Water Cluster Ions Research Articles

Ab Initio Path Integral Simulations for the Fluoride Ion–Water Clusters: Competitive Nuclear Quantum Effect Between F––Water and Water–Water Hydrogen Bonds

Small hydrated fluoride ion complexes, F(-)(H2O)n (n = 1-3), have been studied by ab initio hybrid Monte Carlo (HMC) and ab initio path integral hybrid Monte Carlo (PIHMC) simulations. Because of the quantum effect, our simulation shows that the average hydrogen-bonded F(-)···HO distance in the quantum F(-)(H2O) is shorter than that in the classical one, while the relation inverts at the three water molecular F(-)(H2O)3 cluster. In the case of F(-)(H2O)3, we have found that the nuclear quantum effect enhances the formation of hydrogen bonds between two water molecules. In F(-)(H2O)2 and F(-)(H2O)3, the nuclear quantum effect on two different kinds of hydrogen bonds, F(-)-water and water-water hydrogen bonds, competes against each other. In F(-)(H2O)3, thus, the nuclear quantum effect on the water-water hydrogen bond leads to the elongation of hydrogen-bonded F(-)···HO distance, which we suggest this as the possible origin of the structural inversion from F(-)(H2O) to F(-)(H2O)3.

Enhancing Secondary Ion Yields in Time of Flight-Secondary Ion Mass Spectrometry Using Water Cluster Primary Beams

Low secondary ion yields from organic and biological molecules are the principal limitation on the future exploitation of time of flight-secondary ion mass spectrometry (TOF-SIMS) as a surface and materials analysis technique. On the basis of the hypothesis that increasing the density of water related fragments in the ion impact zone would enhance proton mediated reactions, a prototype water cluster ion beam has been developed using supersonic jet expansion methodologies that enable ion yields using a 10 keV (H2O)1000+ beam to be compared with those obtained using a 10 keV Ar1000+ beam. The ion yields from four standard compounds, arginine, haloperidol, DPPC, and angiotensin II, have been measured under static+ and high ion dose conditions. Ion yield enhancements relative to the argon beam on the order of 10 or more have been observed for all the compounds such that the molecular ion yield per a 1 μm pixel can be as high as 20, relative to 0.05 under an argon beam. The water beam has also been shown to partially lift the matrix effect in a 1:10 mixture of haloperidol and dipalmitoylphosphatidylcholine (DPPC) that suppresses the haloperidol signal. These results provide encouragement that further developments of the water cluster beam to higher energies and larger cluster sizes will provide the ion yield enhancements necessary for the future development of TOF-SIMS.

Specific Reaction Path Hamiltonian for Proton Transfer in Water: Reparameterized Semiempirical Models.

The semiempirical MNDO-based AM1 and PM3 methods and the orthogonalization-corrected OM1, OM2, and OM3 models were reparameterized to improve their description of bulk water and of proton transfer in water. Reference data included the gas-phase geometries and energies of the water molecule, small water clusters, the hydronium ion, and small hydronium ion-water clusters, as well as the gas-phase potential energy surface for proton transfer between the two water molecules in a Zundel ion, all calculated at the MP2/aug-cc-pVTZ level of theory. Combined QM/MM molecular dynamics simulations were carried out for bulk water and for a proton solvated in water using large cluster models. Both the authentic and reparameterized semiempirical models were employed in the simulations. The reparameterization led to significantly better results in all cases. The new set of OM3 parameters gave the best overall results for the structural and dynamic properties of water and the hydrated proton, with a small but finite barrier of 0.1-0.2 kcal/mol in the potential of mean force for proton transfer, in agreement with ab initio path-integral molecular dynamics simulations. The reparameterized OM3 model is expected to be useful for efficient modeling of proton transfer in aqueous solution.

Structural and Infra Red Spectroscopic Aspects of Ion-Water Clusters: A Study Based on a Combined Stochastic and Quantum Chemical Approach

In the present communication, we explore a strategy based on using a stochastic technique, namely Simulated Annealing (SA), in conjunction with quantum chemical calculations to arrive at good candidate structures for ion-water clusters. We investigate if our approach is equally suited to handle structures in which the encapsulated ion is cationic (Na+) as well as anionic (Cl−). To critically analyse the quality of structure being generated, we perform spectroscopic and thermochemical studies on these systems and compare our results with the available experimental ones. We conclude that our idea is both computationally cost effective as well as quantitatively accurate in providing good quality structures for these systems. Structure of (H2O)6Na+ obtained after Simulated Annealing optimization (using an emperical potential) followed by further optimization at MP2 level of theory

Electrokinetic Charging and Evidence for Charge Evaporation in Liquid Microjets of Aqueous Salt Solution

Electrokinetic charging of aqueous microjets was characterized by measuring streaming currents as a function of sodium iodide salt concentration. Measured streaming currents at high salt concentrations (up to 0.5 M) varied nonmonotonically with the jet velocity and can be explained by a multipolar charge distribution at the nozzle-water interface. In the case of potassium fluoride no multipolar charge distribution is observed. Electrokinetic potentials were estimated from the streaming currents, under the assumption that all excess charges are confined within the liquid jet. Measured photoelectron spectra indicate much smaller streaming potentials. To resolve the apparent discrepancy, we propose that a significant fraction of excess charges evaporates in the form of ion-water clusters.

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.

Surface processing using water cluster ion beams

Vaporized water clusters were produced by an adiabatic expansion phenomenon, and various substrates such as Si(100), SiO2, polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), and polycarbonate (PC) were irradiated by water cluster ion beams. The sputtered depth increased with increasing acceleration voltage, and the sputtering rate was much larger than that obtained using Ar monomer ion irradiation. The sputtering yield for PMMA was approximately 200 molecules per ion, at an acceleration voltage of 9kV. X-ray photoelectron spectroscopy (XPS) measurements showed that high-rate sputtering for the PMMA surface can be ascribed to the surface erosion by the water cluster ion irradiation. Furthermore, the micropatterning was demonstrated on the PMMA substrate. Thus, the surface irradiation by water cluster ion beams exhibited a chemical reaction based on OH radicals, as well as excited hydrogen atoms, which resulted in a high sputtering rate and low irradiation damage of the substrate surfaces.

Communication: Electron impact ionization of binary H2O/X clusters in helium nanodroplets: An ab initio perspective

In a recent experiment (H(2)O)(n)∕X(m) binary clusters (where X = Ar, N(2), CO, CO(2), and several other molecules) were formed in superfluid helium nanodroplets and investigated by electron impact mass spectrometry [Liu et al., Phys. Chem. Chem. Phys. 13, 13920 (2011)]. The addition of dopant X was found to affect the branching ratio between H(3)O(+)(H(2)O)(n) and (H(2)O)(+)(n+2) formation. Specifically, the addition of CO increased the proportion of protonated water cluster ions, whereas dopants such as Ar, N(2), and CO(2), had the opposite effect. In this work ab initio calculations have been performed on [X(H(2)O)(2)](+) ions, where X = Ar, N(2), CO, and CO(2), to try and explain this distinct behavior. CO is found to be unique in that it forms a HOCO-H(3)O(+) unit in the most stable cationic complexes where the binding between HO and CO is stronger than that between H(3)O(+) and OH. Thus, on purely energetic grounds, loss of HOCO rather than CO should be the preferred fragmentation process. No comparable chemistry occurs when X = Ar, N(2), or CO(2) and so the co-dopant requires less energy to depart than OH. The calculations therefore account for the experimental observations and provide evidence that HOCO formation is induced in helium droplets containing (H(2)O)(n) clusters and co-doped with CO when subject to electron impact ionization.

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.

Highly Charged Ion - Induced Water Cluster Fragmentation

Interaction between slow multiply charged ions and small to medium-size neutral water clusters is investigated. Strong projectile charge dependence is found for all of the fragmentation patterns. A variation of the projectile charge dramatically changes the proportion of multi-electron removal, the fragment kinetic energies as well as the mean size of the emitted fragments. First results for highly charged ion - water cluster ion collisions will also be presented.

Hydrocarbon analysis using desorption atmospheric pressure chemical ionization

Characterization of the various petroleum constituents (hydronaphthalenes, thiophenes, alkyl substituted benzenes, pyridines, fluorenes, and polycyclic aromatic hydrocarbons) was achieved under ambient conditions without sample preparation by desorption atmospheric pressure chemical ionization (DAPCI). Conditions were chosen for the DAPCI experiments to control whether ionization was by proton or electron transfer. The protonated molecule [M+H]+ and the hydride abstracted [MH]+ form were observed when using an inert gas, typically nitrogen, to direct a lightly ionized plasma generated by corona discharge onto the sample surface in air. The abundant water cluster ions generated in this experiment react with condensed-phase functionalized hydrocarbon model compounds and their mixtures at or near the sample surface. On the other hand, when naphthalene was doped into the DAPCI gas stream, its radical cation served as a charge exchange reagent, yielding molecular radical cations (M+) of the hydrocarbons. This mode of sample ionization provided mass spectra with better signal/noise ratios and without unwanted side-products. It also extended the applicability of DAPCI to petroleum constituents which could not be analyzed through proton transfer (e.g., higher molecular PAHs such as chrysene). The thermochemistry governing the individual ionization processes is discussed and a desorption/ionization mechanism is inferred.

On the stability of ion water clusters at atmospheric conditions: Open system Monte Carlo simulation

The formation of water clusters on Li(+), Na(+), K(+), Cl(-), and I(-) ions from water vapor at atmospheric conditions have been studied using Monte Carlo simulations. The extended simple point charge model has been employed for water molecules. The polarization of ions in the field of molecules and the polarization of molecules in the field of ions have been considered explicitly in the total Hamiltonian of the molecular system. The cluster formation work and the Gibbs free energy and enthalpy of attachment reactions of one water molecule to the cluster have been calculated via the bicanonical ensemble method. Our results reveal the formation of stable clusters in equilibrium with the moist atmosphere in a wide range of vapor pressure values, with largest clusters are formed around cations. Decreasing the temperature, from 293 K to 253 K, leads to the formation of larger equilibrium clusters, and enhances the stability of systems as whole. According to clusters' molecular structures, negative ions are expected to be more active in atmospheric processes, including chemical reactions and cloud formation, than positive ones.

Precipitating radiation belt electrons and enhancements of mesospheric hydroxyl during 2004–2009

Energetic particle precipitation leads to enhancement of odd hydrogen (HOx) below 80 km altitude through water cluster ion chemistry. Using measurements from the Microwave Limb Sounder (MLS/Aura) and Medium Energy Proton and Electron Detector (MEPED/POES) between 2004–2009, we study variations of nighttime OH caused by radiation belt electrons at geomagnetic latitudes 55–65°. For those months with daily mean 100–300 keV electron count rate exceeding 150 counts/s in the outer radiation belt, we find a strong correlation (r ≥ 0.6) between OH mixing ratios at 70–78 km (0.046–0.015 hPa) and precipitating electrons. Correlations r ≥ 0.35, corresponding to random chance probability p ≤ 5%, are observed down to52 km (0.681 hPa), while no clear correlation is observed at altitudes below. This suggests that the fluxes of ≥3 MeV electrons were not high enough to cause observable changes in OH mixing ratios. At 75 km, in about 34% of the 65 months analyzed we find a correlation r ≥ 0.35. Although similar results are obtained for both hemispheres in general, in some cases the differences in atmospheric conditions make the OH response more difficult to detect in the South. Considering the latitude extent of electron forcing, we find clear effects on OH at magnetic latitudes 55–72°, while the lower latitudes are influenced much less. Because the time period 2004–2009 analyzed here coincided with an extended solar minimum, and the year 2009 was anomalously quiet, it is reasonable to assume that our results provide a lower‐limit estimation of the importance of energetic electron precipitation at the latitudes considered.

Proton Mobility in Water Clusters

Proton mobility in water occurs quickly according to the so-called Grotthuss mechanism. This process and its elementary reaction steps can be studied in great detail by applying suitable mass spectrometric methods to ionic water clusters. Careful choice of suitable core ions in combination with analysis of cluster size trends in hydrogen/deuterium isotope exchange rates allows for detailed insights into fascinating dynamical systems. Analysis of the experiments has been promoted by extensive and systematic quantum chemical model calculations. Detailed low-energy mechanistic pathways for efficient water rearrangement and proton transfer steps, in particular cases along short preformed "wires" of hydrogen bonds, have been identified in consistency with experimental findings.

Reactions of H<sup>+</sup>(pyridine)<sub><i>m</i></sub>(H<sub>2</sub>O)<sub><i>n</i></sub> and H<sup>+</sup>(NH<sub>3</sub>)<sub>1</sub>(pyridine)<sub><i>m</i></sub>(H<sub>2</sub>O)<sub><i>n</i></sub> with NH<sub>3</sub>: experiments and kinetic modelling

Abstract. Reactions between pyridine containing water cluster ions, H+(pyridine)1(H2O)n, H+(pyridine)2(H2O)n and H+(NH3)1(pyridine)1(H2O)n (n up to 15) with NH3 have been studied experimentally using a quadrupole time-of-flight mass spectrometer. The product ions in the reaction between H+(pyridine)m(H2O)n (m = 1 to 2) and NH3 have been determined for the first time. It is found that the reaction mainly leads to cluster ions of the form H+(NH3)1(pyridine)m(H2O)n-x, with x = 1 or 2 depending on the initial size of the reacting cluster ion. For a given number of water molecules (from 5 to 15) in the cluster ion, rate coefficients are found to be slightly lower than those for protonated pure water clusters reacting with ammonia. The rate coefficients obtained from this study are used in a kinetic cluster ion model under tropospheric conditions. The disagreement between ambient ground level measurements and previous models are discussed in relation to the results from our model and future experimental directions are suggested.

Irradiation effects of water cluster ion beam on PMMA surface

We investigated the irradiation effects of a water cluster ion beam on a PMMA surface. The incident angle dependence of the sputter depth of the PMMA surface induced by the irradiation of a water cluster ion beam was measured. The sputter depth had a broad peak around the incident angle of 60°. The surface roughness caused by the irradiation of the water cluster ion beam was small compared with the sputter depth.

On Quantitative Determination of Volatile Organic Compound Concentrations Using Proton Transfer Reaction Time-of-Flight Mass Spectrometry

Proton transfer reaction - mass spectrometry (PTR-MS) has become a reference technique in environmental science allowing for VOC monitoring with low detection limits. The recent introduction of time-of-flight mass analyzer (PTR-ToF-MS) opens new horizons in terms of mass resolution, acquisition time, and mass range. A standard procedure to perform quantitative VOC measurements with PTR-ToF-MS is to calibrate the instrument using a standard gas. However, given the number of compounds that can be simultaneously monitored by PTR-ToF-MS, such a procedure could become impractical, especially when standards are not readily available. In the present work we show that, under particular conditions, VOC concentration determinations based only on theoretical predictions yield good accuracy. We investigate a range of humidity and operating conditions and show that theoretical VOC concentration estimations are accurate when the effect of water cluster ions is negligible. We also show that PTR-ToF-MS can successfully be used to estimate reaction rate coefficients between H(3)O(+) and VOC at PTR-MS working conditions and find good agreement with the corresponding nonthermal theoretical predictions. We provide a tabulation of theoretical rate coefficients for a number of relevant volatile organic compounds at various energetic conditions and test the approach in a laboratory study investigating the oxidation of alpha-pinene.

Ionization of Water Clusters Mediated by Exciton Energy Transfer from Argon Clusters.

The exciton energy deposited in an argon cluster (Arn, ⟨n = 20⟩) using VUV radiation is transferred to softly ionize doped water clusters ((H2O)n, n = 1-9), leading to the formation of nonfragmented clusters. Following the initial excitation, electronic energy is channeled to ionize the doped water cluster while evaporating the Ar shell, allowing identification of fragmented and complete water cluster ions. Examination of the photoionization efficiency curve shows that cluster evaporation from excitons located above 12.6 eV is not enough to cool the energized water cluster ion and leads to their dissociation to (H2O)n-2H(+) (protonated) clusters.

Surface irradiation and materials processing using polyatomic cluster ion beams

Abstract

Surface Interaction and Processing Using Polyatomic Cluster Ions

We developed two types of polyatomic cluster ion sources, one of which was a liquid cluster ion source using organic materials with a high-vapor pressure. Vapors of liquid material such as ethanol and water were ejected through a nozzle into a vacuum region, and liquid clusters were produced by an adiabatic expansion phenomenon. Another type was a cluster ion source using ionic liquids with a relatively low-vapor pressure. Positive and negative cluster ions were produced by a high-electric field emission. In addition, the interaction of polyatomic cluster ions with solid surfaces such as Si(100), SiO2, glass, and PMMA surfaces was investigated, and chemical sputtering was predominant for the Si(100) surfaces irradiated by ethanol cluster ion beams. Also, the irradiation damage of the Si(100) surfaces by ethanol and water cluster ion beams was smaller than that by Ar monomer ion irradiation at the same acceleration voltage. With regard to surface modification, PMMA surfaces were chemically modified by water cluster irradiation. Also, glass surfaces changed to electrically conductive surfaces by ionic liquid cluster ion irradiation. Furthermore, to demonstrate engineering applications of high-rate sputtering and low-damage irradiation by ethanol cluster ion beams, micro-patterning was performed on the Si surfaces.