Cluster Ion Distributions Research Articles

Argon clusters embedded in helium nanodroplets

Electron impact ionization of argon clusters embedded in helium droplets is investigated. Superior mass resolution makes it possible to distinguish between nominally isobaric cluster ions. An abundance maximum for ArHe(12)(+) is unambiguously confirmed; the spectra also prove the formation of Ar(2)He(n)(+) complexes that had been claimed to fragment into pure Ar(2)(+). Distributions of larger argon cluster ions containing up to 60 atoms closely resemble distributions observed upon electron impact or photoionization of bare argon clusters; caging and evaporative cooling provided by the helium matrix do not suffice to quench fragmentation of the nascent argon cluster ions. Intriguing abundance anomalies are observed in distributions of argon cluster ions that contain water, nitrogen or oxygen impurities. The strong abundance of Ar(55)H(2)O(+), Ar(54)O(2)(+) and Ar(54)N(2)(+) contrasts with the virtual absence of slightly larger cluster ions containing the corresponding impurities. The features are probably related to enhanced cluster ion stability upon closure of the second icosahedral shell but the difference in magic numbers (54 versus 55) and the well-known reactivity of charged argon-nitrogen complexes suggest structural differences.

Cluster ion formation of alkali halides by electrospray droplet impact

Positive and negative cluster ions formed by electrospray droplet impact on the alkali halides (MX) were measured for all alkali halides. Small amounts of powder samples deposited and crushed into thin films on the metal substrate were served for the measurement. The size distribution of cluster ions is very limited for LiF, CsF, and LiI. Little or no fluoride cluster ions F −(MF) n were detected. In general, distribution of n for negative cluster ions X −(MX) n is narrower than that for positive cluster ions M +(MX) n , probably due to the instability caused by the mutual Coulomb repulsion between the larger-size halide ions than alkali ions. While the cubic-like structures are preferred for alkali halides with smaller ionic radii, the stacked hexagonal structures with n = 3, 6, and 9 are preferred for those with larger ionic radii.

Direct Formation of Silver Cluster Ions from Silver Salts by Laser Desorption/Ionization

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) has been used to characterize a wide range of polymers. In a MALDI mass spectrum, the protonated molecule and cation adduct of an analyte give an important information about the molecular ion mass. Some researchers reported that sliver clusters can be effectively produced under MALDI conditions from various silver salts in the presence of various matrices. During the analysis of organic molecules when Ag is used as the cationization agent, it can be recognized that besides the desired adduct ions, [M + Ag], an additional silver cluster ions, Agn is formed. Keki and coworkers studied the matrix effect on the formation of the silver cluster ions under MALDI conditions using reductive polar organic matrices and silver trifluoroacetate (AgTFA) and reported that the matrix greatly influenced the resulting cluster ion abundances. Metal clusters are very important in both theoretical and practical points of view. Investigation of cluster formation in mass spectrometry provides valuable information on the stability and electronic properties of clusters of different size. Studies on the formation of silver cluster ions under MALDI conditions have been performed in the presence of matrices such as 2,5-dihydroxybenzoic acid (DHB), 3,5dimethoxy-4-hydroxycinnamic acid (sinapinic acid, SA), trans-3-indoleacrylic acid (IAA), 2-(4-hydroxyphenylazo)benzoic acid (HABA), all-trans-retinoic acid (RTA), and 1,8,9-trihydroxyanthracene (dithranol, DIT). Formation of silver cluster ions directly from silver salts in the absence of matrix was not studied. In the present work, we investigated the silver cluster ions formed directly from silver salts of silver benzoate (AgBz), AgTFA, silver nitrate (AgNO3), and silver p-toluenesulfonate (AgTS) in the absence of any matrices. Scheme 1 shows their chemical structures. Figures 1, 2, 3, and 4 are the laser desorption/ionization (LDI) mass spectra of AgBz, AgTFA, AgNO3, and AgTS, respectively. Only the mass spectrum of AgBz shows a clear distribution of the silver cluster ions. It is a surprising result that the silver cluster ions are directly formed from AgBz in the absence of any matrix. Macha and coworkers investigated the formation of silver clusters from silver salts such as AgTFA, AgNO3, and silver acetylacetonate [Ag(acac)], but silver clusters were not detected. The cluster ion intensity distribution of AgBz shows an odd-even alternation pattern as shown in Figure 1. In other words, the intensity of the odd-number cluster ions tends to be stronger than that of the neighboring even-number cluster ions, especially in the low-mass range. The mass spectrum also shows a steep decrease of the ion intensities after some cluster ions called magic numbers. Figure 1 shows clear magic numbers of n = 9 and 21 of Agn. The specific ion intensity distribution of the silver cluster ions can be explained with the theoretical models. Since odd-number silver cluster ions have even numbers of valance electrons that become paired in the electronic shell of the formed clusters, they show stronger stabilities and higher ion intensities compared to those of even-number ones which have odd-number of electrons in their electronic shells. In addition, odd-number silver cluster ions tended to lose two silver atoms in their dissociation while even-number clusters lose a single atom. The magic numbers are in good agreement with the jellium model theory. In the jellium model one assumes that the positive ions of the cluster are smeared into a uniform background of spherical shape with density equal to that in the bulk. The electrons respond to this uniform background and fill the successive angular momentum states in accordance with the rules of quantum mechanics. A cluster shows pronounced stability when one of the angular momentum levels is filled. This shell closure occurs for alkali atoms at n = 2, 8, 20, 40, 58, 92, ... Formation of the silver cluster ions has been studied in the presence of matrix. In general, formation of the silver

Waterfalls as sources of small charged aerosol particles

Abstract. In this study, we measured the mobility distributions of cluster and intermediate ions with an ion spectrometer near a waterfall. We observed that the concentration of negative 1.5–10 nm ions was one-hundred fold higher than a reference point 100 m away from the waterfall. Also, the concentration of positive intermediate ions was found to be higher than that at the reference point by a factor of ten. This difference was observed only at the smallest sizes; above 10 nm the difference was insignificant.

Cluster size dependence on energy and velocity distributions of gas cluster ions after collisions with residual gas

Cluster size dependence on energy distribution and velocity of gas cluster ion beam (GCIB) after collision with residual gas was studied using size-controlled GCIB system to make it clear the irradiation energy and cluster size under poor vacuum conditions. From the energy distribution measurements, an Ar cluster ion which was accelerated at high voltage (30kV) was easier to lose its kinetic energy than that accelerated at low voltage (10kV). Also, at the same acceleration voltage, the smaller Ar cluster ion lost its energy rapidly than larger one. These results indicated that the energy loss during beam transport is determined by the velocity of cluster ions. If cluster was originated from the gas which has strong binding force such as CO2, large cluster ions kept their kinetic energy at constant even after frequent collisions with residual gas.

Desorption of water cluster ions from the surface of solid rare gases

Electron or photon irradiation of H2O adsorbed on the surface of rare gas solids induces the desorption of protonated water clusters, (H2O)nH+. The yield and the size n distribution of cluster ions depend on the coverage, the deposition temperature of the water, and the thickness of the rare gas film. These results indicate that the (H2O)nH+ ions originate from the isolated water clusters, and the most important factor determining the size n distribution of desorbed (H2O)nH+ is the sizes of water islands on the rare gas solid. The measurement of kinetic energy distributions indicated that the desorbing energy of clusters depend on the rare gas species of the substrates and the cluster size. It is suggested that the (H2O)nH+ desorption is due to Coulomb repulsion between the ionic water cluster and the rare gas ion.

Low-energy(5–250eV)electron-stimulated desorption ofH+,H2+, andH+(H2O)nfrom low-temperature water ice surfaces

Low-energy $(5--250\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ electron-stimulated desorption (ESD) has been used to study the production and removal of ${\mathrm{H}}^{+}$, $\mathrm{H}_{2}{}^{+}$, and ${\mathrm{H}}^{+}{({\mathrm{H}}_{2}\mathrm{O})}_{n=1--8}$ from porous amorphous solid water (PASW), amorphous solid water (ASW), and crystalline (CI) water ice films. The threshold energies for ESD of ${\mathrm{H}}^{+}$ and $\mathrm{H}_{2}{}^{+}$ from CI and ${\mathrm{H}}^{+}$ and ${\mathrm{H}}^{+}({\mathrm{H}}_{2}\mathrm{O})$ from both PASW and ASW are $\ensuremath{\sim}22\ifmmode\pm\else\textpm\fi{}3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. There is also a $\mathrm{H}_{2}{}^{+}$ yield increase at $\ensuremath{\sim}40\ifmmode\pm\else\textpm\fi{}3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and a $\ensuremath{\sim}70\ifmmode\pm\else\textpm\fi{}3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ threshold for ESD of ${\mathrm{H}}^{+}{({\mathrm{H}}_{2}\mathrm{O})}_{n=2--8}$ for all phases of ice. $\mathrm{H}_{2}{}^{+}$ production and desorption involves direct molecular elimination and reactive scattering of an energetic proton. Both of these channels likely involve localized two-hole one-electron and∕or two-hole final states containing $4{a}_{1}$, $3{a}_{1}$, and∕or $2{a}_{1}$ character. The $70\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ cluster ion threshold implicates either an initial $(2a_{1}{}^{\ensuremath{-}2})$ state localized on a monomer or the presence of at least two neighboring water molecules each containing a single hole. The resulting correlated two-hole or two-hole, one-electron configurations are localized within a complex and result in an intermolecular Coulomb repulsion and cluster ion ejection. The ${\mathrm{H}}^{+}{({\mathrm{H}}_{2}\mathrm{O})}_{n}$ yields are highest from PASW relative to ASW and CI and decrease with temperature, whereas the $\mathrm{H}_{2}{}^{+}$ yields are highest for CI and increase with temperature. The temperature effects and cluster ion distributions are accounted for by distance and temperature dependent hole screening. Changes in screening, hole lifetimes and hopping probabilities are greatest for ${a}_{1}$ levels. This is supported by valence band photoemission studies of ice as a function of temperature.

Collision energy transfer in collision of NH4+(NH3)n−1 (n=3–9) with ND3

An incorporation of ND(3) into protonated ammonia cluster ions NH(4)(+)(NH(3))(n-1) (n=3-9), together with a dissociation of the cluster ions, was observed in the collision of the cluster with ND(3) at collision energies ranging from 0.04 to 1.4 eV in the center-of-mass frame. The branching fractions of the cluster ion species produced in the reactions were obtained as a function of the collision energy. The branching fractions of the incorporation products were successfully explained in terms of the Rice-Ramsperger-Kassel (RRK) theory at collision energies lower than the binding energy of the cluster ion. In addition, the internal energy distributions of the parent cluster ions were determined, and found to be in good agreement with those predicted using the evaporative ensemble model. In incorporations at collision energies lower than the binding energy of the cluster ion, all of the collision energy was transferred to the internal energy of the cluster ions; subsequently, an evaporation of ammonia molecules occurred in an equilibrium process after a complete energy redistribution in the clusters. In contrast, at collision energies higher than the binding energy of the cluster ion, a release of an ammonia molecule from the incorporation products occurred in a nonequilibrium process. The transition from the complex mode to the direct mode in the incorporation was observed at collision energies approximately equal to the binding energy. On the other hand, the collision energy dependence of the cross sections for the dissociation and for a nonreactive collision were estimated by a RRK simulation in which the collision energy transfer was interpreted by using the classical hard-sphere collision model. A relationship between reactivity and reaction modes in the collision of NH(4)(+)(NH(3))(4) with ND(3) is discussed via a comparison of the experimental results with the RRK simulation.

Ejection of Clusters from Liquid Beam Surface by IR Laser Irradiation

AbstractA continuous liquid flow of an aqueous solution of phenol (Phe) in a vacuum (a liquid beam) was irradiated with an IR laser at 2.85 μm, which is resonant to the vibrational absorption band of the liquid water. The Phe (H2O)N, ejected from the liquid beam surface into the gas phase, was ionized by a UV laser at 270 nm into hydrated phenol cluster ions, Phe+ (H2O)n (n = 0 − 30), and analyzed by a time‐of‐flight mass spectrometer. The velocity distributions of the product cluster ions were derived from the spatial distributions measured at different elapsed times after the IR laser irradiation. The results and the analysis show that dense neutral clusters are ejected from surface regions locally heated by the intense IR laser.

Inter-comparison between airborne measurements of methanol, acetonitrile and acetone using two differently configured PTR-MS instruments

Proton-transfer-reaction mass spectrometry (PTR-MS) has emerged as a useful tool to study the atmospheric chemistry of volatile organic compounds (VOCs). The technique combines a fast response time with a low detection limit, and allows atmospheric measurements of many important VOCs and their oxidation products. Here, we inter-compare the results obtained with two differently configured PTR-MS instruments operated onboard a Falcon aircraft during the Mediterranean Intensive Oxidants Study (MINOS) campaign in the Mediterranean region. One PTR-MS was operated at a drift tube pressure of 2.3 mbar and an electric field divided by gas number density value ( E/ N) of 120 Td for the detection of VOCs and aromatic hydrocarbons. The other PTR-MS was operated at an increased pressure of 2.8 mbar and a reduced E/ N of 97 Td for the detection of peroxyacetyl nitrate (PAN). As a consequence, more H 3O +(H 2O) n cluster ions were present in the drift tube, which undergo proton-transfer reactions with VOCs similar to H 3O + ions. The results for methanol (CH 3OH), acetonitrile (CH 3CN) and acetone (CH 3COCH 3) obtained with the instruments compared very well. The agreement between the two results was relatively independent of the ambient mixing ratio of water, which influences the H 3O +(H 2O) n cluster ion distribution, and of ozone, which has been implicated in the artificial formation of aldehydes and ketones.

Size and energy distribution of gas cluster ion beam measured by energy resolved time of flight mass spectroscopy

Energy resolved time of flight (ERTOF) mass spectroscopy has been developed to measure the energy and size distributions in a gas cluster ion beam (GCIB). Gas cluster ions, being aggregates of a few to several thousands of atoms, were generated by the ionization of neutral clusters, which were created by the adiabatic expansion of a high pressure gas into a vacuum. As a result of the large cross section of cluster ions, the cluster size can be easily reduced by collision with fast neutrals and monomer ions. An ERTOF system consisted of a retarding grid in front of a drift tube with a pair of deflection electrodes that acted as a gate. This system was attached to an ionization chamber, where Ar gas clusters, generated by a nozzle in a source chamber, were ionized by electron impact. Current waveforms were measured with ERTOF as an increasing voltage was applied to the retarding grid. The size distribution of clusters having a particular kinetic energy was calculated from the difference in the current waveform obtained at a corresponding retarding voltage to that at a larger voltage. Using ERTOF, it was found that the GCIB consisted of two components; small sized clusters with low kinetic energy and large sized clusters with high kinetic energy. It was also noted that the depth to which a substrate, on which the cluster beam impinged, was etched increased with the proportion of large clusters to the GCIB, since the small clusters, resulting from disintegration of large sized clusters by collisions with monomer ions, fast neutrals and residual gas molecules, have too low a kinetic energy to remove atoms from the surface. Note that the etched depth was affected by both the energy and size distribution of cluster ions. Therefore, ERTOF measurement could be utilized to assure the reproducibility of processes for the manufacture of semiconductors and magnetic recording heads.

Cluster composition distribution at the liquid surface of alcohol–water mixtures and evaporation processes studied by liquid ionization mass spectrometry

Liquid ionization mass spectrometry (LPI-MS) gives information about hydrogen-bonded clusters at a liquid surface. By improving the way in which samples are introduced, mass spectra showing clusters at the liquid surface or in a gas phase were obtained for 20% (v/v) ethanol and methanol aqueous solutions. Observed cluster ions were expressed as (ROH) m (H 2O) n H +, R = CH 3 or C 2H 5, and mass spectra gave the molar ratios of methanol to water ( M/ W) or ethanol to water ( E/ W) close to the ratio corresponding to the concentration of the respective solution. Binary cluster ions containing two molecules of alcohol ( m = 2) were abundant for both solutions. The molar ratios calculated from the cluster compositions, m − n, of the most abundant cluster ions were also close to the ratio corresponding to the concentration of the solution. The results suggest that the composition distribution of cluster ions observed as LPI mass spectrum may be similar to the composition distribution of clusters existing at the liquid surface. The cluster compositions at the liquid surface vary very quickly due to evaporation of the liquid. The clusters in the vapor were also measured using another device for sample introduction. The evaporation processes occurring in the nano-space at and above the liquid surface were mainly the loss of water molecules from larger clusters. The following collisions between smaller clusters leaded to the association of alcohol molecules accompanied with further loss of water molecules, resulting in the increase of the number of alcohol molecules in each cluster. Even ethanol clusters, (C 2H 5OH) m H +, were formed from the aqueous ethanol solutions. Reasonable differences between ethanol–water and methanol–water were observed for the mass spectra measured in the gas phase. Liquid ionization mass spectrometry gives nano-scale information concerning the cluster compositions at the liquid surface and the evaporation processes.

On the iron oxide neutral cluster distribution in the gas phase. I. Detection through 193 nm multiphoton ionization.

Iron oxide (FemOn) neutral clusters are generated in the gas phase through laser ablation of the metal and reaction with various concentrations of O2 in He. The mixture of expansion gas and neutral FemOn cluster species is expanded through a supersonic nozzle into a vacuum system, in which the clusters are ionized by an ArF excimer laser at 193 nm, and the ions are detected and identified in a time-of-flight mass spectrometer. In this report, the experimental parameters that influence the observed cluster distributions, such as ablation laser power, expansion pressure, vacuum system pressure, and 193 nm ArF ionization laser power, are explored. In the second paper in this series, the effect of the ionization laser wavelength (355 nm, 193 nm, 118 nm) on the observed cluster ion distribution is explored. The cluster ion distribution observed employing 193 nm laser ionization, is sensitive to the neutral cluster distribution as evidenced by the change in the observed time-of-flight mass spectra with changes in laser power, growth conditions, and expansion conditions. The thermodynamically stable neutral clusters for saturated O2 growth conditions are suggested to be of the forms FemOm, FemO(m+1), and FemO(m+2); which one of these series of neutral clusters is most stable depends on the size of the cluster. For m < 10, FemOm is the most stable neutral cluster series, for 10 < or = m < or = 20, FemO(m+1) is the most stable neutral cluster series, and for 21 < or = m < = 30, FemO(m+2) is the most stable neutral cluster series. Some neutral cluster fragmentation is clearly present for 193 nm ionization due to multiphoton absorption in both the neutral and ionic cluster species.

High resolution measurements of kinetic energy release distributions of neon, argon, and krypton cluster ions using a three sector field mass spectrometer.

Using a newly constructed three sector field mass spectrometer (resulting in a BE1E2 field configuration) we have measured the kinetic energy release distributions of neon, argon, and krypton cluster ions. In the present study we used the first two sectors, B and E1, constituting a high resolution mass spectrometer, to select the parent ions in terms of mass, charge, and energy, and studied the decay of those ions in the third field free region. Due to the improved mass resolution we were able to extend earlier studies carried out with a two sector field machine, where an upper size limit arose from the fact that several isotopomers contribute to a decaying parent ion beam when the cluster size exceeds a certain value. Furthermore we developed a new data analysis. It allows us to model also fragment ion peaks that are a superposition of different decay reactions and thus we can determine the average kinetic energy release for all decay reactions of a given cluster ion. In a further step we used these results to determine the binding energies of cluster ions Rg(n) (n> or =10) by applying finite heat bath theory. The smaller sizes have not been included in this analysis, because the validity of finite heat bath theory becomes questionable below n approximately 10. The present average kinetic energy releases and binding energies are compared with other experiments and various calculations.

Measurement of cluster ions and residue nanoparticles from water samples with an electrospray/differential mobility analyzer.

Cluster ions and residue nanoparticles with sizes below 30 nm were generated by electrospraying (ES) and drying droplets of pure water, tap water, and aqueous solutions of salts. The mobility spectra of the cluster ions between 9.1 and 9.3 x 10(-5) m2/(V s) were measured using a differential mobility analyzer (DMA) operated at room temperature and atmospheric pressure. A modified Faraday cup and a condensation nucleus counter were used for detection. The concentrations of total residue/contaminants in the water were determined as a function of sizes of measured aerosol particles and of the initial droplets. Method detection limits were at sub-ppb level for pure water and sub-ppm level for tap water. ES/DMA is capable of simultaneously measuring the mobility distribution of cluster ions and concentration of total residue present in water samples.

The Lifetime Distribution of Cluster Ions from Sputtering Ion Source

The lifetime distribution of silver and mercury-silver binary cluster ions from a sputtering ion source was investigated. The cluster ions were produced by 10 keV Xe ion bombardment on a mercury-silver amalgam for mercury-silver binary clusters, and a silver plate for silver clusters. The fragmentation rates were obtained from experimental data at several positions of the mass spectrometer. The lifetime distribution was calculated using fragmentation rates. The distribution was found to be rather power functional than exponential, because of the wide distribution of lifetimes for cluster ions. The cluster ion, Hg12Ag+, which might have an icosahedron structure turned out to be more stable than neighboring clusters.

Chemical ionization mass spectrometry technique for detection of dimethylsulfoxide and ammonia

A chemical ionization mass spectrometer (CIMS) was used to study reactions of protonated ethanol clusters (C2H5OH)nH+ with dimethylsulfoxide (DMSO), dimethylsulfone (DMSO2), ammonia (NH3), and a series of nonmethane hydrocarbons (NMHCs) and volatile organic compounds (VOCs). The reactivity of the (C2H5OH)nH+ cluster ions is a function of cluster size with reactivity decreasing as cluster size increases. Ethanol cluster ion distributions that formed at atmospheric pressure from 24 ppbv, 900 ppmv, and 1% ethanol/N2 gas mixtures were studied. Small (C2H5OH)nH+ clusters, those formed using the 24 ppbv ethanol/N2 mixture, react at or near the collisional rate with DMSO, NH3, acetone, and methyl vinyl ketone (MVK). The effective ion molecule rate coefficients are 1.8 × 10−9, 1.5 × 10−9, 1.0 × 10−9, and 1.6 × 10−9 cm3 molecule−1 s−1, respectively. Only DMSO and NH3 react efficiently with the two larger (C2H5OH)nH+ cluster ion distributions studied. The effective rate coefficients for DMSO and NH3 with the 900 ppmv ethanol cluster ion distribution are 1.5 × 10−9 and 0.7 × 10−9 cm3 molecule−1 s−1, respectively. The effective rate coefficient for DMSO with the 1% ethanol/N2 mixture is 0.35 × 10−9 cm3 molecule−1 s−1, while NH3 reaches equilibrium with this cluster ion distribution. Experiments show that large (C2H5OH)nH+ ion clusters must be used at relative humidities greater than 50% at 20°C to prevent formation of and subsequent interferences from H3O+ ions. These results demonstrate that the (C2H5OH)nH+ ion chemistry can selectively detect DMSO and NH3 under most ambient atmospheric conditions with high sensitivity.

Kinetic energy distributions of molecular and cluster ions sputtered from self-assembled monolayers of octanethiol on gold

Self-assembled monolayers (SAMs) of alkanethiols are an ideal model system to study the mechanisms that lead to emission of organic species under keV ion bombardment. In this contribution, we focus on the emission processes of gold-molecule cluster ions, which are not fully understood yet. To gain insights into these processes, monolayers of octanethiol CH 3(CH 2) 7SH adsorbed on gold are investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). First, the static SIMS conditions are verified using the degradation of the SAM signals as a function of ion fluence. Second, the kinetic energy distributions (KEDs) of fragment, parent and cluster ions ejected upon 15 keV Ga + ion bombardment are measured. The peak maxima, FWHMs and high energy tails of the distributions are analyzed for Au-thiolate clusters, as well as thiol fragments. After calibration of the energy spectra using monoatomic ions, we find that the KEDs of all the clusters containing the thiolate molecule peak at about 1.2 eV. Besides, the distributions of the gold-molecule cluster ions including Au(M–H) 2 −, the most intense cluster peak in the spectrum, are significantly narrower than that of the hydrocarbon fragments.

Cluster ions derived from sodium and potassium tetrafluoroborate and their collision induced dissociation in an ion trap mass spectrometer.

Electrospray ionization was used to produce distributions of gas-phase cluster ions from solutions of sodium and potassium tetrafluoroborate. The majority of the cluster species followed the trend (MBF(4))(n)M(+), where M=Na and K. The values of n, for both salts, ranged from 1-15. Collision induced dissociation (MS/MS and MS(n)) in an ion trap mass spectrometer was used to determine the dissociation pathways for the cluster ions. The (NaBF(4))(n)Na(+) cluster ions fragmented via two pathways: (a) the loss of one or multiple neutral BF(3) molecules and (b) the loss of one or more NaBF(4) units. Of the two, the product ions corresponding to the loss of BF(3) units were more prominent. Unlike the Na salt, the (KBF(4))(n)K(+) cluster ions decomposed primarily by the loss of one or multiple KBF(4) units. Similar differences in dissociation behavior were observed when the heated transfer capillary, normally used to desolvate ions, was used to investigate cluster ion stability to thermal degradation and dissociation. The dissociation profiles (decrease in ion abundance with increasing activation amplitude) for several (NaF)(n)Na(+) and (KF)(n)K(+) cluster ions were measured and compared to probe the influence of the relative stability of the alkali fluorides (NaF and KF) on the dissociation behavior exhibited by the tetrafluoroborate cluster distributions. We found that the (NaF)(n)Na(+) cluster ions required higher activation amplitudes to induce fragmentation than the corresponding (KF)(n)K(+) species, indicative of stronger ionic bonding and higher gas-phase stability for the former. This in turn indicates that the reaction pathway involving only the loss of one or multiple units of BF(3), favored for the (NaBF(4))(n)Na(+) cluster series, but not for the analogous (KBF(4))(n)K(+) series, may be due to the high gas-phase stability of NaF, and relatively lower stability of KF, towards dissociation.

Collision-induced fragmentation and neutralization of methanol cluster cations

This contribution addresses the inelastic interaction of positively charged molecular cluster ions with a solid surface at kinetic energies up to 30 eV/molecule. We report experimental results on the scattering of mass-selected, protonated methanol cluster cations (CH3OH)nH+, n = 4-32, off a diamond-coated silicon surface. In particular we provide fragment size distributions of methanol cluster ions following their impact on the target, as well as surface-induced neutralization probabilities of methanol cluster ions as a function of the size and the kinetic energy of the parent clusters.