Alkali Halide Cluster Ions Research Articles

Emission of Alkali Halide Cluster Ions from the Charged Droplets Generated from Electrospray Ionization.

In this study, the formation and emission of alkali halide cluster ions in charged droplets generated by electrospray ionization (ESI) was investigated using mass spectrometry (MS). We focus on ion emission at the air-solution interface of charged droplets, distinguishing between two mechanisms: the ion evaporation model (IEM), where ions are released directly from the interface, and the charge residue model (CRM), where ions are generated after complete solvent evaporation. Using an iodide/chloride mixture, we analyzed how interfacial affinity influences the composition of the generated alkali halide cluster cations and anions. With the knowledge that iodides have much higher interfacial affinities than chlorides, a relative faction of iodide in the cluster ion enables us to distinguish between IEM and CRM. Small cluster anions and cations exclusively containing iodides are suggested to be from IEM, while the larger cluster ions containing more chlorides are expected to be from CRM. This work clarifies the distinctions between IEM and CRM in alkali halide cluster ion formation and also establishes a robust analytical approach for assessing interfacial affinities of ions using ESI-MS, which may potentially enhance our understanding of interfacial chemistry and its implications in atmospheric and analytical sciences.

Isomerism of Cyclodextrin Tetramer Induced by Alkali Halide Cluster Ions Observed by Ion Mobility Spectrometry-Mass Spectrometry.

Cyclodextrins (CDs) exhibit versatile self-assembly properties due to their hydrophilic and hydrophobic components, with applications such as drug delivery and selective binding. While research on CD self-assembly is extensive, limited studies have explored their aggregation behavior, particularly in interactions with small ionic guests. The present work investigates the structure of β-CD tetramers aggregated with alkali metal chloride clusters using ion mobility spectrometry-mass spectrometry (IMS-MS). The results revealed that diverse structures emerge in the tetramer depending on the alkali metal cluster size. Notably, the doubly charged tetramer exhibits distinct aggregation trends with specific numbers of MCl clusters for Na+ and K+ ions. After initially adopting a bucket-wheel structure with two internal cations, the structure transforms into a new isomer with a tetrahedral configuration upon cluster addition. The formation of the new isomer structure is closely linked to filling the cavity volume with MCl clusters and ionic interactions, which possibly compensate for the weakened hydrogen bonds between CDs. Theoretical calculations further support the structures, showing well-matched collision cross-section (CCS) values compared with the experimental CCS values. This study highlights the role of alkali metal chloride clusters as potential templates, leading to the formation of novel CD assemblies.

Geometrical Structures and Molecular Adsorption Reactions of Alkali-halide Cluster Ions Investigated by Ion Mobility-mass Spectrometry

Geometrical Structures and Molecular Adsorption Reactions of Alkali-halide Cluster Ions Investigated by Ion Mobility-mass Spectrometry

Very high critical energy fragmentations observed in CID

High energy CID fragmentations of triatomic alkali halide cluster ions were examined to estimate the energy transfer during the CID process. A detailed CID study using keV collisions revealed that M 2X + clusters yield not only the expected M + ions but practically all possible combinations of mono- and biatomic ions. In the case of Cs 2I +, for example Cs +, Cs 2 +, CsI +, and even I + fragment ions are observed. The results suggest that collisional energy transfer in keV collisions has a monotonously decreasing shape (full width at half height is ∼2 eV) and has a long high energy tail extending well over 10 eV (with at least 5% probability), the average collisional energy transfer being about 5 eV.

Structures and stabilities of doubly charged (MgO)nMg2+ (n=1–29) cluster ions

Ab initio perturbed ion plus polarization calculations are reported for doubly charged nonstoichiometric (MgO)nMg2+ (n=1–29) cluster ions. We consider a large number of isomers with full relaxations of the geometries, and add the correlation correction to the Hartree–Fock energies for all cluster sizes. The polarization contribution is included at a semiempirical level also for all cluster sizes. Comparison is made with theoretical results for neutral (MgO)n clusters and singly charged alkali–halide cluster ions. Our method is also compared to phenomenological pair potential models in order to assess their reliability for calculations on small ionic systems. The large coordination-dependent polarizabilities of oxide anions favor the formation of surface sites, and thus bulk-like structures begin to dominate only after n=24. The relative stabilities of the cluster ions against evaporation of an MgO molecule show variations that are in excellent agreement with the experimental abundance spectra.

Evaporative processes of sputtered alkali halide cluster ions

Metastable cluster ions produced by 8 keV atom sputtering of alkali halides have been measured. The dominant evaporative mechanism of the metastable ( MX) n M + cluster ions involves both monomer and dimer loss, and these processes have been examined using semi-empirical self-consistent-field molecular orbital methods to determine the geometry-optimized structures and decomposition energetics.

Experimental and theoretical studies of the structure of alkali halide clusters

We have investigated the structures and properties of alkali halide cluster ions produced by laser vaporization of solid samples. In many alkali halide cluster ions, we observe the appearances of bulk-like characteristics even at sub-nanometer sizes:fcc crystalline structures (including surface terraces), ionic binding, and a susceptibility to common bulk defects such as F and H color-centers. To understand the origins of cluster structures, we have made calculations of ground state energetics, high-temperature molecular dynamics, and the electronic structure of clusters having excess electrons.

Alkali-halide cluster ions produced by laser vaporization of solids

The mass spectra of alkali-halide cluster ions M(MX${)}_{\mathit{n}}^{+}$ and X(MX${)}_{\mathit{n}}^{\mathrm{\ensuremath{-}}}$, produced by laser vaporization of a solid sample, were measured by tilted-plate time-of-flight mass spectrometry. The measured abundances of sodium chloride clusters (n=1--200), sodium iodide clusters (n=1--125), cesium chloride clusters (n=1--125), and cesium iodide clusters (n=1--100) confirm that surface terraces are the most stable additions to a cuboid base lattice. Also, the spectra show that the relative stabilities of certain cluster ions are dependent on the ionic radii of the constituent atoms.

High-pressure fast-atom bombardment mass spectrometry: Collisional stabilization and reactions of alkali halide cluster ions

A high-pressure/fast-atom bombardment ion source has been used for detailed investigations of collisional stabilization of sputtered ions. Specifically, alkali halide cluster ions, which acquire significant internal energies from the sputtering process, are stabilized to a large extent through collisions with a buffer gas, as demonstrated by reduction of “magic number” behavior in the cluster ion abundance distributions and in the unimolecular dissociations of mass-selected species. The extent of collisional stabilization correlates well with the collisional stabilization efficiency of the gas and buffer gas pressure. Reactions between the alkali halide samples and certain buffer gases result in unexpected cluster product ions, for example the observation of only [(CsF) n Cs] + from sputtering CsI in SF 6. The origin of these product ions has been investigated with a set of experiments involving mass spectrometry and X-ray photoelectron spectroscopy. The formation mechanism for these cluster product ions is not clear, but in part it must consist of fast-atom beam-assited reactions between the substrate and buffer gas reactant species in the selvedge.

Do the new cluster sources also produce isomers?

The experimental and theoretical effort in the Chemistry Division of the Naval Research Laboratory in the area of clusters is briefly reviewed. We have focused on the effects of structural isomers and “magic number” structures on distributions of cluster sizes and, more recently, on the role of isomers in chemical reactions. The focus of this work is on whether or not the existence of a magic cluster size precludes there being more than one isomer of that size. Various theoretical methods have been used to study the sometimes magic 9-atom alkali-halide cluster ions. It is possible, but not compelling, that isomers of this cluster size are produced in sputtering sources of clusters. Under certain experimental conditions the relative intensity of the 60-carbon-atom ion appears so great as to suggest that no isomer other than the proposed icosahedral structure can be present. The geometry of this unique structure has been optimized using two local density functionals to predict some of its special properties. For the case of the magic C ion, theoretically the two lowest-lying isomers are clearly linear and monocyclic. Experimentally, both forms are observed with cyclic form predominant.

Mechanisms of ion-induced desorption of large molecules and clusters

Our model, which describes the final states of molecules and clusters desorbed from ion beam-irradiated surfaces of molecular solids and dielectrics, is reviewed. The desorption is considered to be induced by a relaxation process in which the deposited energy is transferred to the internal vibration modes of the perturbed region. The model is used to describe the mass distribution of the insulin molecular ions desorbed by MeV primary ions, the mass distribution of various alkali halide cluster ions desorbed by keV primary ions, and the energy distribution of desorbed neutrals from frozen SF 6. The agreement with theory for experimental mass and energy distributions of desorbed particles is reasonable. The energy deposited per normal mode calculated by our model suggests that the MeV primary ions are more efficient in promoting desorption than keV primary ions, in agreement with experimental observations.

The role of alternative geometries in alkali–halide clusters

The relative importance of the cubic structures that were proposed to explain magic numbers for alkali–halide cluster ions from cluster sources is examined via total-energy calculations on nine-atom cluster ions of various optimized geometries. The relative energies of the planar, tetrahedral, quasioctahedral, lowest energy nonplanar nine-atom clusters for LiF, LiI, NaI, KI, RbI, CsI, NaF, NaCl, NaBr, and NaI are computed using Martin’s Coulomb plus the Born–Mayer potential model. The most stable structure is invariably a slightly puckered plane. The relative energies of these clusters for LiF have also been tested using Hartree–Fock and density functional theory. Other comparisons are made for NaCl clusters and eight-atom LiF clusters. The computationally more tractable Born–Mayer potentials rather accurately predict the relative energies of the clusters in the ab initio calculations. The largest problem is too strong a repulsion between like atoms which overestimates the energy difference between the planar and quasioctahedral structure proposed by Morgan et al. These calculations suggest a greater population of noncubic structures for the larger and more polarizable alkali–halide cluster ions in beams from cluster sources.

The magic number nine‐atom alkali halide cluster ion. Is the nine‐atom planar structure the most stable?

AbstractThe question of whether the cubic structures proposed for the alkali halide cluster ions are the most stable has been re‐examined. Geometry/energy optimization calculations were performed on other candidate structures for the [M(MX)4]+ species. The square‐planar structure was found to be more than 3 eV lower in energy than a proposed quasi‐octahedral structure. The relative energies of the square‐planar and other structures for a series of nine‐atom alkali halide cluster ions are found to correlate with published experimental data.

Investigation of the relative stability of positive alkali halide cluster ions generated by fast atom bombardment

AbstractThe stabilities of alkali halide cluster ions [M(MX)n]+ (M  Li, Na, K, Rb, Cs; X  F, Cl, Br, I) have been studied by measuring the fragment ion yields following dissociation of the ions in the second field free region of a ZAB‐2F mass spectrometer. Extractable cluster ions were observed for certain values of n. It was found that the stabilities of the neutral fragment species formed are also of importance in determining the fragmentation rates. Possible configurations of M and X in the stable ions are discussed.