@Nb10O15- cluster exhibits a large HOMO-LUMO gap (1.46 eV) and effective multicenter bonds with remarkable superatom orbitals for all the 26 valence electrons of the Nb@Nb10 core corresponding to well-staggered energy levels. We illustrate the superatomic features in the Nb@Nb10 metallic core for which the adaptive natural density partitioning (AdNDP) analysis unveils thirteen 11c-2e bonds. Among them, one of the 11c-2e bonds accounts for the superatomic S orbital, three bonds correspond to superatomic P orbitals, another five display vivid D orbital characteristics, and the remaining four 11c-2e bonds are assigned to F orbital features. In addition, the net atomic charge of the center Nb atom is as high as -0.804 |e| rendering core-shell electrostatic interactions and the shielding effect of the Nb10O15 shell.

Hydrogen release from a single water molecule on Vn+ (3 ≤ n ≤ 30)

Water and its interactions with metals are closely bound up with human life, and the reactivity of metal clusters with water is of fundamental importance for the understanding of hydrogen generation. Here a prominent hydrogen evolution reaction (HER) of single water molecule on vanadium clusters V_n⁺ (3 ≤ n ≤ 30) is observed in the reaction of cationic vanadium clusters with water at room temperature. The combined experimental and theoretical studies reveal that the wagging vibrations of a V-OH group give rise to readily formed V-O-V intermediate states on V_n⁺ (n ≥ 3) clusters and allow the terminal hydrogen to interact with an adsorbed hydrogen atom, enabling hydrogen release. The presence of three metal atoms reduces the energy barrier of the rate-determining step, giving rise to an effective production of hydrogen from single water molecules. This mechanism differs from dissociative chemisorption of multiple water molecules on aluminium cluster anions, which usually proceeds by dissociative chemisorption of at least two water molecules at multiple surface sites followed by a recombination of the adsorbed hydrogen atoms.

Plasma-Assisted Chain Reactions of Rh3+ Clusters with Dinitrogen: N≡N Bond Dissociation

Dinitrogen activation is known as one of the most challenging subjects in chemistry. A number of well-defined metal complexes, nitrides, and clusters have been studied that show catalysis for dinitrogen activation. However, direct evidence of a complete cleavage of the N≡N triple bond at mild conditions is rather limited to date. Herein, we report a study on the dissociation of N₂ on small rhodium clusters assisted by surface plasma radiation. From mass spectrometry observation, a few rhodium nitride clusters with an odd number of nitrogen atoms are produced, such as the Rh₃N_2m-1⁺ (m = 1-5) series, indicative of N≡N bond dissociation in the mild plasma atmosphere. Interestingly, Rh₃N₇⁺ is identified with outstanding mass abundance among the Rh_nN_2m-1⁺ products, and its ground-state structure is in the form of Rh₃N(N₂)₃⁺ by capping a nitrogen atom on the top of Rh₃⁺ plane and hanging three N₂ molecules beneath the three Rh atoms respectively, giving rise to a C_3v symmetry and excellent stability. We demonstrate the catalysis of such a three-atom rhodium cluster and reveal a dinitrogen activation strategy by thermodynamics- and dynamics- favorable chain reactions of multiple N₂ molecules with two rhodium clusters under plasma atmosphere.

Co13O8—metalloxocubes: a new class of perovskite-like neutral clusters with cubic aromaticity

Exploring stable clusters to understand structural evolution from atoms to macroscopic matter and to construct new materials is interesting yet challenging in chemistry. Utilizing our newly developed deep-ultraviolet laser ionization mass spectrometry technique, here we observe the reactions of neutral cobalt clusters with oxygen and find a very stable cluster species of Co₁₃O₈ that dominates the mass distribution in the presence of a large flow rate of oxygen gas. The results of global-minimum structural search reveal a unique cubic structure and distinctive stability of the neutral Co₁₃O₈ cluster that forms a new class of metal oxides that we named as ‘metalloxocubes’. Thermodynamics and kinetics calculations illustrate the structural evolution from icosahedral Co₁₃ to the metalloxocube Co₁₃O₈ with decreased energy, enhanced stability and aromaticity. This class of neutral oxygen-passivated metal clusters may be an ideal candidate for genetic materials because of the cubic nature of the building blocks and the stability due to cubic aromaticity.

Reactivity of Cobalt Clusters Con±/0 with Ammonia: Co3+ Cluster Catalysis for NH3 Dehydrogenation

A customized reflection time-of-flight (Re-TOF) mass spectrometer combined with a 177 nm deep-ultraviolet laser has enabled us to observe well-resolved cobalt clusters Co_n^±/0 and perform a comprehensive study of their reactivity with ammonia (NH₃). The anions Co_n^- are found to be inert, the neutrals allow the adsorption of multiple NH₃ molecules, while the cationic Co_n⁺ clusters readily react with NH₃ giving rise to dehydrogenation. However, incidental dehydrogenation of NH₃ on Co_n⁺ is only observed for n ≥ 3. The dramatic charge- and size-dependent reactivities of Co_n^±/0 clusters with NH₃ are studied by the density functional theory (DFT)-calculation results of energetics, density of states, orbital interactions, and reaction dynamics. We illustrate the dehydrogenation from two NH₃ molecules, where a significantly reduced transition-state energy barrier is found pertaining to the dimolecular co-catalysis effect. The reactivity of Co₃⁺ with NH₃ is illustrated showing effective catalysis for N-H dissociation to produce hydrogen applicable for designing ammonia fuel cells.

Anionic Copper Clusters Reacting with NO: An Open-Shell Superatom Cu18. J Phys Chem Lett 2020;11:5807-5814. [PMID: 32597656 DOI: 10.1021/acs.jpclett.0c01643]

Gas-phase metal clusters have been a subject of research interest for allowing reliable strategies to explore the stability and reactivity of materials at reduced sizes with atomic precision. Here we have prepared well-resolved copper cluster anions Cu_n^- (n = 7-37) and systematically studied their reactivity with O₂, NO, and CO. We found remarkable stability of an open-shell cluster Cu₁₈^-, which is comparable with the closed-shell clusters Cu₁₇^- and Cu₁₉^- within the picture of an electronic shell model. Even without having a magic number of valence electrons, intriguingly, the unpaired electron on the singly occupied molecular orbital of Cu₁₈^- is mainly contributed by the central copper atom, while the other 18 delocalized valence electrons occupy the lower-energy superatomic orbitals of the cluster. The finding of such an open-shell superatom Cu₁₈^-, with an electron configuration of 1S²1P⁶1D¹⁰2S¹||1F⁰, is interesting in the sense that an elementary cluster of coinage metal atoms could still behave as a superatom mimicking coinage metals like silver or gold atoms with an empty f orbital. The superatomic stability of this Cu₁₈^- cluster is reinforced by the unique electrostatic interaction between the Cu^- core and Cu₁₇ shell, which provides new insights into the chemistry of metal clusters.

Infrared + vacuum ultraviolet two-color ionization spectroscopy of neutral metal complexes based on a tunable vacuum ultraviolet free-electron laser

This paper describes an experimental technique for studying neutral metal complexes using infrared + vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy based on a tunable VUV free-electron laser (VUV-FEL). The preliminary IR spectroscopy results of mass-selected nickel tetracarbonyl are reported in this work. The results demonstrate that the tunable VUV-FEL light allows the selective ionization of a given neutral cluster free of confinement along with the recording of well-resolved IR spectra. As the ionization energies of many neutral clusters are accessible by a broadly tunable VUV-FEL (50-150 nm) and near-threshold ionization can be readily achieved, the proposed experimental method offers unique possibilities for the size-specific study of a wide variety of confinement-free neutral clusters.