Ionization of water clusters by collision with surface

New results of the investigations of the phenomenon of the ion formation and separation at the neutral water cluster scattering by solid surface are reported. First, molecular dynamics simulation has shown the possibility of polar dissociation of water molecules in (H ₂ O)_n cluster with n = 34 and 64 during their impact with a rigid surface. Second, the current of ions rebound from target and the current to the target at scattering of (H2O)_n clusters (up to n = 50000) by various targets are measured. Third, we have measured the water-cluster-induced electrification of the electric field mill sensor, which has been used for the rocket measurement of mesospheric electric field structure in the vicinity of noctilucent clouds (NLC). A comparison between these and rocket measurements shows that the fluctuations of the field mill signal detected when a rocket was passing the NLC layer is a result of the impact of NLC particles on the field mill electrodes.     

Ionization of water clusters by collisions with graphite surfaces

We present experimental results on the scattering of neutral water clusters from graphite surfaces. We use cluster beams with an average cluster size up to 3700 molecules and an incident velocity of 1300 m/s, and study the emission of negatively and positively charged cluster fragments from the surface. The ionization probability is found to depend on cluster size and surface temperature, and for a given mean cluster size the emission rate of positive and negative cluster ions follows the Arrhenius equation. In the surface temperature range 950–1450 K, activation energies of 0.52±0.02 and 3.1±0.3 eV are determined for the emission of positive and negative ions, respectively. The emission of negative cluster fragments is attributed to electron transfer from the surface, and we estimate an electron affinity of 1.4±0.3 eV for large water clusters. Positive cluster fragments are proposed to be formed by dissocative ionization inside the cluster, followed by removal of the negative ion during surface contact.     

Nucleation of water vapor on Na<Superscript>+</Superscript>Cl<Superscript>−</Superscript> ion pairs: Computer simulation

The Monte Carlo method is used to calculate the free energy, entropy, and work of water cluster formation in the field of Na⁺Cl⁻ ion pairs. A detailed model is used that allows for polarization and covalent many-particle interactions, as well as the effects of ion charge reversal. The model is matched to the experimental data on the free energy of ion hydration and the results of the quantum-chemical calculations of stable configurations. The hydration leads to the cleavage of an ion pair in a molecular cluster after approximately ten water molecules are captured. As vapor molecules are added, the stable interion distance monotonically elongates. The low free energy barrier separating the dissociated and nondissociated states of the ion pair in an equilibrium cluster does not hinders the reversible spontaneous transitions between the states, which are responsible for strong fluctuations and the instability of the system. Unlike hydroxonium-containing ion pairs, the formation of long-lived metastable states of hydrated Na⁺Cl⁻ pairs is impossible.     

Measuring the extent and width of internal energy deposition in ion activation using nanocalorimetry

The recombination energies resulting from electron capture by a positive ion can be accurately measured using hydrated ion nanocalorimetry in which the internal energy deposition is obtained from the number of water molecules lost from the reduced cluster. The width of the product ion distribution in these experiments is predominantly attributable to the distribution of energy that partitions into the translational and rotational modes of the water molecules that are lost. These results are consistent with a singular value for the recombination energy. For large clusters, the width of the energy distribution is consistent with rapid energy partitioning into internal vibrational modes. For some smaller clusters with high recombination energies, the measured product ion distribution is narrower than that calculated with a statistical model. These results indicate that initial water molecule loss occurs on the time scale of, or faster than energy randomization. This could be due to inherently slow internal conversion or it could be due to a multi-step process, such as initial ion-electron pair formation followed by reduction of the ion in the cluster. These results provide additional evidence for the accuracy with which condensed phase thermochemical values can be deduced from gaseous nanocalorimetry experiments.     

Self-assembly formation of the magic ion of (H2O)20O+: observation of nanoscale cages of oxygenated water clusters induced from iron nanoparticles

For the first time, we observed a stable and intense ion (m/z 376) of the oxygenated water cluster ion ((H(2)O)(20)O(+)) produced from simply spraying an aqueous solution of iron nanoparticles (Fe NPs) into an electrospray mass spectrometry (ESI-MS) system. Tandem mass spectrometric (MS/MS and MS/MS/MS) results were applied to identify the assignments of the fragment ions of m/z 376 in order to explore the possible structures of this cluster ion. The possible structures of the (H(2)O)(20)O(+) ions are proposed as pentagonal dodecahedron water clathrate cages from the results of tandem mass spectrometry since eliminations of five water molecules were frequently observed in the MS/MS results for many subsequent fragment ions of m/z 376. The formation of this oxygenated water cluster ion ((H(2)O)(20)O(+)) in ESI-MS is attributed to the high surface reactivity and surface energy of Fe NPs during ESI processes (under high temperature and high voltage (5 kV) of ESI spray environment). We believe that the observation of self-assembly formation of oxygenated water clusters is an important issue in nanoscience as well as in the fields of water clusters.     

Microsolvation of the sodium and iodide ions and their ion pair in acetonitrile clusters: a theoretical study

The structural and thermodynamic properties of Na+(CH3CN)n, I-(CH3CN)n, and NaI(CH3CN)n clusters have been investigated by means of room-temperature Monte Carlo simulations with model potentials developed to reproduce the properties of small clusters predicted by quantum chemistry. Ions are found to adopt an interior solvation shell structure, with a first solvation shell containing approximately 6 and approximately 8 acetonitrile molecules for large Na+(CH3CN)n and I-(CH3CN)n clusters, respectively. Structural features of Na+(CH3CN)n are found to be similar to those of Na+(H2O)n clusters, but those of I-(CH3CN)n contrast with those of I-(H2O)n, for which "surface" solvation structures were observed. The potential of mean force calculations demonstrates that the NaI ion pair is thermodynamically stable with respect to ground-state ionic dissociation in acetonitrile clusters. The properties of NaI(CH3CN)n clusters exhibit some similarities with NaI(H2O)n clusters, with the existence of contact ion pair and solvent-separated ion pair structures, but, in contrast to water clusters, both types of ion pairs adopt a well-defined interior ionic solvation shell structure in acetonitrile clusters. Whereas contact ion pair species are thermodynamically favored in small clusters, solvent-separated ion pairs tend to become thermodynamically more stable above a cluster size of approximately 26. Hence, ground-state charge separation appears to occur at larger cluster sizes for acetonitrile clusters than for water clusters. We propose that the lack of a large Na+(CH3CN)n product signal in NaI(CH3CN)n multiphoton ionization experiments could arise from extensive stabilization of the ground ionic state by the solvent and possible inhibition of the photoexcitation mechanism, which may be less pronounced for NaI(H2O)n clusters because of surface solvation structures. Alternatively, increased solvent evaporation resulting from larger excess energies upon photoexcitation or major solvent reorganization on the ionized state could account for the observed solvent-selectivity in NaI cluster multiphoton ionization.     

Removal of disturbances in carbon determination by activation analysis for lowering the detection limit

A sensitive method of carbon determination in silicon by activation with deuterons and³He particles followed by chemical separation of the measured nuclides¹³N and¹¹C has been developed. Aside from the etch removal after irradiation, it is essential for a low detection limit to avoid too much thermal exposure of the target surface during the irradiation process, or else microflaw formation and diffusion on the surface have to be anticipated. The ion current must have an optimal rather than a maximum value.     

Liquid chromatography particle beam-mass spectrometry with massive cluster impact

A new liquid chromatography-mass spectrometry technique is described that utilizes a particle beam interface to transport and deposit desolvated analyte molecules onto a target surface that is bombarded by a primary beam of massive multiply charged glycerol cluster ions to generate secondary ions for mass analysis. The massive cluster ion beam is generated by electrohydrodynamic emission from a solution of 1. 5-M ammonium acetate in glycerol. In the present instrumental configuration the massive cluster ion gun is placed above the target probe and the particle beam interface is connected through a side port of the mass spectrometer. The massive cluster ion beam and particle beam are intercepted by a target surface substituted for a conventional ion volume. The target surface is positioned such that it is ～ 45 ° to the primary cluster ion beam, the particle beam, and the mass analyzer axis. This geometric orientation represents a compromise among the performances of these three elements. The feasibility of this liquid chromatography-particle beam on-line with massive cluster impact is demonstrated by flow injections of acetylcholine chloride and gramicidin S. Spectra generated from this preliminary study indicate promise for routine liquid chromatography-mass spectrometry of polar compounds by using a robust inlet and an effective generation of secondary ions without an added matrix.     

Interaction of water with LiCl, LiBr, and LiI in the deeply supercooled region

The hydration mechanism of lithium halides was studied using time-of-flight secondary ion mass spectrometry as a function of temperature. The lithium halides embedded in thin films of amorphous solid water segregate to the surface at temperatures higher than 135-140 K, with efficiency increasing in the order of LiCl, LiBr, and LiI. A monolayer of LiCl and LiI adsorbed on the surface of amorphous solid water tends to diffuse into the bulk at 160 K. The infrared absorption band revealed that the aqueous lithium-halide solutions and crystals are formed simultaneously at 160 K; these phenomena are explicable as a consequence of the evolution of supercooled liquid water. The strong surfactant effect is inferred to arise from hydration of a contact ion pair having hydrophilic (lithium) and hydrophobic (halide) moieties. Furthermore, bulk diffusion of lithium halides might result from the formation of a solvent-separated ion pair in supercooled liquid water. The presence of two liquid phases of water with different local structures is probably responsible for the formation of these two hydrates, consistent with the calculated result reported by Jungwirth and Tobias[J. Phys. Chem. B 106, 6361 (2002)].     

Total cross sections for K + Br2 at relative energies between 0 and 8000 eV

Trajectory Surface Hopping (TSH) calculations have been applied to the non-elastic scattering in the K + Br₂ collision system over a wide range of relative kinetic energies from 0 to 8000 eV. Absolute total cross sections have been computed for the formation of various collision products with an accuracy of 5% with respect to statistical errors. The following non-elastic processes have been studied: chemical reaction, inelastic neutral scattering, neutral dissociation and ion pair formation, yielding atomic as well as molecular negative bromine ions together with PC ions. The absolute values of the respective total cross sections, obtained from the TSH calculations, are in close agreement with the available experimental data, both for chemical reaction and for ion pair formation, over the whole energy range considered. The three particle character of the collision system is important in describing the experimental results quantitatively at relative kinetic energies below 100 eV.     

On the dissolution processes of Na2I+ and Na3I2+ with the association of water molecules: mechanistic and energetic details

The sequential association energies for one through six water molecules clustering to Na(2)I(+), as well as one and two water molecules clustering to Na(3)I(2)(+), are measured. The association energies show a pairwise behavior, indicating a symmetric association of water molecules to the linear Na(2)I(+) and Na(3)I(2)(+) ions. This pairwise behavior is well reproduced by Density Functional Theory (DFT) calculations. DFT calculations also suggest that a significant separation of charge for the Na-I ion pair occurs when four or more water molecules cluster to a single sodium center. Two different solvent-separated ion pairs have been identified with the DFT calculations. Experiments also show that the dissolution processes, loss of a neutral NaI unit, occurs when six or more water molecules have been added to Na(2)I(+) cluster. However, one or two water molecules are able to detach an NaI unit from the Na(3)I(2)(+) cluster. The difference in solubility of the Na(2)I(+) and Na(3)I(2)(+) ions is due to the difference in the energies required to lose an NaI unit from these two species. The experiment also confirms that the loss of a neutral NaI unit, instead of an Na(+) ion, occurs during the dissolution processes of Na(3)I(2)(+). The microsolvation schemes proposed to explain our experimental observations are supported by DFT and phase space theory (PST) calculations.     

Polarization effects and charge separation in AgCl-water clusters

Structural, energetic, vibrational, and electronic properties of salt ion pairs (AgCl and NaCl) in water (W) clusters were investigated by density functional theory. In agreement with recent theoretical studies of NaCl-water clusters, structures where the salt ion pair is separated by solvent molecules or solvent separated ion pair (SSIP) were found in AgCl-W(6) and AgCl-W(8) aggregates. Our results indicate that for small AgCl-water clusters, contact ion pair (CIP) structures are energetically more stable than SSIP, whereas an opposite tendency was observed for NaCl-water clusters. In comparison with CIP, SSIP are characterized by extensive electronic density reorganization, reflecting enhanced polarization effects. A major difference between AgCl-water and NaCl-water CIP aggregates concerns charge transfer. In AgCl-water CIP clusters, charge is transferred from the solvent (water) to the ion pair. However, in NaCl-water CIP clusters charge is transferred from the ion pair to the water molecules. The electronic density reorganization in the aggregates was also discussed through the analysis of electronic density difference isosurfaces. Time dependent density functional theory calculations show that upon complexation of AgCl and NaCl with water molecules, excitation energies are significantly blueshifted relative to the isolated ion pairs ( approximately 2 eV for AgCl-W(8) SSIP). In keeping with results for NaI-water clusters [Peslherbe et al., J. Phys. Chem. A 104, 4533 (2000)], electronic oscillator strengths of transitions to excited states are weaker for SSIP than for CIP structures. However, our results also suggest that the difference between excitation energies and oscillator strengths of CIP and SSIP structures may decrease with increasing cluster size.     

Large- and small-angle ion-scattering spectroscopy for studying cold, clean and gas-covered metal surfaces

New possibilities of the large-angle and small-angle ion scattering spectroscopy are demonstrated. If the target material consists of two or more isotope components it was found that the ratio of isotope parts of the quasi-single-scattering peak reaches a maximum at some incidence angle. The maximum is attributed to the fact that the ions doubly scattered from the atoms with smaller masses can have the same energy and exit angle as the ions singly scattered from the atoms with larger masses. Based on the atomic pair model (shadowing atom plus scattering atom) the mean distance between two atoms of the topmost atomic layer can be found. In the range of small scattering angles the substrate quasi-single-scattering peak as well as the recoil peak show a fine structure if the cold metal surface is covered by a thin film of condensed gases. It is postulated that such effect is connected with a multiple scattering of projectiles and recoils inside the condensed film. Multiple scattering direct the ions to the detector after single scattering from the substrate atom over the angles larger or smaller that the detection angle. This effect can be used for determination of a cleanliness of the target surface or parameters of the condensed layer (thickness or density).     

Secondary ion emission induced by fission fragment impact in CO-NH(3) and CO-NH(3)-H(2)O ices: modification in the CO-NH(3) ice structure

CO-NH(3) and CO-NH(3)-H(2)O ices at 25-130 K were bombarded by (252)Cf fission fragments ( approximately 65 MeV at the target surface) and the emitted secondary ions were analyzed by time-of-flight mass spectrometry (TOF-SIMS). It is observed that the mass spectra obtained from both ices have similar patterns. The production of hybrid ions (formed from CO and NH(3) molecules) emitted from CO-NH(3) ice has already been reported by R. Martinez et al., Int. J. Mass. Spectrom. 262 (2006) 195; here, the secondary ion emission and the modifications of the CO--NH(3) ice structure during the temperature increase of the ice are addressed. These studies are expected to throw light on the sputtering from planetary and interstellar ices and the possible formation of new organic molecules in CO-NH(3)-H(2)O ice by megaelectronvolt ion bombardment. The presence of water in the CO-NH(3) ice mixture generates molecular ion series such as (NH(3))(p-q)(H(2)O)(q)CO(+) and replaces the cluster series (NH(3))(n)NH(4) (+) emission by the hybrid series (NH(3))(I-i)(H(2)O)(i=1, 2...I)H(+). The distribution of NH(3) and H(2)O molecules within the cluster groups indicates that ammonia and water mix homogeneously in the icy condensate at T = 25 K. The desorption yield distribution of the cluster series (NH(3))(n)NH(4) (+) is described by the sum of two exponential functions: one, slow-decreasing, attributed to the fragmentation of the solid target into clusters; and another, fast-decreasing, due to a local sublimation followed by recombination of ammonia molecules. The analysis of the time-temperature dependence of these two yield components gives information on the formation process of molecular ions, the transient composition of the ice target and structural changes of the ice. Data suggest that the amorphous and porous structure of the NH(3) ice, formed by the condensation of the CO--NH(3) gas at T = 25 K, survives CO sublimation until the occurrence of a phase transition around 80 K, which produces a more fragile ice structure.     

Electron attachment to molecules in a cluster environment

Low-energy dissociative electron attachment (DEA) to the CF(2)Cl(2) and CF(3)Cl molecules in a water cluster environment is investigated theoretically. Calculations are performed for the water trimer and water hexamer. It is shown that the DEA cross section is strongly enhanced when the attaching molecule is embedded in a water cluster, and that this cross section grows as the number of water molecules in the cluster increases. This growth is explained by a trapping effect that is due to multiple scattering by water molecules while the electron is trapped in the cluster environment. The trapping increases the resonance lifetime and the negative ion survival probability. This confirms qualitatively existing experiments on electron attachment to the CF(2)Cl(2) molecule placed on the surface of H(2)O ice. The DEA cross sections are shown to be very sensitive to the position of the attaching molecule within the cluster and the orientation of the electron beam relative to the cluster.     

Structural transition in the OH<Superscript>−</Superscript>(H<Subscript>2</Subscript>O)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> cluster in water vapors

The hydration of hydroxyl ion in water vapors at temperatures corresponding to seasonal variations in natural air medium is studied by the Monte Carlo simulation in grand canonical statistical ensemble using the detailed model of intermolecular forces that takes into account many-particle covalent interactions, polarization, and charge transfer. An increase in the number of water molecules in a cluster is accompanied by a structural transition from strongly asymmetric ion environment of water molecules to the formation of enveloping shell composed of these molecules. This transition is accompanied by an abrupt increase in cluster size and qualitative changes in its structural characteristics. The displacement of ion on the surface of clusters with extremely small sizes is an entropy effect. Results of simulation are compared with data on the hydration of hydroxonium at which similar structural transition is not observed, and with data of quantum-chemical calculations.     

What is the minimum number of water molecules required to dissolve a potassium chloride molecule?

This work answers an unsolved question that consists of determining the least number of water molecules necessary to separate a potassium chloride molecule. The answer based on accurate quantum chemical calculations suggests that tetramers are the smallest clusters necessary to dissociate KCl molecules. The study was made with Møller‐Plesset second‐order perturbation theory modified with the cluster theory having single, double, and perturbative triple excitations. With this extensive study, the dissociation of KCl molecule in different water clusters was evaluated. The calculated results show that four water molecules stabilize a solvent separated K⁺/Cl^? ion‐pair in prismatic structure and with six water molecules further dissociation was observed. Attenuated total reflection infrared spectroscopy of KCl dissolved in water establishes that clusters are made of closely bound ions with a mean of five water molecules per ion‐pair [K⁺(H₂O)₅Cl^?]. (Max and Chapados, Appl Spectrosc 1999, 53, 1601; Max and Chapados, J Chem Phys 2001, 115, 2664.) The calculated results tend to support that five water molecules leads toward the formation of contact ion‐pair. The structures, energies, and infrared spectra of KCl molecules in different water clusters are also discussed. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Excited-state proton transfer and ion pair formation in a Cinchona organocatalyst

The excited-state proton transfer and subsequent intramolecular ion pair formation of a cupreidine-derived Cinchona organocatalyst () were studied in THF-water mixtures using picosecond time-resolved fluorescence together with global analysis. Full spectral and kinetic characterization of all the fluorescent species allowed us to monitor the 3-step process for the ion pair dissociation. In the first step, proton transfer occurs through a water "wire" from the 6-hydroxyquinoline unit (excited-state acid) to the covalently bonded basic quinuclidine moiety, resulting in a hydrogen bonded ion pair. This was confirmed by the observed kinetic isotope effect in the presence of heavy water. In the second step, the formed ions are further solvated by a few solvent molecules, producing the solvent separated ion pair. Finally, a fully solvated ion pair is formed. The 5-exponential global model derived from the reaction scheme describes the experimental data very well.     

Specific interactions including imidazolium andN-methylimidazolium lons. I. Interactions with the picrate anion

The conductances of solutions of methylimidazolium and imidazolium picrate (MeImHPic and ImHPic) in nitrobenzene-benzene mixtures (27.2–. These triple ions are highly stabilized by the hydrogen bond between the second NH group of the ion pair and the second picrate ion. Values of the formation constants for the ion pair ImHPic and for the triple ion PicImHPic^– have been calculated and are discussed.     

Ab initio molecular dynamics studies of ionic dissolution and precipitation of sodium chloride and silver chloride in water clusters, NaCl(H2O)n and AgCl(H2O)n, n = 6, 10, and 14

An ab initio molecular dynamics method was used to compare the ionic dissolution of soluble sodium chloride (NaCl) in water clusters with the highly insoluble silver chloride (AgCl). The investigations focused on the solvation structures, dynamics, and energetics of the contact ion pair (CIP) and of the solvent-separated ion pair (SSIP) in NaCl(H(2)O)(n) and AgCl(H(2)O)(n) with cluster sizes of n = 6, 10 and 14. We found that the minimum cluster size required to stabilize the SSIP configuration in NaCl(H(2)O)(n) is temperature-dependent. For n = 6, both configurations are present as two distinct local minima on the free-energy profile at 100 K, whereas SSIP is unstable at 300 K. Both configurations, separated by a low barrier (<10 kJ mol(-1)), are identifiable on the free energy profiles of NaCl(H(2)O)(n) for n = 10 and 14 at 300 K, with the Na(+)/Cl(-) pairs being internally solvated in the water cluster and the SSIP configuration being slightly higher in energy (<5 kJ mol(-1)). In agreement with the low bulk solubility of AgCl, no SSIP minimum is observed on the free-energy profiles of finite AgCl(H(2)O)(n) clusters. The AgCl interaction is more covalent in nature, and is less affected by the water solvent. Unlike NaCl, AgCl is mainly solvated on the surface in finite water clusters, and ionic dissolution requires a significant reorganization of the solvent structure.