Positive Cluster Ions Research Articles

Simulation of the sublimation process in the preparation of photochromic WO 3 film by laser microprobe mass spectrometry

It is known that electrophotochromism of a WO 3 film dramatically depends on its microstructure. To probe the influence of the microcomposition and the microstructure on the electrophotochromism of the VE-WO 3 film, we employed a method based on laser microprobe mass spectrometry to simulate the sublimation process of WO 3 with conditions mimicking those in vacuum evaporation. Both positive and negative ion clusters containing no more than six tungsten atoms were generated from direct laser vaporization of WO 3 powder. The fragment valence calculation on the ion clusters showed that a valence of six was the ideal oxidation state for tungsten. The X-ray photoelectron spectroscopy results also confirmed the formation of stoichiometric units in the deposited film. Based on experimental results, (WO 3) x , especially (WO 3) 3 and (WO 3) 4, were found to be the major component units in the vacuum evaporated (VE) film.

Relative yields, mass distributions and energy spectra of cluster ions sputtered from niobium under keV atomic and polyatomic gold ion bombardment

In the present work, the comparative studies of relative yields, mass distributions and kinetic energy spectra of secondary Nbn+ ions (n=1–16) sputtered from niobium target by atomic and polyatomic Aum− projectiles (m=1–3) with the energy E0=6–18 keV/atom have been carried out. The strong effect of anomalously high non-additivity of metal sputtering as positive large cluster ions under polyatomic ion bombardment was found. The comparison and discussion of the results obtained for Nb and for Ta are presented.

High non-additive sputtering of silicon as large positive cluster ions under polyatomic ion bombardment

In the present work comparative studies of relative yields and mass spectra of secondary cluster Si + n ions ( n=1–17) and carbon-containing cluster Si n C + ( n=1–12) and Si n C + 2 ( n=1–6) ions sputtered from a silicon target by atomic and polyatomic Au − m projectiles ( m=1–3) with energy of 9 and 18 keV have been carried out. Anomalously high non-additivity in silicon sputtering as large positive cluster ions under polyatomic ion bombardment has been found. On the basis of the results obtained, a new method of “cluster-SIMS-cluster” registration of impurities in materials under study is proposed. This method makes it possible to increase the element analysis sensitivity more than three orders of a magnitude as compared to traditional SIMS methods.

高質量分解能と広質量範囲の性能をもつ小型サイズの飛行時間式質量分析器の開発

A time-of-flight mass spectrometer with a reflector has been constructed in a compact device (<1.8 m long). Neutral clusters are photoionized and accelerated by dc electric potential up to 25 kV. Positive and negative ion clusters are accelerated by a fast rising (<80 ns) pulsed electric potential up to ±10 kV. The ion clusters are detected by a two-stage multi-channel-plate and the detection signals are recorded by a microcomputer with a multi-stop time-to-digital converter. Both a high mass resolution up to 5,000 and a wide mass range from 1 u to more than 200,000 u are attained.

Observation of Na(+)-bound oligomers of quercetin in the gas phase.

While developing a liquid chromatography/tandem mass spectrometry method for the analysis of the flavonoid quercitin, it was observed that quercetin (3,3',4',5,7-pentahydroxyflavone) exhibited clustering in both the positive and negative ion mode. Two series of positive ion clusters were observed; the first series corresponds to singly charged [2M + Na](+) at m/z 627.2 to [13M + Na](+) at m/z 3947.5, while the second series corresponds to doubly charged [7M + 2Na](2+) at m/z 1080.4 to [25M + 2Na](2+) at m/z 3798.5. In the negative ion mode, the behavior of quercetin parallels that of apigenin (4',5,7-trihydroxyflavone) in that [M + NO(3)](-), [2M + NO(3)](-), and [3M + NO(3)](-) were observed at m/z 364.1, 666.0, and 968.9, respectively; in addition, quercitin clusters with chloride ions ([2M + Cl](-) at m/z 638.9 and [3M + Cl](-) at m/z 940. 9) were observed. The results of tandem mass spectrometric examination of several cluster ions are reported.

Detection of pesticides from liquid matrices by ion mobility spectrometry

The detection of different pesticide compounds from liquid matrices was achieved by aspiration ion mobility spectrometry. This technique is based on ion mobility which is proportional to the molecular weight and charge. The ion mobility spectrometer is able to measure mobility changes of product ions as well as mobility changes of reactant ions. In the ion mobility spectrometer used, it is possible to measure positive and negative ion clusters at the same time in six different electrodes. Each measuring electrode detects a different portion of the ion mobility distribution formed within the cell’s radioactive source. The pattern recognition used is based on differences in the gas phase profiles of the different compounds. The results of this study reveal that the ion mobility spectrometer MGD-1 can also be used to measure pesticides even from liquid matrices. On the basis of projection calculation, the responses for 2-propanol (= background) and pesticide compounds were easily separated from each other. The greatest responses for pesticides were seen in the 2nd positive channel. Correspondingly, only minor background signals were measured in the 1st and 2nd positive channels. The detection limits of pesticides were at ng or μg levels. The sensitivity of detection of the ion mobility spectrometer for different pesticides decreased in the order: diazinon, aldicarb, dimethoate and parathion. In comparison with the traditional ion mobility spectrometric technique, the main advantages of this aspiration technique are its fast response, high sensitivity, real time vapor monitoring, straightforward maintenance and low cost. In addition, the cell tolerates high chemical concentrations and still recovers quickly.

Carbon nitride thin-film growth by pulsed laser deposition

Thin films of carbon nitride were deposited using pulsed laser deposition techniques both with and without an atomic nitrogen source. In situ characterization of chemical composition and atomic bonding were studied using scanning Auger, x-ray photoelectron spectroscopy, and electron energy-loss spectroscopy. The influence of growth parameters including substrate temperature, substrate bias, nitrogen partial pressure, and atomic nitrogen on the film composition were studied. Nitrogen content x for CNx films ranged from 0.3 to 0.6 (25–40 at. % nitrogen). Time-of-flight mass spectrometry of the species ejecting from a nitrided carbon target was performed. Neutrals of CN4 and C3N4 clusters and positive ions of CxNy clusters were observed in addition to carbon neutral and positive ion clusters.

Studies of the chromium oxygenated cluster ions produced during the laser ablation of chromium oxides by laser ablation/ionization Fourier transform ion cyclotron resonance mass spectrometry

Trivalent and hexavalent chromium oxides have been examined by time-of-flight laser microprobe mass spectrometry and Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). The positive-ion spectra show a strong correlation with the valency of chromium in the oxide. Thus, the positive cluster ion distribution and the presence of characteristic chromium cluster ions allow us to distinguish the two oxides. Because of their important role in the formation of large chromium cluster ions, we have investigated the formation mechanism of precursor ions such as CrO+. In this context, FTICRMS has been used to detect the presence of singlet oxygen 1O2(1Δg) and to show its importance in the clustering process. Finally, our hypothesis concerning CrO+ formation involving the Cr+ ion and singlet oxygen 1O2(1Δg) was confirmed. © 1997 John Wiley & Sons, Ltd.

Comparison of Positive and Negative Ion Clusters of Methanol and Ethanol Observed by Low Temperature Secondary Ion Mass Spectrometry

Positively and negatively charged cluster ions were produced from methanol and ethanol samples under low temperature secondary ion mass spectrometric conditions. Neat alcohol clusters [Mn. H+], [Mn − H]− and mixed alcohol–water cluster sets [Mn. (H2O)m.H+] and [Mn.(H2O)m − H]− sets were recorded as the most prominent peaks in the spectra. Clusters composed of up to 30 molecules have been observed. Distributions of these cluster series, and also some satellite ion clusters observed with lower abundances, are discussed. © 1997 John Wiley & Sons, Ltd.

Comparison of Positive and Negative Ion Clusters of Methanol and Ethanol Observed by Low Temperature Secondary Ion Mass Spectrometry

Comparison of Positive and Negative Ion Clusters of Methanol and Ethanol Observed by Low Temperature Secondary Ion Mass Spectrometry

Comparison of Positive and Negative Ion Clusters of Methanol and Ethanol Observed by Low Temperature Secondary Ion Mass Spectrometry

Comparison of Positive and Negative Ion Clusters of Methanol and Ethanol Observed by Low Temperature Secondary Ion Mass Spectrometry

Structures and formation mechanism of potassium cyanide clusters

Formation of potassium cyanide clusters in the direct laser vaporization of K 3[Fe(CN) 6] was studied by use of a TOF mass spectrometer. It was found that the positive cluster ions are due to [K(KCN) n ] +, n = 0–37, in which magic numbers are n = 4, 13, 22, 37, whereas the negative cluster ions were assigned to [(KCN) n CN] −, n = 0–13, where magic numbers are n = 4, 13. These magic numbers are identical with those of alkali-halide clusters. This indicates that they have the structures 1 × 3 × 3 ( n = 4), 3 × 3 × 3 ( n = 13), 3 × 3 × 5 ( n = 22), 3 × 5 × 5 ( n = 37) as those of alkali-halide clusters. Thus the formation process of potassium cyanide clusters should go along the growth course of the rock salt structure.

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.

Experimental and Theoretical Studies of Laser-Generated Sulfur Polycarbon Hydride Ions: Collision-Induced Dissociation and ab Initio Calculations

Both sulfur polycarbon hydride cations and anions were generated by laser ablating the mixture of carbon and sulfur powders and were further investigated experimentally and theoretically by means of collision-induced dissociation and ab initio calculations respectively. The computation results agree very well with the mass spectrometric results in the stability and connectivity of these species. Both experimental and theoretical results illustrate that the connectivity of the cluster ions is H−C−···−C−S, their stability alternates with the number of the clustering carbon atoms, and C−S is the weakest bond in all species. The results also reveal the opposite parity of the alternation effect between the positive and negative cluster ions: HCnS+ with odd n is more stable, while HCnS- with even n is more stable.

Ions and electrons of the lower‐latitude D region

A positive and negative ion chemical steady state model which combines about 400 ion‐chemical and photochemical reactions with a two‐dimensional neutral model for minor atmospheric constituents is presented in the height range 50–84 km, for latitudes −50° to +50° in January and September. The ion model yields concentrations of the main positive and negative ions and of electrons. Two important parameters for the description of the lower ionosphere were deduced, namely, the ratio λ of electrons to negative ions and the ƒ+ parameter, describing the ratio of the positive cluster ions to positive molecular ions. In addition, the parameterization of ƒ+ indicates a strong dependency on the water vapor concentration. At the transition height (ƒ+ = 1) at midlatitudes the water concentration can be expressed as a function of the electron density and the temperature: [H2O] = 1.09 × 10−6[ρ]0.6T−3[e], with the units of the water vapor density [H2O], the air density [ρ], and the electron concentration [e] in cm−3 and T in degree kelvin.

Production and mass spectroscopic characterization of ammonium halide clusters

The ammonium halide clusters from solid crystals of NH 4X (X = F, Cl, I) have been investigated by using a laser vaporization method and time of flight mass spectrometry. Positive ion clusters of NH 4 +(NH 3) n (HF) m ( n≤5, m≤5) and NH 4 +(NH 3) n (HI) m ( n≤10, m≤5) are detected with remarkable size effects. However, essentially no NH 4 +(NH 3) n (HCl) m and negative ammonium halide clusters are observed. These results are interpreted within the framework of intra-cluster interactions and transitions from a hydrogen bonded complex to an ionic crystal of the ammonium halide clusters.

Temperature and density effects on the properties of a long positive streamer in air

The 1.5D simulation is used to study the positive-streamer properties in air in a 20 cm sphere - plane gap versus gas temperature and density. It is shown that an increase in temperature up to 900 K at atmospheric pressure and a decrease in density by a factor of three at room temperature strongly affect the average electric field required for bridging the gap and the charge transferred through the streamer to the cathode; the temperature effect is much more pronounced than is the density effect. The calculated results qualitatively conform to available experimental data. The density effect is associated with a decrease in the rates of three-body processes such as three-body electron attachment to or conversion of ions into ions. The temperature effect is due to the decomposition of positive cluster ions which decreases the rate of dissociative electron - ion recombination. Electron attachment and detachment processes become important only when the electron density greatly decreases (by a factor of ten and more).

Bare phosphorus and binary phosphide cluster ions generated by laser ablation

Both positive and negative phosphorus cluster ions were generated from the laser ablation of a red phosphorus sample. The mass distribution of phosphorus cluster ions was found to be very sensitive to the power density of the ablation laser. The P 7 + species exhibits the highest signal intensity in the recorded mass spectra of bare phosphorus cluster cations, as does P 5 - among the anions. Their special structural stability can be attributed to their planar configuration and their aromatic character. As the phosphorus cluster size increases, the odd/even alternation of the signal intensity becomes more pronounced. For the P n + species with n > 24, the relative abundance varies in the order of 8 and P n + with n = 8k + 1 (k = 3–11) are more intense than their neighbors. For comparison, some binary phosphide cluster ions, including CnP m - , SinP m - , BnP m + and AlnP m + , were produced as well. The mass distribution of binary phosphide cluster ions changes with different components. From analysis of the recorded mass spectra of the phosphide cluster ions, the larger clusters may be in a polyhedral configuration and tend to have all valence electrons paired.

Deuteration of positive hydrogen cluster ions H5+ to H17+ at 10 K

The dynamics of the sequential deuteration of hydrogen cluster cations Hn+, n odd, is studied in a 22-pole radio frequency ion trap at a nominal temperature of 10 K. Rate coefficients for the step by step replacement of hydrogen by deuterium in initially mass selected clusters, Hn+, for n = 3, /3., 17 are extracted from the measured temporal evolution of the product ion intensities. The rates for the exchange of a single proton or alternatively for the replacement of a hydrogen molecule are strongly cluster size dependent. This is additional evidence for a previously proposed dynamical shell structure [W. Paul et al., Intern. J. Mass Spectrom. Ion Processes 150 (1995) 373]. Measurements are carried out in normal deuterium and deuterium with admixtures of helium, para hydrogen or normal hydrogen in order to investigate thermalization and the influence of back reactions. Results are discussed on the basis of calculated equilibrium structures, zero point energies and simple dynamical considerations. Constraints for intracluster rearrangement processes are found.

Comparison of mobility equivalent diameter with Kelvin-Thomson diameter using ion mobility data

Mobility distributions of both positive and negative cluster ions were studied in nitrogen gas enriched by ethanol, isopropanol, n-butanol and acetone vapours, respectively. The ions were generated into the system by a radioactive source normally used as a bipolar aerosol charger. The ion mobility distributions were measured with a time-of-flight mobility spectrometer (TOF). The measured ion mobility can be used to estimate the mobility equivalent diameter via conventional Millikan–Fuchs formula based on Stokes law corrected by molecule slip effects. When the vapour concentration is increased the cluster ions will get larger due to enhanced molecule attachment onto ions. Here the vapour concentration is assumed to be high enough so that the chemical tracer effects would become of minor importance. Therefore the Kelvin–Thomson equation for ion induced nucleation is first assumed to be valid. The Kelvin–Thomson diameter of the ions in all cases was found to be systematically smaller than the corresponding mobility equivalent diameter according to Millikan–Fuchs, indicating of inconsistencies between the formulas based on macroscopic quantities at 1 nm size range. Thereafter, other available mobility vs diameter relationships were surveyed. The closest agreement between mobility equivalent and Kelvin–Thomson diameters at this size was obtained using the relationship given by Tammet [J. Aerosol Sci. 26, 459 (1995)].