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In the past few decades, a development of organic magnets with room-temperature strong ferromagnetism is challenged by the difficulty of creating three-dimensional (3D) long-range magnetic orderings in organic materials at a temperature higher than room temperature. Here, we report room-temperature ferrimagnetism of a tetragonal organic–inorganic hybrid Fe14Se16(tepa)III (tepa = tetraethylenepentamine), where III represents a coordination of a tepa molecule with a Fe3+ ion for an organic complex. The layered hybrid in a nanoplate-like shape is formed by periodic incorporation of tetragonal β-Fe3Se4 inorganic layers and organic spacing layers consisting of tepa and Fe3+. Fe14Se16(tepa)III shows a saturation magnetization MS of 7.2 emu g−1 at 300 K and a record-high Néel temperature TN (>560 K) in the organic magnets reported experimentally. A Mössbauer spectrum confirms a 3D long-range magnetic ordering of Fe2+ [S = 2 (71.4%)] and Fe3+ ions [S = 5/2 (21.7%) and 1/2 (4.0%)] in β-Fe3Se4 layers and organic spacing layers of Fe14Se16(tepa)III,9. First-principles calculations explain that the 3D long-range antiferromagnetic interactions between interlayer and intralayer irons result in the strong ferrimagnetism of Fe14Se16(tepa)III. This study unveils the possibility of tuning magnetic couplings of interlayer and intralayer high-spin Fe3+ and Fe2+ for enhancing the ferrimagnetism of layered hybrids and, thus, for future room-temperature magnetic/spintronic applications. [ABSTRACT FROM AUTHOR]

The fragmentation of ion clusters within the accelerating fields of ionic liquid ion sources (ILISs) is well documented and degrades ILIS performance and lifetime. Some of the most popular ILIS liquids, such as EMI-BF4 (1-Ethyl-3-methylimidazolium tetrafluoroborate) and EMI-Im (1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), emit clusters with lifetimes as low as ∼1 ns. Studies of fragmentation within the accelerating field typically rely on measuring the plume energy distribution averaged over all plume species and comparing those measurements with numerical simulations to estimate ion cluster lifetimes. Here, for the first time, we estimate EMI-BF4 cluster lifetimes by analyzing the energy distributions of individual plume species. We use this novel analysis method to estimate mean lifetimes of positive EMI-BF4 ion clusters from previously published experimental data. We find that the mean lifetime ranges from τ = 3.7 ns to τ = 124 ns for [ EM I + ] [ EMI - B F 4 ] dimers and ranges from τ = 1.5 ns to τ = 23 ns for [ EM I + ] [ EMI - B F 4 ] 2 trimers. Fitting those data to an analytical fragmentation model, we estimate the binding energy and temperature as Δ G S 0 = 0.49 eV and T = 394 K for dimers and Δ G S 0 = 0.40 eV and T = 365 K for trimers. Comparing our results with previous studies supports the conclusion that clusters are emitted with a wide distribution of internal energies, contrary to the common assumption of single internal energy for each species. [ABSTRACT FROM AUTHOR]

A porous electrospray ion source was tested with three ionic liquids in order to investigate the effects of ionic liquid properties on the sizes of ion clusters emitted by purely ionic electrospray sources. Two of the ionic liquids, bis(1-ethyl-3-methylimidazolium) tetrathiocyanatocobaltate and 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide, were selected due to them having a dication or dianion, which were termed multiply charged ionic liquids due to them containing anions or cations with more than one charge within them. These were selected in order to investigate ionic clustering within electrospray ion emission and were compared against one of the most common ionic liquids, EMI-BF4. The current–voltage data showed that EMI-BF4 emitted similar levels of current to bis(1-ethyl-3-methylimidazolium) tetrathiocyanatocobaltate, even though the latter liquid had significantly lower conductivity and higher viscosity, suggesting an improvement in current speculated to be due to the extra charges contained by the ions. Time-of-flight and retarding potential analysis data are provided, showing all three liquids emitting ions comprising of monomers, dimers, trimers, and quadramers, with some of the 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide data showing heavier species emission. The data also suggested that the multiply charged ionic liquids produced ions that have two anions or cations attached, termed "double ions," with these ions not been previously reported using porous electrospray sources. Furthermore, it was found that the dimers emitted by both of the multiply charged ionic liquids seemed to be more stable than EMI-BF4 dimers, providing insight into ion cluster formation. [ABSTRACT FROM AUTHOR]

The outstanding radiation damage stability of an NiCoCrFePd high entropy alloy (HEA) as compared to conventional alloys poses the question for the mechanism of an ion–matter interaction. The positron annihilation lifetime spectroscopic and TEM (transmission electron microscopic) measurements are implemented to trace different kinds of defects produced by 120 MeV Au+9 ion irradiation and their evolution as a function of ion fluence. The variation of lifetimes and corresponding intensities with the ion fluence indicates the formation of dislocation-type defects at a lower ion fluence and vacancy clusters at a higher ion fluence caused by coalescence or agglomeration of dislocation defects. Formation of different types of defects in turn modulates the strain development inside the crystal. Additionally, the HR-TEM investigation of NiCoCrFePd HEA also exhibits the formation of dislocation and vacancy clusters with the average size of vacancy clusters increases from ∼2.9 ± 0.1 to ∼3.8 ± 0.1 nm with the increases in the ion fluence. Surprisingly, the average defect cluster size in NiCoCrFePd HEA is suppressed compared to NiCoCrFe MEA, thereby showing the enhanced radiation stability on Pd incorporation due to the high defect recombination caused by reduced thermal conductivity and high lattice distortion. Nano-indentation measurement shows that the radiation hardening behavior of the NiCoCrFePd HEA responded slowly owing to its damage suppression property as compared to the NiCoCrFe MEA. Additionally, softening behavior also appeared at an early fluence in NiCoCrFe MEA compared to the NiCoCrFePd HEA signifying its excellent resistance to defect accumulation. [ABSTRACT FROM AUTHOR]

Molecular cluster ions, which are stored in an electromagnetic trap under ultra-high vacuum conditions, undergo blackbody infrared radiative dissociation (BIRD). This process can be simulated with master equation modeling (MEM), predicting temperature-dependent dissociation rate constants, which are very sensitive to the dissociation energy. We have recently introduced a multiple-well approach for master equation modeling, where several low-lying isomers are taken into account. Here, we experimentally measure the BIRD of CO4●–(H2O)1,2 and model the results with a slightly modified multiple-well MEM. In the experiment, we exclusively observe loss of water from CO4●–(H2O), while the BIRD of CO4●–(H2O)2 leads predominantly to loss of carbon dioxide, with water loss occurring to a lesser extent. The MEM of two competing reactions requires empirical scaling factors for infrared intensities and the sum of states of the loose transition states employed in the calculation of unimolecular rate constants so that the simulated branching ratio matches the experiment. The experimentally derived binding energies are ΔH0(CO4●––H2O) = 45 ± 3 kJ/mol, ΔH0(CO4●–(H2O)–H2O) = 41 ± 3 kJ/mol, and ΔH0(CO2–O2●–(H2O)2) = 37 ± 3 kJ/mol. Quantum chemical calculations on the CCSD(T)/aug-cc-pVTZ//CCSD/aug-cc-pVDZ level, corrected for the basis set superposition error, yield binding energies that are 2–5 kJ/mol higher than experiment, within error limits of both experiment and theory. The relative activation energies for the two competing loss channels are as well fully consistent with theory. [ABSTRACT FROM AUTHOR]

The abundance of extraterrestrial methanol makes the reaction between methanol molecules in a molecular cluster a possible key step in the search for mechanisms for the formation of more complex molecules under the conditions of the interstellar medium as well as circumstellar and planetary atmospheres. The reaction leading to the formation of the dimethyl ether ion from a methanol molecule interacting with a protonated methanol ion via the elimination of a water molecule is a basic mechanism for the formation of complex organic molecules. Here, we experimentally examine such reactions in the gas phase, analyzing the production and reactivity of protonated cluster ions formed by the ionization of a supersonic jet of methanol. Focusing especially on the post-collisional relaxation of the protonated methanol dimer and trimer ions after high-energy single collisions, the results indicate a strong size selectivity favoring the occurrence of this reaction only in the dimer ion. To elucidate this behavior, the velocity distribution of the eliminated water molecule was measured using an event-by-event coincidence analysis. These results are interpreted using quantum chemical calculations of the dissociation pathways. It turns out that in the dimer case, two transition states are able to contribute to this intracluster reaction. In the trimer case, methanol evaporation appears as the most energetically favorable relaxation pathway. [ABSTRACT FROM AUTHOR]

Intermolecular Coulombic decay or electron transfer-mediated decay are the autoionization processes through which a molecule can relax. This relaxation is only possible if the inner valence's ionization potential (IP) exceeds the system's double ionization potential (DIP). To study the effects of charge and solvation shell, we have calculated the IP, DIP values, and lifetime of Na-2s and Mg-2s temporary bound states in various optimized structures of Na+-(H2O)n and Mg2+-(H2O)n (n = 1–5) micro-solvated clusters, where n water molecules are distributed in a way that some are directly bound to the metal ion and the rest to the water molecules. The first and second solvation shells are the names for the former and the latter water-binding positions, respectively. For a given n, the lifetime of decaying states is longer when water molecules are in the second solvation shell. We found that the Mg-2p state can decay for all n values in Mg2+-(H2O)n clusters, whereas the Na-2p state's decay is possible for n ≥ 2 in Na+-(H2O)n clusters. Our findings highlight the influence of metal ions' charge, different solvation shell structures, and the number of water molecules on the decay rate. These systems are relevant to the human body, which makes this study significant. [ABSTRACT FROM AUTHOR]

Ultrafast transient vibrational action spectra of cryogenically cooled Re(CO)3(CH3CN)3+ ions are presented. Nonlinear spectra were collected in the time domain by monitoring the photodissociation of a weakly bound N2 messenger tag as a function of delay times and phases between a set of three infrared pulses. Frequency-resolved spectra in the carbonyl stretch region show relatively strong bleaching signals that oscillate at the difference frequency between the two observed vibrational features as a function of the pump–probe waiting time. This observation is consistent with the presence of nonlinear pathways resulting from underlying cross-peak signals between the coupled symmetric–asymmetric C≡O stretch pair. The successful demonstration of frequency-resolved ultrafast transient vibrational action spectroscopy of dilute molecular ion ensembles provides an exciting, new framework for the study of molecular dynamics in isolated, complex molecular ion systems. [ABSTRACT FROM AUTHOR]

We report a joint negative ion photoelectron spectroscopy (NIPES) and quantum chemical computational study on glycine-chloride/bromide complexes (denoted Gly·X−, X = Cl/Br) in close comparison to the previously studied Gly·I− cluster ion. Combining experimental NIPE spectra and theoretical calculations, various Gly·X− complexes were found to adopt the same types of low-lying isomers, albeit with different relative energies. Despite more congested spectral profiles for Gly·Cl− and Gly·Br−, spectral assignments were accomplished with the guidance of the knowledge learned from Gly·I−, where a larger spin–orbit splitting of iodine afforded well-resolved, recognizable spectral peaks. Three canonical plus one zwitterionic isomer for Gly·Cl− and four canonical conformers for Gly·Br− were experimentally identified and characterized in contrast to the five canonical ones observed for Gly·I− under similar experimental conditions. Taken together, this study investigates both genericity and variations in binding patterns for the complexes composed of glycine and various halides, demonstrating that iodide-tagging is an effective spectroscopic means to unravel diverse ion-molecule binding motifs for cluster anions with congested spectral bands by substituting the respective ion with iodide. [ABSTRACT FROM AUTHOR]

The rhombohedral phase of gallium oxide (α-Ga2O3) is of interest because of its highest bandgap among the rest of the Ga2O3 polymorphs, making it particularly attractive in applications. However, even though the ion beam processing is routinely used in device technology, the understanding of radiation phenomena in α-Ga2O3 is not mature. Here, we study non-linear effects for radiation disorder formation in α-Ga2O3 by varying both the defect generation rate and the density of collision cascades, enabled by comparing monoatomic and cluster ion implants, also applying systematic variations of ion fluxes. In particular, we show that the collision cascade density governs the surface amorphization rates, also affected by the ion flux variations. These trends are explained in terms of the non-linear in-cascade and inter-cascade defect interactions occurring during ballistic and dynamic defect annealing stages. As such, these data reveal new physics of the radiation phenomena in α-Ga2O3 and may be applicable for more predictive ion beam processing of α-Ga2O3-based devices. [ABSTRACT FROM AUTHOR]

In this work, we present coordinated molecular dynamics, ion cluster acceleration, and retarding potential analysis simulations to determine cluster fragmentation behavior in a realistic emitter geometry for electrosprays operating in the pure ionic regime. Molecular dynamics simulations are used to determine the fragmentation rates of ionic liquid clusters as a function of internal energy, electric field strength, and cluster size. A simplified model of electrospray cluster acceleration is developed from previous electrohydrodynamic emission models and used to simulate retarding potential analysis curves. Fragmentation rates and beam composition are inferred for experimental data based on the molecular dynamics and cluster acceleration simulations. We find that for these experimental data, temperatures of EMI-BF 4 dimers likely range between 590 and 687 K while trimer temperatures are larger between 989 and 1092 K. The percentage of monomers, dimers, and trimers in the beam is approximately 45%, 30%–43%, and 13%–25%, respectively. Both ionic liquid cluster temperatures and beam composition agree with previous analysis of this experimental work, supporting the use of coordinated molecular dynamics and retarding potential analysis as a method of inferring electrospray beam parameters. Insights gained from this simulation process are discussed in the context of currently unexplained electrospray emitter behavior and experimental results including the presence of tetramers and trimers in the beam and fragmentation rates in high electric field regions. [ABSTRACT FROM AUTHOR]

The structures of magnesium chloride dimer-water clusters, (MgCl2)2(H2O)n−/0, were investigated with size-selected anion photoelectron spectroscopy and theoretical calculations to understand the dissolution of magnesium chloride in water. The most stable structures were confirmed by comparing vertical detachment energies (VDEs) with the experimental measurements. A dramatic drop of VDE at n = 3 has been observed in the experiment, which is in accordance with the structural change of (MgCl2)2(H2O)n−. Compared to the neutral clusters, the excess electron induces two significant phenomena in (MgCl2)2(H2O)n−. First, the planar D2h geometry can be converted into a C3v structure at n = 0, making the Mg–Cl bonds easier to be broken by water molecules. More importantly, a negative charge-transfer-to-solvent process occurs after adding three water molecules (i.e., at n = 3), which leads to an obvious deviation in the evolution of the clusters. Such electron transfer behavior was noticed at n = 1 in monomer MgCl2(H2O)n−, indicating that the dimerization between two MgCl2 molecules can make the cluster more capable of binding electron. In neutral (MgCl2)2(H2O)n, this dimerization provides more sites for the added water molecules, which can stabilize the entire cluster and maintain its initial structure. Specifically, filling the coordination number to be 6 for Mg atoms can be seen as a link between structural preferences in the dissolution of the monomers, dimers, and extended bulk-state of MgCl2. This work represents an important step forward into fully understanding the solvation of MgCl2 crystals and other multivalent salt oligomers. [ABSTRACT FROM AUTHOR]

The design of clusters featuring non-classical planar hypercoordinate atoms (phAs) often depends on the delocalized multicenter bonds involving reactive electron-deficient elements, which both destabilize the clusters and lead to difficulty in achieving the phA arrangement for electronegative elements such as nitrogen due to their preference for localized bonds. In this work, we computationally designed a series of aluminum chalcogenide clusters NAl4X4+ (X = S, Se, Te) with a desired planar tetracoordinate nitrogen and meaningfully improved chemical stability, as evidenced by the wide gaps (6.51–7.23 eV) between their highest occupied molecular orbitals and lowest unoccupied molecular orbitals, high molecular rigidity (dynamically stable up to 1500 K), and exclusively low global energy minima nature (their isomers locate at least 51.2 kcal/mol higher). Remarkably, these clusters are stabilized by peripheral chalcogen atoms, which not only sterically protect the NAl4 core moiety but also electronically compensate for the electron-deficient aluminum atoms via X → Al π back bonds, meeting the description of our recently proposed "electron-compensation" strategy. [ABSTRACT FROM AUTHOR]

AlGaN-based heterostructures are promising candidates for the fabrication of ultraviolet light-emitting diodes. The analysis of the atomic composition of the grown epitaxial films is important from a technological point of view, allowing precise control over the wavelength and intensity of the emitted light. In this work, the depth profiling of AlN(0001), AlGaN(0001), and AlGaN quantum dot surfaces grown by molecular beam epitaxy was carried out by using angle-resolved x-ray photoelectron spectroscopy (XPS) combined with Ar gas cluster ion source (GCIS) sputtering. Core level shifts in XPS spectra measured by Al Kα and Ag Lα photon sources were determined. We found that sputtering by Ar1000+ ion clusters with an energy of 10 keV creates disorder on the surface and induces changes in surface atomic composition. As a result, depth profiling with a typical surface-sensitive Al Kα photon source is affected by the damaged layer. The application of a less surface-sensitive Ag Lα photon source with high photon energy could suppress the contribution from the damaged surface layer. Combining GCIS sputtering with Ag Lα line XPS measurements is, therefore, very promising for the quantification of atomic composition in the buried epitaxial layers or heterostructures with thicknesses of several tens of nm. [ABSTRACT FROM AUTHOR]

The kinetics of electrons and negative ions was numerically studied under the conditions, which are typical for multi-pulsed guided ionization waves in He flows ejected into ambient air. It was shown that, in He with a small admixture of O2 or air, O2− ions formed due to three-body electron attachment in the discharge afterglow are rapidly converted to O4− cluster ions in pulse off time. We suggested that seed electrons are generated at the leading edge of a pulse in two steps, and these steps are dissociation of O4− ions to form O2− ions followed by electron detachment from the O2− ions. The rate of O4− dissociation was calculated in He–O2 mixtures for various reduced electric fields on the basis of a Monte Carlo simulation. This rate, as well as the rate of electron detachment from O2− ions calculated previously, was used to simulate seed electron generation in a high-voltage nanosecond pulse in He–O2 mixtures for different gas pressures. It was shown that the formation of O4− ions drastically hinders the generation of seed electrons in multi-pulsed guided ionization waves in He–O2 and He–air mixtures. [ABSTRACT FROM AUTHOR]

In this work, we investigate the ion pair tetramethylphosphonium cation, [P1,1,1,1]+, and bis(oxalato)borate anion, [BOB]−, as a model system for the study of ionic liquids interacting with both hydroxylated and oxygen terminated α-SiO2 (001) surfaces, using first-principles electronic structure theory. We use a single ionic pair and clusters of ion pairs, in order to have exclusively neutral supercell slab models. We use dispersion-corrected density functional theory (DFT) to ascertain that both the strong physical binding between the ions, dominated by ionic binding, and the weaker physical binding of ions to the different surfaces are correctly described. We have found that the binding of ion pairs is stronger to the hydroxylated α-SiO2 (001) surface compared to the oxygen terminated surface, which is attributed to the formation of H-binding with the oxygen atom(s) of the [BOB]− anion. Through rotation of ionic pair(s), we estimate the surface-ions energy barrier for translational movement and, thus, the strength of H-binding of the ions. At the surface of hydroxylated α-SiO2 (001), we have studied how water molecules form a network of H-binding with the OH groups of the surface and the [BOB]− anion, which offers an explanation for the reduction in the friction of ionic liquids on the inclusion of water. We suggest modeling protocols for simulation of ion pairs on surfaces, which can open up the possibility to use DFT to aid in designing and understanding the physicochemical mechanism of interactions of ionic materials (including ionic liquids) in various technological applications. [ABSTRACT FROM AUTHOR]

In plasma-driven solution electrolysis (PDSE), gas-phase plasma-produced species interact with an electrolytic solution to produce, for example, nanoparticles. An atmospheric pressure plasma jet (APPJ) directed onto a liquid solution containing a metallic salt will promote reduction of metallic ions in solution, generating metallic clusters that nucleate to form nanoparticles. In this article, results from a computational investigation are discussed of a PDSE process in which a radio-frequency APPJ sustained in helium impinges on a silver nitrate solution, resulting in growth of silver nanoparticles. A reaction mechanism was developed and implemented in a global plasma chemistry model to predict nanoparticle growth. To develop the reaction mechanism, density functional theory was used to generate probable silver growth pathways up to Ag9. Neutral clusters larger than Ag9 were classified as nanoparticles. Kinetic reaction rate coefficients for thermodynamically favorable growth pathways were estimated based on an existing, empirically determined base reaction mechanism for smaller Ag particle interactions. These rates were used in conjunction with diffusion-controlled reaction rate coefficients that were calculated for other Ag species. The role of anions in reduction of Agn ions in forming nanoparticles is also discussed. Oxygen containing impurities or admixtures to the helium, air entrainment into the APPJ, and dissociation of saturated water vapor above the solution can produce additional reactive oxygen species in solution, resulting in the production of anions and O 2 - in particular. For a given molarity, delivering a sufficient fluence of reducing species will produce similar nanoparticle densities and sizes for all applied power levels. Comparisons are made to alternate models for nanoparticle formation, including charged nanoparticles and use of direct current plasmas. [ABSTRACT FROM AUTHOR]

Ultraviolet photodissociation processes of gas phase Mg+–NO complex were studied by photofragment ion imaging experiments and theoretical calculations for excited electronic states. At 355 nm excitation, both Mg+ and NO+ photofragment ions were observed with positive anisotropy parameters, and theoretical calculations revealed that the two dissociation channels originate from an electronic transition from a bonding orbital consisting of Mg+ 3s and NO π* orbitals to an antibonding counterpart. For the NO+ channel, the photofragment image exhibited a high anisotropy (β = 1.53 ± 0.07), and a relatively large fraction (∼40%) of the available energy was partitioned into translational energy. These observations are rationalized by proposing a rapid dissociation process on a repulsive potential energy surface correlated to the Mg(1S) + NO+(1Σ) dissociation limit. In contrast, for the Mg+ channel, the angular distribution was more isotropic (β = 0.48 ± 0.03) and only ∼25% of the available energy was released into translational energy. The differences in the recoil distribution for these competing channels imply a reaction branching on the excited state surface. On the theoretical potential surface of the excited state, we found a deep well facilitating an isomerization from bent geometry in the Franck–Condon region to linear and/or T-shaped isomer. As a result, the Mg+ fragment was formed via the structural change followed by further relaxation to lower electronic states correlated to the Mg+(2S) + NO(2Π) exit channel. [ABSTRACT FROM AUTHOR]

Ionic-liquid ion sources produce beams of charged particles through evaporation and acceleration of ions and charged droplets from the surface of an ionic liquid. The composition of the emitted beam can impact the performance of ion sources for various applications such as focused beams for microfabrication and space propulsion. Numerical inference is considered for quantification of the beam composition of an ionic-liquid ion source through determining the current fraction of different species along with providing uncertainty in inferred values. An analysis of previously presented data demonstrates the ability to quantify the presence of ion clusters, including the distinct presence of heavy ion clusters such as heptamers. Quantification of beam composition will be an important technique for quantitative comparison of different time-of-flight data. [ABSTRACT FROM AUTHOR]

Helium droplets are unique hosts for isolating diverse molecular ions for infrared spectroscopic experiments. Recently, it was found that electron impact ionization of ethylene clusters embedded in helium droplets produces diverse carbocations containing three and four carbon atoms, indicating effective ion–molecule reactions. In this work, similar experiments are reported but with the saturated hydrocarbon precursor of ethane. In distinction to ethylene, no characteristic bands of larger covalently bound carbocations were found, indicating inefficient ion–molecule reactions. Instead, the ionization in helium droplets leads to formation of weaker bound dimers, such as (C2H6)(C2H4)+, (C2H6)(C2H5)+, and (C2H6)(C2H6)+, as well as larger clusters containing several ethane molecules attached to C2H4+, C2H5+, and C2H6+ ionic cores. The spectra of larger clusters resemble those for neutral, neat ethane clusters. This work shows the utility of the helium droplets to study small ionic clusters at ultra-low temperatures. [ABSTRACT FROM AUTHOR]

A multi-scale approach to electrospray ion source modeling has been developed. The evolution of a single-emitter electrospray plume in a pure ionic regime is simulated with a combination of electrohydrodynamic fluids and n-body particle modeling. Simulations are performed for the ionic liquid, EMI-BF4, firing in a positive pure-ion mode. The metastable nature of ion clusters is captured using an ion fragmentation model informed by molecular dynamics simulations and experimental data. Results are generated for three operating points (120, 324, and 440 nA) and are used to predict performance relevant properties, such as the divergence angle and the extractor surface impingement rate. Comparisons to experimental data recorded at similar operating points are provided. [ABSTRACT FROM AUTHOR]

Surface charging caused by the bombardment of samples with positive ion beams is a significant problem in material processing and surface analysis. The charging potential of an electrically isolated sample is commonly believed to increase with the acceleration voltage of a positive ion beam in the absence of charge compensation. Contrary to the common belief, however, this paper reports that the charging potential of an electrically isolated ionic liquid target decreases with increasing acceleration voltage of a positive cluster ion beam. A typical ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)amide (EMI-TFSA), was used as the target. It was placed on a metal plate that was electrically isolated from the ground, and its charging potential during cluster ion bombardment was measured with a high-impedance electrometer. For comparison, an electrically isolated metal plate was used. This study demonstrates that the charging potential varies significantly depending on cluster ion species and target materials. The charging potential of the metal plate increased monotonically with the irradiation time, whereas that of the ionic liquid target saturated at a lower voltage. The charging potential of the ionic liquid target decreased with increasing acceleration voltage of the ion beam even though the beam current increased. Larger cluster ions (m/z 502) caused less charge buildup than lighter cluster ions (m/z 111). The results obtained are explained by considering secondary ion emission from the ionic liquid target, which reduce the incoming net charge into the target, resulting in reduced surface charging. [ABSTRACT FROM AUTHOR]

We present a study of stepwise cryogenic N2 adsorption on size-selected Fen+ (n = 8–20) clusters within a hexapole collision cell held at T = 21–28 K. The stoichiometries of the observed adsorption limits and the kinetic fits of stepwise N2 uptake reveal cluster size-dependent variations that characterize four structural regions. Exploratory density functional theory studies support tentative structural assignment in terms of icosahedral, hexagonal antiprismatic, and closely packed structural motifs. There are three particularly noteworthy cases, Fe13+ with a peculiar metastable adsorption limit, Fe17+ with unprecedented nitrogen phobia (inefficient N2 adsorption), and Fe18+ with an isomeric mixture that undergoes relaxation upon considerable N2 uptake. [ABSTRACT FROM AUTHOR]

The Greifswald multi-reflection time-of-flight setup has been extended with a magnetron sputtering gas aggregation source for the production of atomic cluster ions with sizes ranging from a single to thousands of atoms. This source, combined with a newly added quadrupole mass filter and a linear Paul trap, opens up the possibility of many new atomic-cluster studies not feasible with the setup before. The new components and their interfacing with the previous setup are described, and benchmarking as well as the first experimental results are presented. The capability of the system to handle singly charged ions with masses of several ten thousand atomic mass units is demonstrated. [ABSTRACT FROM AUTHOR]

Vibrational predissociation processes of the H2O+Ar complex ion following mid-infrared excitations of the OH stretching modes and bending overtone of the H2O+ unit were studied by photofragment ion imaging. The anisotropy parameters, β, of the angular distributions of the photofragment ions were clearly dependent on the type (branch) of rotational excitation, β > 0 for the P-branch excitations, while β < 0 for the Q-branch excitations, which were consistent with the previous theoretical predictions for the rotationally resolved optical transition of a prolate symmetric top. The translational energy distributions had a similar form, irrespective of the excitation modes. This result suggests that the prepared excited states underwent a common relaxation pathway via the bending or bending overtone state of the H2O+ unit. In addition, the available energy was preferentially distributed into the rotational energy of the H2O+ fragment ions rather than the translational energy. The mechanism of the rotational excitations of the H2O+ fragment ions was discussed based on the steric configuration of the H2O+ and Ar units at the moment of dissociation. [ABSTRACT FROM AUTHOR]

Hydrated molecular anions are present in the atmosphere. Revealing the structure of the microsolvation is key to understanding their chemical properties. The infrared spectra of CO3•−(H2O)1,2 and CO4•−(H2O)1,2 were measured via infrared multiple photon dissociation spectroscopy in both warm and cold environments. Redshifted from the free O–H stretch frequency, broad, structured spectra were observed in the O–H stretching region for all cluster ions, which provide information on the interaction of the hydrogen atoms with the central ion. In the C–O stretching region, the spectra exhibit clear maxima, but dissociation of CO3•−(H2O)1,2 was surprisingly inefficient. While CO3•−(H2O)1,2 and CO4•−(H2O) dissociate via loss of water, CO2 loss is the dominant dissociation channel for CO4•−(H2O)2. The experimental spectra are compared to calculated spectra within the harmonic approximation and from analysis of molecular dynamics simulations. The simulations support the hypothesis that many isomers contribute to the observed spectrum at finite temperatures. The highly fluxional nature of the clusters is the main reason for the spectral broadening, while water–water hydrogen bonding seems to play a minor role in the doubly hydrated species. [ABSTRACT FROM AUTHOR]

Metal–ligand cluster ions are structurally characterized by means of gas-phase infrared multiple photon dissociation spectroscopy. The mass-selected complexes consist of one or two metal cations M3+ (M = Al, Fe, or Ru) and two to five anionic bidentate acetylacetonate ligands. Experimental IR spectra are compared with different density functional theory calculations, namely, PBE/TZVP, B3LYP/6-31G*, and M06/6-31+G**. Frequency analysis was also performed at different levels, namely, scaled static harmonic and unscaled static anharmonic, or with ab initio molecular dynamics simulations at the PBE/TZVP level. All methods lead to simulated spectra that fit rather well with experimental data, and the spectral red shifts of several main bands, in the 1200 cm−1–1800 cm−1 range, are sensitive to the strength of the metal–ligand interaction and to the spin state of the ion. Due to the rigidity of those complexes, first principles molecular dynamics calculations provide spectra similar to that produced by static calculations that are already able to catch the main spectral signatures using harmonic calculations at the B3LYP/6-31G* level. [ABSTRACT FROM AUTHOR]

Neon cluster ions Nes+ grown in pre-ionized, mass-to-charge selected helium nanodroplets (HNDs) reveal a strong enrichment of the heavy isotope 22Ne that depends on cluster size s and the experimental conditions. For small sizes, the enrichment is much larger than previously reported for bare neon clusters grown in nozzle expansions and subsequently ionized. The enrichment is traced to the massive evaporation of neon atoms in a collision cell that is used to strip helium from the HNDs. We derive a relation between the enrichment of 22Ne in the cluster ion and its corresponding depletion factor F in the vapor phase. The value thus found for F is in excellent agreement with a theoretical expression that relates isotopic fractionation in two-phase equilibria of atomic gases to the Debye temperature. Furthermore, the difference in zero-point energies between the two isotopes computed from F agrees reasonably well with theoretical studies of neon cluster ions that include nuclear quantum effects in the harmonic approximation. Another fitting parameter provides an estimate for the size si of the precursor of the observed Nes+. The value is in satisfactory agreement with the size estimated by modeling the growth of Nes+ and with lower and upper limits deduced from other experimental data. On the other hand, neon clusters grown in neutral HNDs that are subsequently ionized by electron bombardment exhibit no statistically significant isotope enrichment at all. The finding suggests that the extent of ionization-induced dissociation of clusters embedded in HNDs is considerably smaller than that for bare clusters. [ABSTRACT FROM AUTHOR]

To better understand the nexus between structure and photophysics in metallo-DNA assemblies, the parallel-stranded duplex formed by the all-cytosine oligonucleotide, dC20, and silver nitrate was studied by circular dichroism (CD), femtosecond transient absorption spectroscopy, and time-dependent-density functional theory calculations. Silver(I) ions mediate Cytosine–Cytosine (CC) base pairs by coordinating to the N3 atoms of two cytosines. Although these silver(I) mediated CC base pairs resemble the proton-mediated CC base pairs found in i-motif DNA at first glance, a comparison of experimental and calculated CD spectra reveals that silver ion-mediated i-motif structures do not form. Instead, the parallel-stranded duplex formed between dC20 and silver ions is proposed to contain consecutive silver-mediated base pairs with high propeller twist-like ones seen in a recent crystal structure of an emissive, DNA-templated silver cluster. Femtosecond transient absorption measurements with broadband probing from the near UV to the near IR reveal an unusually long-lived (>10 ns) excited state in the dC20 silver ion complex that is not seen in dC20 in single-stranded or i-motif forms. This state is also absent in a concentrated solution of cytosine–silver ion complexes that are thought to assemble into planar ribbons or sheets that lack stacked silver(I) mediated CC base pairs. The large propeller twist angle present in metal-mediated base pairs may promote the formation of long-lived charged separated or triplet states in this metallo-DNA. [ABSTRACT FROM AUTHOR]

We report on the experimental observation of the B+2Σ+ state of MgAr+ located below the Mg+(3p²P3/2) + Ar(¹S0) dissociation asymptote. Using the technique of isolated-core multiphoton Rydberg-dissociation spectroscopy, we have recorded rotationally resolved spectra of the B+2Σ+(v′) ← X+ ²Σ+(v″ = 7) transitions, which extend from the vibrational ground state (v′ = 0) to the dissociation continuum above the Mg+(3p²P3/2) + Ar(¹S0) dissociation threshold. The analysis of the rotational structure reveals a transition from Hund’s angular-momentum-coupling case (b) at low v′ values to case (c) at high v′ values caused by the spin–orbit interaction. Measurements of the kinetic-energy release and the angular distribution of the Mg+ fragments detected in the experiments enabled the characterization of the dissociation mechanisms. The vibrational levels of the B+ state above v′ = 6 are subject to predissociation into the Mg+(3p²P1/2) + Ar(¹S0) continuum, and the fragment angular distributions exhibit anisotropy β parameters around 0.5, whereas direct dissociation into the continuum above the Mg+(3p²P3/2) + Ar(¹S0) asymptote is characterized by β parameters approaching 2. Molecular ions excited to the B+ state with v′ = 0–6 efficiently absorb a second photon to the repulsive part of the ²Σ+ state associated with the Mg+(3d ²D3/2,5/2) + Ar(¹S0) continua. The interpretation of the data is validated by the results of ab initio calculations of the low-lying electronic states of MgAr+, which provided initial evidence for the existence of bound vibrational levels of the B+ state and for the photodissociation mechanisms of its low vibrational levels. [ABSTRACT FROM AUTHOR]

We report a global study of the 3p Rydberg complex of the MgAr+ molecular ion. High-resolution spectroscopic data on the two spin–orbit components of the A+ electronic state were obtained by isolated-core multiphoton Rydberg-dissociation spectroscopy up to vibrational levels as high as v′ = 29, covering more than 90% of the potential wells. Accurate adiabatic potential-energy functions of the A+ and B+ states, which together form the 3p Rydberg complex, were obtained in a global direct-potential-fit analysis of the present data and the extensive data on the B+ state reported in Paper I [D. Wehrli et al., J. Chem. Phys. 153, 074310 (2020)]. The dissociation energies of the B+ state, the two spin–orbit components of the A+ state, and the X+ state of MgAr+ are obtained with uncertainties (1 cm−1) more than two orders of magnitude smaller than in previous studies. [ABSTRACT FROM AUTHOR]

In this work, we present an overview of nanopatterning and imaging applications using newly developed workflows with focused ion beams (FIBs) produced with a GaBiLi liquid metal alloy ion source. The primary beam of this source type contains gallium, bismuth, and lithium as well as cluster ions which can be separated quickly using a Wien filter. Lithium ion milling has been applied to generate heptamer-arranged nanohole arrays in gold films with high resolution. Workflows for two-step bowtie nanofabrication using lithium and bismuth ions from the same source have been established. Furthermore, we present ion beam imaging results that were obtained with lithium ions on various sample materials. Combining the large sputter yield and high depth resolution of heavy bismuth ions with the high lateral imaging resolution of light lithium ions enables 3D nanoscale tomography using different ion species generated from the same source. Sample tilt is not required due to the top-down geometry of the FIB. [ABSTRACT FROM AUTHOR]

The investigation of chemical reactions during ion irradiation is a frontier for the study of the ion–material interaction. In order to probe the chemistry of ion produced nanoclusters, valence electron energy loss spectroscopy (VEELS) was exploited to investigate Ga+ ion damage in Al2O3, InP, and InGaAs, where each target material has been shown to react differently to the interaction between impinging ions, recoil atoms, and vacancies: metallic Ga, ternary InGaP clusters, and metallic In clusters are formed in Al2O3, InP, and InGaAs, respectively. Supporting simulations based on Monte Carlo and crystal orbital Hamiltonian calculations indicate that the chemical constitution of cascade induced nano-precipitates is a result of a competition between interstitial/vacancy consumption rates and preferential bond formation due to differing bond strengths. [ABSTRACT FROM AUTHOR]

Molecular gas species for gas cluster ion beams (GCIBs), such as carbon dioxide and water, were examined with a range of beam energies and cluster sizes to compare with the "universal relation" of the sputter yield, Y, per cluster atom against incident beam energy, E, per cluster atom of Arn cluster beam using Irganox 1010. In this work, we compare Arn, (CO2)n, and (H2O)n gas clusters to the universal equations for Arn clusters. To discuss molecular gas species for GCIBs, energy per nucleon (E/N) needs to replace energy per atom. We monitored sputter rate, depth resolution, and secondary ion yield as a function of the beam parameters: gas species, beam energy, and cluster size. (H2O)n GCIB shows reduced sputter rates and improved depth resolution with high sensitivity compared to Arn and (CO2)n GCIBs. These initial results indicate the potential to achieve high-depth resolution with high sensitivity and suggest that (H2O)n cluster ion beam has the potential to play a significant role in surface analysis techniques with organic materials. Results also show that no single set of conditions will provide the "best gas cluster ion beam" for all applications. However, it is possible to choose a set of conditions that will be more or less optimal depending on the experimental goals, such as maximizing the sputter rate, depth resolution, and molecular ion yield. In this work, we recommend the following three guidelines for GCIB users to set their own conditions: (1) to maximize the sputter rate, select a smaller cluster (higher E/N), but be aware that this will increase fragmentation and reduce molecular ion yield; (2) to maximize the depth resolution, select a larger cluster (lower E/N), and use (H2O)n GCIB, if possible; and (3) to maximize the molecular ion signal, use the highest beam energy available, and select a cluster with 0.15–0.25 eV/nucleon for Ar and (CO2)n GCIBs or around 0.1 eV/nucleon if using (H2O)n GCIB. These results are valid for XPS, SIMS, and any technique that utilizes GCIBs. [ABSTRACT FROM AUTHOR]

Interfacial regions attached to hydrophobic and hydrophilic surfaces have very low relative permittivities (ε ∼ 3–5), and these low values induce the formation of ion pairs. A detailed description of ion pairing will be possible only after the development of adequate experimental probing methods. The scheme described in this work detects the ion pair (or multistage ion association) formation in water solutions in the interfacial region. Forces acting on the tip when immersed in the interfacial region attached to hydrophilic substrates, such as mica where ε < 7, are attractive (AFM tip dielectric constant = 7). These attractive force steps with extensions equal or smaller than the Bjerrum length at hydrophilic substrates are associated with ion pair formation in interfacial regions. Measurements show that the interfacial water molecular arrangement and ion pairs form an ordered structure (ε < 7) induced by mica interfacial charges. The interfacial region attached to hydrophobic substrates, however, form separated arrangements of ion pairs and water molecular structures as observed by the repulsive steps (ε > 7) intercalated in between attractive regions (ε < 7). [ABSTRACT FROM AUTHOR]

The surfaces of zirconium dioxide and yttria-stabilized zirconia (YSZ) have been analyzed using x-ray photoelectron spectroscopy after ion sputtering with monoatomic Ar+ or an argon gas cluster ion beam (GCIB). The O/Z ratio and new components in the Zr 3d lines show reduction to lower oxidation states when sputtered with monoatomic Ar+, but significantly less damage is observed when GCIB sputtering is used. The damaged surface layer caused by Ar+ sputtering can be removed by subsequent GCIB sputtering. However, the depth resolution observed in depth profiles of thin YSZ films was significantly better when Ar+ sputtering is used. Differences in the Sn content in the oxidized Zr-4 specimen were also observed when comparing Ar+ and GCIB sputtering, suggesting preferential sputtering. [ABSTRACT FROM AUTHOR]

Manganese (II) oxide (MnO), manganese (IV) oxide (MnO2), cobalt (II,III) oxide (Co3O4), and nickel (II) oxide (NiO) were analyzed with time-of-flight secondary ion mass spectrometry using 70 keV gas cluster ion beams. The obtained mass spectra are influenced by projectile chemistry and to a lesser extent velocity. Gas cluster ion beams containing CO2 or H2O enhanced the relative yield of metal oxide and metal hydroxide secondary ions compared to beams containing only Ar. For all gas cluster ion beams tested, steady-state ion ratios [MxOy]+/[Mx]+ were reached. For manganese oxides, the [MnxOy]+/[Mnx]+ ratio reflected the metal oxidation state whereas the [MnxOyHz]+/[Mnx]+ ion ratios did not. This study demonstrates that secondary ion mass spectrometry using 70 keV gas cluster ion beams provides a novel approach to the quantitative analysis of the surface and subsurface regions of metal oxides related to energy-storage materials. [ABSTRACT FROM AUTHOR]

Ionic liquid ion sources (ILISs) are similar to liquid metal ion sources and use ionic liquids (i.e., room temperature molten salts) instead of liquid metals. Aprotic ionic liquids have been used in most cases, whereas protic ion liquids have hardly been used thus far. In this study, propylammonium nitrate (PAN), which is a protic ionic liquid, was tested as the liquid material for an ILIS. Beam generation was investigated in DC operation using a tungsten needle externally wetted with PAN. Experimental results showed that a stable ion beam was generated without special measures and that the ion beam contained cluster ions with m/z values on the order of 103. Additionally, electrochemical experiments were performed in vacuum to investigate electrochemical reactions involved in beam generation. Residual gas analysis revealed that H2O and O2 were electrochemically produced at a tungsten electrode immersed in PAN. These products will probably result from electrochemical reactions of water in PAN because protic ionic liquids contain a small amount of water even in vacuum. The stability of the ion beam can be explained by the effect of water in PAN. [ABSTRACT FROM AUTHOR]

As the commercial display market grows, the demand for low-toxicity, highly emissive, and size-tunable semiconducting nanoparticles has increased. Indium phosphide quantum dots represent a promising solution to these challenges; unfortunately, they typically suffer from low inherent emissivity resulting from charge carrier trapping. Strategies to improve the emissive characteristics of indium phosphide often involve zinc incorporation into or onto the core itself and the fabrication of core/shell heterostructures. InP clusters are high fidelity platforms for studying processes such as cation exchange and surface doping with exogenous ions since these clusters are used as single-source precursors for quantum dot synthesis. Here, we examined the incorporation of zinc and gallium ions in InP clusters and the use of the resultant doped clusters as single-source precursors to emissive heterostructured nanoparticles. Zinc ions were observed to readily react with InP clusters, resulting in partial cation exchange, whereas gallium resisted cluster incorporation. Zinc-doped clusters effectively converted to emissive nanoparticles, with quantum yields strongly correlated with zinc content. On the other hand, gallium-doped clusters failed to demonstrate improvements in quantum dot emission. These results indicate stark differences in the mechanisms associated with aliovalent and isovalent doping and provide insight into the use of doped clusters to make emissive quantum dots. [ABSTRACT FROM AUTHOR]

Inversion recovery with electron spin-echo detection has been used to study the electron spin-lattice relaxation rates 1 / T 1 for transition metal impurities in heavily cobalt-doped hydrothermally grown ZnO single crystals. The relaxation dynamics of Co 2 + ions dominates the phonon bottleneck effect in the Orbach-Aminov process, which involves the modulation of the zero-field-splitting tensor. The relaxation mechanism may be treated in terms of phonon heating with fast rate of energy pump from Co 2 + spins into the lattice phonon modes. The measurements reveal the cross-relaxation process in which the single Co 2 + ions cross-relax to exchange-coupled clusters of cobalt ions. The higher temperature relaxation of Co 2 + indicates an additional Orbach-Aminov process via a state of ∼ 226 cm − 1 above the ground state Kramers doublet. It is shown that in this system Co 2 + ions play a role of the rapidly relaxing centers, strongly mediating the spin-lattice relaxation of the other transition metal impurities, such as Mn 2 + and Fe 3 + . [ABSTRACT FROM AUTHOR]