Small Cluster Ions Research Articles

Statistical analysis of the atmospheric ion concentrations and mobility distributions at a tropical station, Pune

Measurements of ion mobility spectra in the range of 0.00133–3.16 cm2 s−1 volt−1 were made at a tropical station, Pune, India from March 2010 to December 2012. A total of 12 004 hourly averaged ion mobility spectra were obtained over the entire period. The average mobility spectrum shows three distinct peaks for the traditional categories of small, intermediate and large ions. However, to understand the seasonal variability and statistical characteristics of ions that justify their further division into five different categories, we have divided our data into five mobility ranges of small cluster, big cluster, intermediate, light large and heavy large ions. Our results show that small ions in atmospheric conditions at Pune can be further divided into two classes with the boundary at a mobility of 1.78 cm2 s−1 volt−1 (diameter 0.66 nm) where relative standard deviation of ion concentrations starts increasing instead of decreasing with mobility. Some but not all small cluster ions of negative polarity are likely to grow to the intermediate ion size. Further, the growth from small to big cluster ions is faster for negative than positive ions. In contrast to the observations of higher mean values of ion concentration in warm season than in winter at a midlatitude station, our observations show higher mean values of ion concentration in winter than in the warm season at this tropical station. This contrast in observations at the two sites is explained by trapping of radioactive emanations by snow at the ground during winter at the midlatitude site.Our observations indicate that intermediate ions are generated by two independent processes, probably by growth of big cluster ions and by direct nucleation from trace gases. Further, the results indicate that the sources for light and heavy large ions are different.

Mass and energy distribution of negatively and positively charged small cluster ions sputtered from GaAs(100) by 150 keV Ar+ bombardment

Mass and energy distribution of positively and negatively charged small GaxAsy cluster ions consisting of up to six atoms sputtered from a GaAs(100) surface after 150keV Ar+ ion bombardment are reported. Positively charged ions contain a larger fraction of Ga atoms while negatively charged ions are rich in As. Measured energy distributions display a maximum at low kinetic energies of a few eV followed by a steep decrease with increasing energy which is more pronounced for larger ions.

Effect of Bisulfate, Ammonia, and Ammonium on the Clustering of Organic Acids and Sulfuric Acid.

We investigate the effect of the bisulfate anion HSO4-, ammonium cation NH4+, and ammonia NH3 on the clustering of sulfuric acid and pinic acid or 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA). The systems were chosen based on their expected relevance in atmospheric new particle formation. Using quantum chemical methods together with kinetic calculations, we study the ability of these compounds to enhance cluster formation and growth. The cluster structures are obtained and frequencies are calculated using three different DFT functionals (M06-2X, PW91, and ωB97X-D) with the 6-31++G(d,p) basis set. The electronic energies are corrected using an accurate DLPNO-CCSD(T)/def2-QZVPP level of theory. The evaporation rates are evaluated based on the calculated Gibbs free energies. The interaction between the ions and sulfuric acid or carboxylic acid group is strong, and thereby small two-component ionic clusters are found to be very stable against evaporation. The presence of bisulfate stimulates the cluster formation through addition of the sulfuric acid, whereas the presence of ammonium favors the addition of organic acids. Bisulfate and ammonium enhance the first steps of cluster formation; however, at atmospheric conditions further cluster growth is limited due to the weak interaction and fast evaporation of the larger three-component clusters. On the basis of our results it is therefore unlikely that the studied organic acids and sulfuric acid, even together with bisulfate, ammonia, or ammonium can drive new-particle formation via clustering mechanisms. Other mechanisms such as chemical reactions are needed to explain observed new-particle formation events in the presence of oxidized organic compounds resembling the acids studied here.

Fragmentation of KrN+ clusters after electron impact ionization. Short-time dynamics simulations and approximate multi-scale treatment.

Post-ionization fragmentation of small ionic krypton clusters, KrN+ (N = 3-13), has been investigated using a semiclassical non-adiabatic dynamics approach consisting of classical treatment of atomic nuclei and full quantum treatment of electrons, and an extended diatomics-in-molecules model including the spin-orbit coupling as well as leading three-body interaction corrections. Electronic quantum decoherence has also been considered via a simplified scheme proposed previously. The positive charge has been initially localized on a randomly selected atom in the form of a localized 2P1/2 positive hole. It follows from the calculations that the data are not converged at timescales usually considered in dynamical calculations (t = 200 ps in this work) and that an extension to t ≈ 1 μs is needed. An approximate multi-scale treatment developed recently has been used to provide such an extension of the output of dynamical calculations. A qualitative agreement with available experimental data has been achieved, in particular, the experimental observation that the monomer fragment, Kr+, completely dominates has been reproduced. Interestingly, stabilized neutral dimer and trimer fragments have been observed in our calculations at non-negligible abundances despite extremely weak bonding in these species.

Numerical investigation of depth profiling capabilities of helium and neon ions in ion microscopy

The analysis of polymers by secondary ion mass spectrometry (SIMS) has been a topic of interest for many years. In recent years, the primary ion species evolved from heavy monatomic ions to cluster and massive cluster primary ions in order to preserve a maximum of organic information. The progress in less-damaging sputtering goes along with a loss in lateral resolution for 2D and 3D imaging. By contrast the development of a mass spectrometer as an add-on tool for the helium ion microscope (HIM), which uses finely focussed He+ or Ne+ beams, allows for the analysis of secondary ions and small secondary cluster ions with unprecedented lateral resolution. Irradiation induced damage and depth profiling capabilities obtained with these light rare gas species have been far less investigated than ion species used classically in SIMS. In this paper we simulated the sputtering of multi-layered polymer samples using the BCA (binary collision approximation) code SD_TRIM_SP to study preferential sputtering and atomic mixing in such samples up to a fluence of 1018 ions/cm2. Results show that helium primary ions are completely inappropriate for depth profiling applications with this kind of sample materials while results for neon are similar to argon. The latter is commonly used as primary ion species in SIMS. For the two heavier species, layers separated by 10 nm can be distinguished for impact energies of a few keV. These results are encouraging for 3D imaging applications where lateral and depth information are of importance.

Unexpected Mechanistic Variants in the Thermal Gas-Phase Activation of Methane

In this review gas-phase studies conducted (mostly) at the Berlin laboratory of the authors are presented. The focus will be on describing mechanistic variants we (and others) came across recently in investigating the thermal activation of methane in the gas phase under idealized conditions. Typical examples include the discussion of those hydrogen-atom-transfer processes that do not follow the well-established conventional pathways in which oxyl radicals play a decisive role. This is the case when the spin is located at a metal center, as in [Al2O2]•+, and the C–H bond cleavage follows a proton-coupled electron-transfer mechanism. Also, examples will be presented in which a high spin density at a bridging oxygen atom can be generated by judicious “doping” of the cluster oxides. Further, the particular role Lewis-acidic sites play in the methane activation by closed-shell metal-oxide ions will be highlighted. Then, aspects of the dissociative adsorption of CH4 on rather small cluster ions will be analyzed...

Characterization of Carbon Plasma Evolution Using Laser Ablation TOF Mass Spectrometry**supported by the National Magnetic Confinement Fusion Science Program of China (No. 2013GB109005) and National Natural Science Foundation of China (No. 11175035), Chinesisch-Deutsches Forschungs Project (GZ768), the Fundamental Research Funds for the Central Universities of China (Nos. DUT12ZD(G)01, DUT14ZD(G)04) and MMLab Research Project (DP1051208)

In this work, a time-of-flight (TOF) mass spectrometer has been used to investigate the distribution of intermediate species and formation process of carbon clusters. The graphite sample was ablated by Nd:YAG laser (532 nm and 1064 nm). The results indicate that the maximum size distribution shifted towards small cluster ions as the laser fluence increased, which happened because of the fragmentation of larger clusters in the hot plume. The temporal evolution of ions was measured by varying the delay time of the ion extraction pulse with respect to the laser irradiation, which was used to provide distribution information of the species in the ablated plasma plume. When the laser fluence decreased, the yield of all of the clusters obviously dropped.

Comparison Studies on Sub-Nanometer-Sized Ion Clusters in Aqueous Solutions: Vibrational Energy Transfers, MD Simulations, and Neutron Scattering.

In this work, MD simulations with two different force fields, vibrational energy relaxation and resonant energy transfer experiments, and neutron scattering data are used to investigate ion pairing and clustering in a series of GdmSCN aqueous solutions. The MD simulations reproduce the major features of neutron scattering experimental data very well. Although no information about ion pairing or clustering can be obtained from the neutron scattering data, MD calculations clearly demonstrate that substantial amounts of ion pairs and small ion clusters (subnanometers to a few nanometers) do exist in the solutions of concentrations 0.5 M*, 3 M*, and 5 M* (M* denotes mole of GdmSCN per 55.55 mole of water). Vibrational relaxation experiments suggest that significant amounts of ion pairs form in the solutions. Experiments measuring the resonant energy transfers among the thiocyanate anions in the solutions suggest that the ions form clusters and in the clusters the average anion distance is 5.6 Å (5.4 Å) in the 3 M* (5 M*) Gdm-DSCN/D2O solution.

Ionic polymer actuator based on anion-conducting methylated ether-linked polybenzimidazole

A new type of ionic polymer actuator based on anion-conducting methylated poly[(1-(4,4′-diphenylether)-5-oxybenzimidazole)-benzimidazole] (PBI–OO) membranes and a carbon composite electrode material was successfully developed. The anion-conducting membrane was prepared through N-methylation of PBI–OO with iodomethane. The ion exchange reaction of the methylated PBI–OO membrane to various anions largely affected the ion conductivity and the resulting actuator performance. The methylated PBI–OO membranes possessed unique small ion clusters with the size of 1.71–2.04nm and exhibited high tensile moduli and strengths in the hydrated states (tensile modulus: 553MPa, tensile strength: 32MPa in the iodide form) due to the π–π stacking interaction among the benzimidazole rings in the polymer backbone. The actuators based on PBI–OO bent toward the negatively charged electrode direction reverse to the bending direction of conventional ionic polymer actuators with cation-conducting membranes. They also revealed exceptional charge-specific blocking forces (at least 17 times larger than those of Nafion-based actuators) because of a combinatorial effect of the larger modulus of PBI–OO-based actuators and the smaller size of ion clusters inside the PBI–OO layer, and the electrolysis of water restrained using a carbon electrode as well. Moreover, the actuators showed no back relaxation within the examined time period even under a high dc voltage beyond the electrolysis voltage of water. The distinct characteristics of the PBI–OO-based actuators are expected to be used as a unique anion-conducting polymer actuator.

Thermodynamics of small alkali metal halide cluster ions: comparison of classical molecular simulations with experiment and quantum chemistry.

We evaluate the ability of selected classical molecular models to describe the thermodynamic and structural aspects of gas-phase hydration of alkali metal halide ions and the formation of small water clusters. To understand the effect of many-body interactions (polarization) and charge penetration effects on the accuracy of a force field, we perform Monte Carlo simulations with three rigid water models using different functional forms to account for these effects: (i) point charge nonpolarizable SPC/E, (ii) Drude point charge polarizable SWM4-DP, and (iii) Drude Gaussian charge polarizable BK3. Model predictions are compared with experimental Gibbs free energies and enthalpies of ion hydration, and with microscopic structural properties obtained from quantum DFT calculations. We find that all three models provide comparable predictions for pure water clusters and cation hydration but differ significantly in their description of anion hydration. None of the investigated classical force fields can consistently and quantitatively reproduce the experimental gas-phase hydration thermodynamics. The outcome of this study highlights the relation between the functional form that describes the effective intermolecular interactions and the accuracy of the resulting ion hydration properties.

Molecular dynamics simulations of 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide clusters and nanodrops.

Atomistic molecular dynamics simulations of small clusters and nanodroplets of the ionic liquid 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [EMIM-Tf2N] subject to an external electric field were performed. A 125-ion-pair droplet was found to be nearly spherical with an isotropic distribution of cations and anions under vacuum conditions. The droplet was subjected to external electric fields of varying strength, and ion emission events were observed. The initially spherical droplet is elongated along the electric field axis, resulting in nonspherical behavior and increased net dipole values after the application of strong electric fields. The critical electric field required for ion field emission was determined to be 0.985 V/nm, in agreement with the experimental value of 1.0 V/nm. Excellent agreement is found in the prediction of ionic emission products for a neutral 125-ion-pair droplet of the ionic liquid at an electric field strength of 1.2 V/nm when compared to the results of two independent experiments. Small ionic liquid clusters were investigated with respect to their thermal stabilities and were found to be thermally stable well above room temperature. The role of electric fields in the dissociation of small charged ion clusters was also investigated.

Surface modification using ionic liquid ion beams

We developed an ionic liquid (IL) ion source using 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) and produced IL ion beams by applying a high electric field between the tip and the extractor. Time-of-flight measurements showed that small cluster and fragment ions were contained in the positive and negative ion beams. The positive and negative cluster ions were deposited on Si(100) substrates. X-ray photoelectron spectroscopy measurements showed that the composition of the deposited layers was similar to that of an IL solvent. This suggests that a cation (A+) or an anion (B−) was attached to an IL cluster (AB)n, resulting in the formation of positive cluster ions (AB)nA+ or negative cluster ions (AB)nB−, respectively. The surfaces of the IL layers deposited on Si(100) substrates were flat at an atomic level for positive and negative cluster ion irradiation. Moreover, the contact angles of the deposited layers were similar to that of the IL solvent. Thus, surface modification of Si(100) substrates was successfully demonstrated with BMIM-PF6 cluster ion beams.

Structure of Membranes for Fuel Cells: SANS and SAXS Analyses of Sulfonated PEEK Membranes and Solutions

The microstructure of sulfonated poly(ether ether ketone) (sPEEK) membranes was investigated by combining small-angle neutron and X-ray scattering techniques (SANS and SAXS) for large and low water contents, respectively. The ion-exchange capacity, the water content and the nature of the counterion were varied for a better understanding of the membrane microstructure. SAXS and SANS contrast variation experiments reveal a significantly more complex structure than it is commonly believed with a delicate balance of the contrasts. At low water content, the structure can be depicted by the presence of small ionic clusters and larger more or less connected core–shell domains between crystallites for low values of the sulfonation degree. The first step of the swelling process corresponds to the filling without significant structural changes of the porosity created by the solvent evaporation during the casting process. The second step is associated with a major structural reorganization induced by a large increase of the membrane water content over a small range of temperature. This reorganization is attributed to an ionic domain percolation on a large scale. The third step corresponds to the swelling of lamellar ionic domains around the crystallites as revealed by the study of the dilution laws and of the structure of sPEEK ionomer dispersions. The sizes of the ribbon-like polymer particles were determined. They do not depend linearly with the membrane ion content suggesting a nonhomogeneous distribution of the ionic groups along the polymer chain during the sulfonation process associated with the semicrystalline nature of the polymers. Finally, the effect of the degradation in oxidative media on the membrane structure is shown to correspond to an increase of the membrane water content.

Morphology and Electrochemical Properties of Perfluorosulfonic Acid Ionomers for Vanadium Flow Battery Applications: Effect of Side‐Chain Length

Perfluorosulfonic acid ionomers (PFSI) with different side-chain lengths have been investigated with respect to their morphology and electrochemical properties in vanadium flow batteries (VFB). The results indicated that the membrane with the shortest side chains (SSC-M2) displayed small ion clusters and a low degree of hydrophobic-hydrophilic separation, which is favourable to reduce the cross-over of vanadium ions in the VFB. SSC-M2 shows a similar proton conductivity to Nafion, which carries longer ionic side chains but with much lower ion permeability. As a result, the VFB assembled with SSC-M2 exhibited a superior coulombic efficiency and a voltage efficiency close to that of Nafion115. In situ mass transfer revealed that SSC-M2 had a remarkably low degree of vanadium and water transfer across the membrane, which resulted in lower capacity fading than in the case of Nafion115. These results indicate that a membrane with short side chains is an ideal option in the fabrication of high-performance VFBs with low capacity loss.

A Model Potential for Acetonitrile: from Small Clusters to Liquid

A portable model potential, representing the intermolecular interaction of acetonitrile with itself and with ions, is proposed. Such model, formulated as a combination of a few effective components, given in terms of the polarizability and dipole moment values of the molecular partners, is here adopted as a building block of the force field of acetonitrile clusters in molecular dynamics simulations. Its reliability is tested by comparing the predicted features for both small ionic and neutral clusters containing acetonitrile with ab initio results and experimental information. Its application to molecular dynamics simulations of liquid acetonitrile and of the solvated Li(+), Na(+), K(+), and I(-), performed at several values of the temperature, discloses an ample and interesting phenomenology, described in an internally consistent way. Such model will be useful to assess the effect of intermolecular interactions on the dynamics of acetonitrile processes occurring in various environments of applied relevance, with emphasis on the dye-sensitized solar cell framework.

Stability and rigidity/flexibility—Two sides of the same coin?

Protein molecules require both flexibility and rigidity for functioning. The fast and accurate prediction of protein rigidity/flexibility is one of the important problems in protein science. We have determined flexible regions for four homologous pairs from thermophilic and mesophilic organisms by two methods: the fast FoldUnfold which uses amino acid sequence and the time consuming MDFirst which uses three-dimensional structures. We demonstrate that both methods allow determining flexible regions in protein structure. For three of the four thermophile–mesophile pairs of proteins, FoldUnfold predicts practically the same flexible regions which have been found by the MD/First method. As expected, molecular dynamics simulations show that thermophilic proteins are more rigid in comparison to their mesophilic homologues. Analysis of rigid clusters and their decomposition provides new insights into protein stability. It has been found that the local networks of salt bridges and hydrogen bonds in thermophiles render their structure more stable with respect to fluctuations of individual contacts. Such network includes salt bridge triads Agr-Glu-Lys and Arg-Glu-Arg, or salt bridges (such as Arg-Glu) connected with hydrogen bonds. This ionic network connects alpha helices and rigidifies the structure. Mesophiles can be characterized by stand alone salt bridges and hydrogen bonds or small ionic clusters. Such difference in the network of salt bridges results in different flexibility of homologous proteins. Combining both approaches allows characterizing structural features in atomic detail that determine the rigidity/flexibility of a protein structure. This article is a part of a Special Issue entitled: The emerging dynamic view of proteins: Protein plasticity in allostery, evolution and self-assembly.

Reactions of Hydrated Singly Charged First‐Row Transition‐Metal Ions M+(H2O)n (M=V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) toward Nitric Oxide in the Gas Phase

Reactions of M(+) (H2 O)n (M=V, Cr, Mn, Fe, Co, Ni, Cu, Zn; n≤40) with NO were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Uptake of NO was observed for M=Cr, Fe, Co, Ni, Zn. The number of NO molecules taken up depends on the metal ion. For iron and zinc, NO uptake is followed by elimination of HNO and formation of the hydrated metal hydroxide, with strong size dependence. For manganese, only small HMnOH(+) (H2 O)n-1 species, which are formed under the influence of room-temperature black-body radiation, react with NO. Here NO uptake competes with HNO formation, both being primary reactions. The results illustrate that, in the presence of water, transition-metal ions are able to undergo quite particular and diverse reactions with NO. HNO is presumably formed through recombination of a proton and (3) NO(-) for M=Fe, Zn, preferentially for n=15-20. For manganese, the hydride in HMnOH(+) (H2 O)n-1 is involved in HNO formation, preferentially for n≤4. The strong size dependence of the HNO formation efficiency illustrates that each molecule counts in the reactions of small ionic water clusters.

Theoretical and experimental investigation on the stability of Cn=1–6H− and Cn=1–4Hx+ clusters

A theoretical study on the molecular structure of CnH− clusters, where 1⩽n⩽6, and CnHx+ clusters, where 1⩽n⩽4 and 0⩽x⩽2n+2, has been performed at the DFT level. More than 300 configurations were examined and their total electronic energies ordered for each cluster mass. The D-plot methodology was employed for classifying different structures into cluster families and identifying the most stable structures. For each family, the binding energies of the analyzed cluster species have been determined and displayed as a function of the cluster mass. The fluctuations of these curves are then compared to experimental data, namely, to the dependence of relative ion desorption yields on cluster mass, obtained from Plasma Desorption Mass Spectroscopy (PDMS) of quite distinct types of organic samples: solid methane, cholesterol and pump oil vapor adsorbed on polycrystalline LiF. It is found that theoretical predictions and experimental data are in good agreement, showing that the abundance of the emitted small cluster ions is determined by their own structure and not by the structure of the original sample.

XUV photofragmentation of small water cluster cations at FLASH

Photofragmentation of small water cluster ions H+(H2O)n (n = 1..4) has been investigated at 35eV using fast beam momentum imaging in a crossed beams setup at the free electron laser FLASH at DESY in Hamburg, revealing the dominant fragmentation pathways of these systems under ionizing radiation.

Initial- and final-state effects on screening and branching ratio in2px-ray absorption of size-selected free3dtransition metal clusters

A comprehensive study of size-dependent initial- and final-state effects in $2p$ x-ray absorption of free, homonuclear $3d$ transition metal clusters from titanium through nickel in a size range of 1--200 atoms is presented. From atom to bulk the $2p\ensuremath{\rightarrow}3d$ excitation in $3d$ transition metals shifts to higher photon energy, which can be explained by increasing valence electron delocalization in the initial state. A cluster-size-dependent decrease of the ${L}_{2,3}$ branching ratio at a constant $2p$ spin-orbit splitting can be attributed to core-hole screening effects for all investigated $3d$ transition metal clusters. For small manganese cluster ions, highly localized $3d$ orbitals with ferromagnetic spin coupling and a filled $3d$ majority spin subband are suggested by the experimental $2p$ x-ray absorption spectra.