Abundant Positive Ions Research Articles

Self-Organized Fractal Structures on Plasma-Exposed Silver Surface.

Planar fractal microstructure is observed on the silver film treated by positive corona discharge for the first time. Due to the abundant positive ions driven by the electrical field of positive polarity, surface modification is mainly induced by the plasma oxidation effect, resulting in a large scale of dendritic pattern with self-similarity and hierarchy. In contrast, negative ions dominate the plasma-film interaction under negative corona discharge condition, leading to a different surface morphology without fractal characteristics. A growth model based on the modified diffusion-limited aggregation (DLA) theory is proposed to describe the formation of the dendritic fractal structure, whilst the physics behind is attributed to the electric field directed diffusion of the positive ions around the surface roughness. Numerical simulation verifies the high density of the hot spot in the dendritic pattern, which may enable potential applications in fractal photonic metamaterials.

Super rogue wave catalysis in Titan’s ionosphere

Super rogue ion-acoustic waves are proposed as a physical catalyst for the heavy hydrocarbon ions formation in the Titan ionosphere. We justified that analytically and numerically by probing a Titan referenced plasma system, consists of the most abundant positive ions and superthermal electrons. A solution of the nonlinear Schrödinger equation (NLSE) has provided us by the plasma (un) stable regions at altitude 900–1200 km from the Titan surface with superthermal parameter values, relative ion to electron densities, and temperature ratio variations. Our results are not only agreed with the Cassini data but also predict a chemistry independent approach for the heavy hydrocarbons’ formation conditions.

Reprint Of: Enhanced spatially-resolved trace analysis using combined SIMS–single-stage AMS

Abstract Secondary ion mass spectrometry (SIMS) provides spatially resolved trace analysis of solid materials, but can be complicated by unresolved abundant molecular isobars. By adding a 300-kV single-stage accelerator mass spectrometer (SSAMS) as a detector for a Cameca ims 4f SIMS, one can measure more abundant positive ions from the SIMS while removing molecular isobars, thus improving very low abundance trace element and isotope analysis. This paper describes important features and capabilities of such an integrated system at the Naval Research Laboratory using charge state +1 ions. Transmission loss is compared to molecule destruction as gas flow to the molecule-destruction cell increases. As most measurements tolerate more modest abundance sensitivities than for 14C analysis, a lower gas flow is acceptable, so good transmission of 20–50% for ions of interest can be maintained for a broad range of ion masses. This new instrument has measured isotope ratios for uranium, lead, rare earths, and other elements from particulates and localized regions, with molecule destruction enabling the measurement at low SIMS mass resolving power and thus high transmission, as examples will show. This new and world-unique instrument provides improved capabilities for applications in nuclear and other forensics, geochemistry, cosmochemistry, and the development of optical, electronic, multifunctional, and structural materials.

Enhanced spatially-resolved trace analysis using combined SIMS–single-stage AMS

Abstract Secondary ion mass spectrometry (SIMS) provides spatially resolved trace analysis of solid materials, but can be complicated by unresolved abundant molecular isobars. By adding a 300-kV single-stage accelerator mass spectrometer (SSAMS) as a detector for a Cameca ims 4f SIMS, one can measure more abundant positive ions from the SIMS while removing molecular isobars, thus improving very low abundance trace element and isotope analysis. This paper describes important features and capabilities of such an integrated system at the Naval Research Laboratory using charge state +1 ions. Transmission loss is compared to molecule destruction as gas flow to the molecule-destruction cell increases. As most measurements tolerate more modest abundance sensitivities than for 14C analysis, a lower gas flow is acceptable, so good transmission of 20–50% for ions of interest can be maintained for a broad range of ion masses. This new instrument has measured isotope ratios for uranium, lead, rare earths, and other elements from particulates and localized regions, with molecule destruction enabling the measurement at low SIMS mass resolving power and thus high transmission, as examples will show. This new and world-unique instrument provides improved capabilities for applications in nuclear and other forensics, geochemistry, cosmochemistry, and the development of optical, electronic, multifunctional, and structural materials.

Mass spectrometry of positive ions in capacitively coupled low pressure RF discharges in oxygen with water impurities

A capacitively coupled RF oxygen discharge is studied by means of mass spectroscopy. Mass spectra of neutral and positive species are measured in the mid plane between the electrodes at different distances between plasma and mass-spectrometer orifice. In the case of positive ions, as expected, the largest flux originates from . However, a significant number of impurities are detected, especially for low input powers and larger distances. The most abundant positive ions (besides ) are , , and . In particular, for the case of hydrated hydronium ions (n = 1, 2) a surprisingly large flux (for low pressure plasma conditions) is detected. Another interesting fact concerns the ions. Despite the relatively high ammount of water impurities ions are present only in traces. The reaction mechanisms leading to the production of the observed ions, especially the hydrated hydronium ions are discussed.

Effect of electrode spacing on the density distributions of electrons, ions, and metastable and radical molecules in SiH4/NH3/N2/He capacitively coupled plasmas

Semiconductor fabrication often requires the deposition of hydrogenated silicon nitride (SiNxHy) film using SiH4/NH3/N2/He capacitively coupled plasma (CCP) discharge. As analysis of the discharge geometry is essential to understanding CCP deposition, the effect of electrode spacing on the two-dimensional distributions of electrons, ions, and metastable and radical molecules was analyzed numerically using a fluid model. The simulation shows that the spatial variations in the ionization rates near the sheath become more obvious as the electrode spacing increases. In addition, as molecule-molecule gas-phase reactions are significantly affected by the local residence time, large electrode spacings are associated with significant volumetric losses for positive ions. Consequently, an increase of the electrode spacing leads axial density profiles of ions to change from bell shaped to double humped. However, NH4+ persistently maintains a bell-shaped axial density profile regardless of the degree of electrode spacing. We set the mole fraction of NH3 to only 1% of the total flow at the inlet, but NH4+ is the most abundant positive ion at the large electrode spacings. As the gas flow can transport the radicals around the space between the electrodes, we found that radical density distribution shifts toward the grounded electrode. The shift becomes pronounced as the electrode spacing increases. Finally, to validate our model, we compared the calculated deposition rate profile with the experimental data obtained along the wafer radius. According to our numerical results, the SiNxHy deposition rate decreases by approximately 16% when the electrode spacing increases from 9 to 20 mm.

The dominant role of impurities in the composition of high pressure noble gas plasmas

We present in this letter how a molecular gas such as nitrogen at different levels of impurity dominates the ionic composition of an atmospheric pressure noble gas plasma such as in helium. The positive charge in the discharge is only determined by helium ions if the discharge gas contains less than 1ppm of impurity. Above this impurity level, the positive charge is completely determined by the impurity nitrogen. The higher the relative nitrogen concentration, the more N4+ dominates over N2+. If the impurity level is between 1 and about 20ppm, N2+ is clearly the most abundant positive ion but for higher levels of impurity, N4+ almost completely determines the positive charge.

An improved assay for platelet-activating factor using HPLC-tandem mass spectrometry

We describe an improved assay for platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) using HPLC-tandem mass spectrometry (LC-MS/MS). The present method can readily detect as little as 1 pg (1.9 fmol) of PAF, a significant improvement over previously described LC-MS/MS methods, and gives a linear response up to 1,000 pg of PAF. Our method also overcomes the artifacts from isobaric lipids that have limited the usefulness of certain existing LC-MS/MS assays for PAF. In the course of these studies, we detected three novel lipid species in human neutrophils. One of the novel lipids appears to be a new molecular species of PAF, and the other two have chromatographic and mass spectrometric properties consistent with stearoyl-formyl-glycerophosphocholine and oleoyl-formyl-glycerophosphocholine. These observations identify previously unknown potential interferences in the measurement of PAF by LC-MS/MS. Moreover, our data suggest that the previously described palmitoyl-formyl-glycerophosphocholine is not unique but rather is a member of a new and poorly understood family of formylated lipids.

A model of the martian ionosphere below 70 km

The ion-neutral chemistry of the atmosphere of Mars has been studied with a one-dimensional model. Here we present the main reactions that lead to the formation of the most abundant positive and negative ions. Galactic cosmic rays ionize the atmospheric constituents, mainly CO 2, and reactions of charge transfer of CO + 2 with oxygen produces O + 2. The further ion-neutral chemistry is quite similar to that of D-region on Earth. The relative high abundance of water and its higher proton affinity compared with other molecules, lead to the expectation that the most abundant positive ions are hydronium hydrates, H + 3O(H 2O) n . The chemistry includes the production of negative ions, O − and O − 2, by electron capture of oxygen-bearing molecules and sinks of negative ions by electron photodetachment and ion-ion recombination. Oxygen negative ions are transformed through a chain of reactions into the carbon and nitrogen-bearing ions, CO − n and NO − n . The formation of hydrated negative ions is also considered.

Anthropogenic perturbations of tropospheric ion composition

This paper presents a simple model of the vertical distribution of the most abundant positive ions in the troposphere. The model is based on a simple ion chemical scheme, measured rate constants, and observed or calculated atmospheric densities of parent neutral compounds. It suggests that in many cases the most abundant gaseous ions are heavy protonated water cluster ions of pyridine, picoline and lutidine with ammonia molecules attached (pyridinated cluster ions) in the lower troposphere (below 7 km altitude), acetone cluster ions in the upper troposphere (between 7 and 15 km) and methyl cyanide cluster ions above 15 km. The relative abundance of heavy clustered aerosol ions is predicted to be highest near the surface. The model suggests that ion composition could be perturbed as a result of human activities (i.e., increasing emissions of parent neutral species including alkaline compounds), and that in particular, the relative atmospheric abundance of pyridinated water cluster ions is increasing dramatically and should further increase in the future.

Elemental sulfur as a matrix for mass spectrometry of photosynthetic pigments and fullerenes

Elemental sulfur works well as a matrix for laser desorption time-of-flight mass spectrometry of bacteriochlorophylls and carotenoids, the major pigments in photosynthetic bacteria. Because sulfur lacks protons, pigment ionization probably involves electron, rather than proton, transfer. Fragmentation of chlorophylls that are esters of allylic alcohols occurs, but is partially suppressed when sulfur is used as a matrix. Mass spectrometry on sulfur itself shows that the most abundant positive ion is S5 +, while the most abundant negative ion is S3 −, indicating that light absorption causes photodissociation of S8 rings into these products. A similar pattern was observed with red selenium, which also occurs as 8-membered rings. Molecular masses of other hydrophobic analytes, such as fullerene compounds, can also be determined using elemental sulfur as the matrix. Copyright © 1999 John Wiley & Sons, Ltd.

Charged-species profiles in electronegative radio-frequency plasmas.

The negative ion density profile in a low pressure oxygen rf plasma has been measured by a photodetachment technique. At an rf power of 10 W and a neutral pressure of 10 mTorr, a parabolic negative ion density profile is obtained with a peak density of 8\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$ and a maximum ratio of negative ion to electron densities ${\mathit{n}}_{\mathrm{\ensuremath{-}}}$/${\mathit{n}}_{\mathit{e}}$\ensuremath{\approxeq}18. Under these conditions, the most abundant positive ion, determined by ion mass spectrometry, is ${\mathrm{O}}_{2}^{+}$ with ${\mathrm{O}}^{+}$ being less than 10% of the positive ion density. The most abundant negative ion is ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ with ${\mathrm{O}}_{2}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{O}}_{3}^{\mathrm{\ensuremath{-}}}$ being less than 20% of the total negative charge density. The maximum in the density profile of negative ions shifts closer to the powered rf electrode as the pressure is increased in the asymmetric system. Comparison of the results to theory indicates that the asymmetry follows from an enhancement of the ionization rate near the powered electrode sheath. The parabolic profile is also obtained in ${\mathrm{CCl}}_{2}$${\mathrm{F}}_{2}$ at low pressure. Simulations and measurements show a rapid drop in ion density near the sheath that may be related to the recently discussed ``stratification'' phenomenon in electronegative plasmas.

Combined capillary column gas chromatography/electron capture negative ion chemical ionization mass spectrometry of dihydropyridine calcium antagonists.

Several dihydropyridine calcium antagonists showed intense [M]-. ion peaks as the base peaks and a few fragment peaks in the electron capture negative ion chemical ionization mass spectra. The peak intensities of the [M]-. ion of these antagonists were about 100 times as high as those of the most abundant positive ion. The use of a capillary column connected directly to the ion source prevents the oxidation of these agents in the column and the transfer line. Furthermore, the presence of plasma extract decreases the oxidation of these agents in the injection port. The limits of sensitivity with this technique are 500 fg-5 pg on the column. Combined capillary column gas chromatography/electron capture negative ion chemical ionization mass spectrometry should be suitable for sensitive and specific assays of dihydropyridine calcium antagonists in biological fluids.

Californium-252 plasma desorption mass spectrometry of cationic, anionic, and neutral dyes.

A series of 31 cationic, anionic, and neutral dyes were examined by californium-252 plasma desorption mass spectrometry in both positive and negative ion modes. Cationic dyes provide abundant positive ions for the expected cation, as in SIMS, while negative ion spectra show only weak peaks corresponding to anions of the dye moiety. Anionic dyes usually show peaks for the expected anion as well as cations corresponding to alkali metal adducts of the alkali metal salts. No doubly charged ions were observed even when the dyes themselves were dicationic ordianionic. Instead, evidence was obtained for neutralization of one of the charges by a variety of processes including electron gain, hydride addition, alkyl loss, and alkali ion gain. Neutral dyes provide abundant cations and anions corresponding to (M - H), M, and (M + H) and are best examined by conversion to their salts where they show intense peaks in the positive ion mode for (M + H)/sup +/. 18 references, 7 figures.

The lower ionosphere of Titan

Ionization of the atmosphere of Titan by galactic cosmic rays is a very significant process throughout the altitude range of 100 to 400 km. An approximate form of the Boltzmann equation for cosmic ray transport has been used to obtain local ionization rates. Models of both ion and neutral chemistry have been employed to compute electron and ion density profiles for three different values of the H 2/CH 4 abundance ratio. The peak electron density is of the order 10 3 cm −3. The most abundant positive ions are C 2H 9 + and C 3H 9 +, while the predicted densities of the negative ions H − and CH 3 − are very small (<10 −4 that of the positive ions). It is suggested that inclusion of the ion chemistry is important in the computation of the H and CH 3 density profiles in the lower ionosphere.

Reaction of the Hydroxonium Ion with Metal Atoms in Flames

THE hydroxonium ion H3O+ is the most abundant positive ion found in the reaction zones of unseeded hydrocarbon flames, where it is produced by ionization processes in amounts much greater than at equilibrium1,2. Downstream from the reaction zone in the region of burnt gas, a steady state of H3O+ is approached by the dissociative recombination process In flames of hydrogen burnt with oxygen, with nitrogen as a diluent, there is a similar state of affairs because of small traces (about 50 p.p.m.) of hydrocarbon impurities in the commercial supplies of gas used2. The introduction of small amounts (about 10 p.p.m.) of potassium into such a hydrogen flame accelerates the rate of disappearance of H3O+ in the burnt gases3. There are two possible explanations of this observation. First, potassium partially ionizes in flames and the additional electrons so produced might be expected to accelerate the rate of disappearance of H3O+ by way of the recombination process in equation (1). Alternatively, H3O+ might disappear by the charge transfer process This has frequently been considered4 to be responsible for the excess ionization and electronic excitation of metals in hydrocarbon flames.

A Mass Spectrometric Study of Ionization and Dissociation by Electron Impact of Perfluoro-methyl Cyclohexane†

ABSTRACT The positive and negative ions, formed by electron impact in perfluoromethyl cyclohexane, have been studied in a mass spectrometer. The observation of the most abundant positive ion CF3+ is in agreement with other studios, as is the appearance potential of this ion. Seven negative ions, the most abundant of which is the parent ion C7F14–, have been observed and possible mechanisms for their formation are discussed.