Positive Surface Ionization Research Articles

Studies of nanosecond pulse surface ionization wave discharges over solid and liquid dielectric surfaces

Surface ionization wave discharges generated by high-voltage nanosecond pulses, propagating over a planar quartz surface and over liquid surfaces (distilled water and 1-butanol) have been studied in a rectangular cross section test cell. The discharge was initiated using a custom-made, alternating polarity, high-voltage nanosecond pulse plasma generator, operated at a pulse repetition rate of 100–500 Hz, with a pulse peak voltage and current of 10–15 kV and 7–20 A, respectively, a pulse FWHM of ∼100 ns, and a coupled pulse energy of 2–9 mJ/pulse. Wave speed was measured using a capacitive probe. ICCD camera images demonstrated that the ionization wave propagated predominantly over the quartz wall or over the liquid surface adjacent to the grounded waveguide placed along the bottom wall of the test cell. Under all experimental conditions tested, the surface plasma ‘sheet’ was diffuse and fairly uniform, both for positive and negative polarities. The parameters of ionization wave discharge propagating over distilled water and 1-butanol surfaces were close to those of the discharge over a quartz wall. No perturbation of the liquid surface by the discharge was detected. In most cases, the positive polarity surface ionization wave propagated at a higher speed and over a longer distance compared to the negative polarity wave. For all three sets of experiments (surface ionization wave discharge over quartz, water and 1-butanol), wave speed and travel distance decreased with pressure. Diffuse, highly reproducible surface ionization wave discharge was also observed over the liquid butanol–saturated butanol vapor interface, as well as over the distilled water–saturated water vapor interface, without buffer gas flow. No significant difference was detected between surface ionization discharges sustained using single-polarity (positive or negative), or alternating polarity high-voltage pulses. Plasma emission images yielded preliminary evidence of charge removal from the liquid surface between the pulses on a microsecond time scale. Products of the plasma chemical reaction that accumulated in the ionization wave discharge over the liquid butanol–saturated butanol vapor interface were detected ex situ, using FTIR absorption spectroscopy. Reaction products identified include CO, alkanes (CH4,C2H6, C3H8), alkynes (C2H2), aldehydes (CH2O) and lighter alcohols (CH3OH).

Gas chromatography with surface ionization detection of nitro pesticides

AbstractA surface ionization gas chromatographic detector, based upon positive surface ionization, was used in capillary gas chromatography to sensitively and selectively detect nitro pesticides: pendimethalin, trifluralin, flumetralin. Higher sensitivity (better detection limit), substance specificity, and advantageous applicability are reported. Sensitivity to pendimethalin, trifluralin, and flumetralin was 1.4 C g−1, 1.1 C g−1, and 1.0 C g−1, respectively, with the linear range of operation greater than 1 × 105 for these compounds. The minimum detectable level was in the range of 10−13 g s−1. Compared with an atomic emission detector, SID provided a 110 times better detection limit for trifluralin.

Evaluation of Halogen‐Specific Detector (XSDTM) for Trace Analysis of Chlorinated Fatty Acids in Fish

The applicability of a halogen‐specific detector (XSD™, manufactured by OI Analytical) was evaluated for GC analysis of chlorinated fatty acid methyl esters (FAMEs) that are present at trace levels in transesterified fish extract, a complex matrix consisting of similar but nonchlorinated compounds. A characteristic of the XSD working principle is that thermal electron emission, negative surface ionization and positive surface ionization are all operative in a concerted manner. While the XSD is not superior to other GC detectors in terms of signal‐to‐noise, its merit is in its high selectivity (106) and low detection limit (2 pg Cl) for chlorinated fatty acids, and ease of operation and maintenance. Its reasonably wide linear range (up to 10 ng Cl) is desirable for trace analysis of chlorinated FAMEs. A major drawback of this detector is a certain degree of peak broadening and peak tailing of eluted compounds with concentrations larger than ∼1 ng Cl/µL in the injected solution even though this value does not exceed the upper limit of the XSD linear range. Nevertheless, in trace analysis of chlorinated compounds, the concentrations of analytes are usually well below 1 ng Cl/µL. Parallel use of the XSD and a universal detector such as FID in gas chromatography is useful for optimizing operation conditions for trace analysis and simultaneously analyzing nonchlorinated major components.

Probe Diagnostics of Meteotron Flame

A movable double probe was used to study the large-scale naked flame of the meteotron installation. The paper gives a description of the installation and experimental technique used and the data obtained. The electron temperature and the degree of ionization of the flame plasma are obtained from current–voltage characteristics. The operation mode of the probe–flame plasma system is determined. The possibility of positive surface ionization of the potassium atoms adsorbed on soot particles and in the flame volume is indicated.

ISOLDE target and ion source chemistry

For the production of radioactive ion beams by means of the ISOL (isotope separation on-line) method in which the nuclei of interest are stopped in a thick target, chemistry plays a crucial role. It serves to separate the nuclear reaction products in atomic or molecular form from the bulk target and to transfer them efficiently to an ion source. This article gives an overview of ISOLDE radiochemical methods where targets (liquid metals, solid metals, carbides and oxides) and ion sources are optimized with respect to efficiency, speed and chemical selectivity. Rather pure beams of non-metals and volatile metals can be obtained with a temperature-controlled transfer line acting as thermo-chromatograph. For less volatile metals the temperature of the target and ion source units needs to be kept as high as possible, but a selective ion source can be used: positive surface ionization for metals with ionization potentials below about 6 eV and the RILIS (resonance ionization laser ion source) technique for most other metals.

A new concept positive (negative) surface ionization source equipped with a high porosity ionizer

A versatile, new concept, spherical-geometry, positive (negative) surface-ionization source has been designed, fabricated, and initial tests completed which can operate in either positive- or negative-ion beam generation modes without mechanical changes to the source. The highly permeable, composite Ir/C has an intrinsic work function of φ=5.29 eV and can be used directly for the generation of positive-ion beams of highly electropositive elements. For negative-surface ionization, the work function is lowered by dynamic flow of a highly electropositive adsorbate such as Cs through the ionizer matrix. The results of initial testing indicate that the source is reliable, stable and easy to operate, with efficiencies for Cs + conservatively estimated to exceed 60% and as high as ∼50% for F − generation. However, the overall ionization efficiency for F − formation and extraction is ∼1% due to the low efficiency for thermal dissociation of the carrier molecules (assumed to be CsF and AlF). The design features, operational principles, and initial performance of the source for generating Cs + and F −, when operated with Cs, are discussed in this article.

A positive (negative) surface ionization source concept for radioactive ion beam generation

A novel, versatile, new concept, spherical-geometry, positive (negative) surface-ionization source has been designed and fabricated which will have the capability of generating both positive- and negative-ion beams without mechanical changes to the source. The source utilizes a highly permeable, high-work-function Ir ionizer ( φ≅ 5.29 eV) for ionizing highly electropositive atoms/molecules; while for negative-surface ionization, the work function is lowered by continually feeding a highly electropositive vapor through the ionizer matrix. The use of this technique to effect low work function surfaces for negative ion beam generation has the potential of overcoming the chronic poisoning effects experienced with LaB 6 while enhancing the probability for negative ion formation of atomic and molecular species with low to intermediate electron affinities. The flexibility of operation in either mode makes it especially attractive for radioactive ion beam (RIB) applications and, therefore, the source will be used as a complementary replacement for the high-temperature electron impact ionization sources presently in the use at the Holifield Radioactive Ion Beam Facility (HRIBF). The design features and operational principles of the source will be described in this report.

Surface ionization detector with a supersonic free jet for gas chromatography some applications

A new design for a gas chromatographic surface ionization detector based upon hyperthermal positive surface ionization has been developed: There were two requirements: supersonic free jet nozzle and the high work function surface of Re-oxide. This detector, which is highly sensitive in response to all organic compounds, can be operated as an universal detector with an additional selectivity towards some species that have low ionization energy, but with selectivity to a much lesser degree than a conventional surface ionization detector. The minimum detectable amount of toluene is ca. 10 −12 g/s with a linearity greater than 10 4. Some applications are demonstrated using three examples for the analysis of different formulations: (1), terpene mixture, (2), polycyclic aromatic hydrocarbon mixture and (3), alkyl alcohol mixture.

A Surface Ionization Detector for Gas Chromatography: Use of a Supersonic Free Jet

A new design of a gas chromatographic surface ionization detector (SID) based on hyperthermal positive surface ionization is described. There are two requirements: use of a supersonic free jet nozzle and the high-work-function surface of rhenium oxide. This detector, which is sensitive in response to all the organics, can be operated as an universal detector with much higher sensitivity toward some species having low ionization energy but with selectivity to a lesser degree than that of a conventional SID. The minimum detectable amount (at S/N = 3) of pyrene is around 4.4 × 10(-)(13) g/s, with linearity greater than 10(6), while that of toluene is around 10(-)(12) g/s. Other unique properties are (1) the ability to control the degree of selectivity through molecular kinetic energy, the surface, and its temperature and (2) a very short response time.

Empirical formulae of the threshold temperature range for dissociative positive ionization of alkali halides on heated metal surfaces

The ionization efficiency (β +) of alkali halide molecules (MX) incident upon W or Nb heated in a high vacuum (∼0.2 μTorr) always attained an appreciable value (∼0.01) at the appearance temperature ( T 0) and steeply increased up to unity at the saturation temperature ( T 1). To find the main factors governing the threshold temperature range ( T 0–T 1), the experimental data onβ +were analyzed by our theory on dissociative positive surface ionization (DPSI), yielding withMX/Wsystems that T 0 =(D 0 + I 0 − Φ 0 +)/(41.5 ± 2.0)kand T 1 =(D 1 + I 1 − Φ 1 +)/(28.7 ± 1.1)k. Here, D, I, andΦ +are the dissociation energy of MX, the ionization energy of M and the effective work function for DPSI, respectively, in eV at T 0 or T 1in K, and k is Boltzmann's constant. The empirical constants are essentially equal to the values (41.1 ± 1.0 and 27.6 ± 0.9) obtained with MX/Nb systems and also to those (41.7 ± 2.2 and 29.1 ± 1.4) found previously with MX/Re systems at ∼0.2-2.0 μTorr, indicating that our empirical formulae expressing the range generally hold with any MX/metal system.

Characterization of a gas chromatographic surface ionization detector based upon hyperthermal positive surface ionization

Characterization of a gas chromatographic surface ionization detector based upon hyperthermal positive surface ionization is described. The newly developed detector consists of a supersonic free-jet nozzle and the high-work-function surface of ReO 2. This detector, which is highly sensitive in response to all organic compounds, can be operated as a universal detector. There will be some additional selectivity toward some species having low ionization energy, but this will be much less than with conventional surface ionization detection. The minimum detectable amount of toluene is around 10 −12 g/s with linearity greater than 10 4.

Thermal dissociation and positive ionization of potassium chloride on tungsten

The efficiency (β +) of dissociative positive surface ionization (DPSI) of KCI impinging on a polycrystalline tungsten surface heated in a high vacuum was found to be unity only in an intermediate temperature range ( T ⋍ 1250–1750K). Analysis of the data by our original theory of DPSI yields the following results: (1) at temperatures above and below this range, β + is determined only by the degree of ionization of potassium and that of dissociation of KCI respectively; (2) both ionization of potassium and dissociation of KCI are strongly governed by the effective work function (Ø +) for DPSI; and (3) Ø + increases from 5.1 to ≈ 6eV mainly owing to residual gas adsorption when T decreases from ≈ 2000 to 1200 K. Temperature dependence of the effective work function for thermal electron emission has virtually the same pattern as that of Ø +, thus giving additional evidence to support our theory.

Temperature dependence of the positive ionization efficiency of sodium chloride molecules incident upon a polycrystalline tungsten surface heated in a high vacuum

In a readily attainable high vacuum (∼2 × 10 −5Pa), the emission current of Na + ions produced from NaCl molecules impinging with a constant flux (1.6 × 10 13-1.4 × 10 14 molecules cm −2 s −1) upon a polycrystalline tungsten surface has been measured as a function of surface temperature ( T). The sticking probability of NaCl to W is ∼1. The ionization efficiency (β +) is ∼1 at T ≅ 1300–1400 K and ∼0.05 at ∼1900–2300 K. The effective work function (φ +) for the dissociative positive surface ionization (DPSI) on the virtually clean surface of W is about 5.2 eV. By adsorption of residual gases below ∼1900 K, φ + is increased up to ∼6.6 eV. Below ∼1300 K, however, φ + is decreased by non-dissociative adsorption of NaCl. The pattern and tendency thus observed with β + and φ + are similar to those in an NaCl/Re system. These results are explained reasonably by our original theory on DPSI.

Rare earth oxide catalyzed oxidation of rhenium to rhenate ions ReO4- and ReO3- as observed by negative surface ionization mass spectrometry

A surface ionization mass spectrometer was used to observe the in situ formation of ReO/sub 4//sup -/ and ReO/sub 3//sup -/ gas-phase ions resulting from the rare earth oxide catalyzed reaction of water with metallic rhenium. Water was found to be 6-10-fold more efficient than oxygen as an oxidizer at temperatures around 1100/sup 0/C. Normal, oxygen-18 enriched, and deuteriated water were used to elucidate the oxidation mechanism. The results were consistent with a mechanism whereby the rare earth oxide goes through a stoichiometric cycle, with the highly stoichiometric oxide functioning as a strong oxidizer and the oxidizing potential being reduced as the rare earth moves into a mixed valency state with reduced oxygen content. The oxygen stoichiometry (and the oxidative potential) is then restored by reaction with water. The oxygen of the water is slowly equilibrated with the original oxygen of the rare earth oxide, with the variations between the rare earths being consistent with known rare earth mixed valency chemistry. The highly stoichiometric oxide readily oxidizes rhenium with high efficiency. The results of this work suggest that both positive and negative surface ionization mass spectromery should be explored as tools for the in situ study of surface catalyzed gas-solidmore » reactions. Few techniques are capable of monitoring the dynamics of solid-state chemical reactions at these temperatures, and surface ionization appears to offer some hope for elucidating at least some reactions at elevated temperatures.« less

Additional evidence for the work function increase due to adsorption of residual gases and for its decrease due to co-adsorption of the gases and alkali metal halide molecules during positive surface ionization in high vacua

Under the various conditions of sample beam flux ( N ⋍ 4×10 12-1×10 14 molecules cm −2 s −1) and of residual gas pressure ( P r ⋍ 0.2–2 μtorr), the emission current of lithium ions produced from lithium iodide molecules impinging upon a heated polycrystalline rhenium surface was measured as a function of surface temperature ( T ⋍ 1000–2100 K) or elapsed time (0–10 min) after flashing the surface. The experimental data thus obtained were analyzed by our theory on dissociative positive surface ionization to determine the change in effective work function (ϕ +) for the ionization. A boundary temperature ( T A) below which ϕ + increased gradually with a decrease in T changed from ≈1750 to ≈1900 K with an increase in P r but depended little upon N, suggesting that completely dissociative adsorption of LiI had little effect upon ϕ +. On the other hand, another boundary temperature ( T B) at which ϕ + began to decrease steeply with a lowering temperature shifted from 1265 to 1180 K according to a decrease in N, but T B was no longer dependent upon P r. Below T B, a rapid accumulation of sample molecules on the surface greatly suppressed the work function increase due to adsorption of residual gas molecules (RGM). These results verify our previous conclusions that the increase in ϕ + below T A is caused mainly by adsorption of RGM and also that the decrease in ϕ + below T B is caused by co-adsorption of sample molecules and RGM.

AIN: a solid Al + ion source

Because of its physical, chemical and electrical properties, AIN is an attractive material for positive surface ionization applications and a likely candidate for Al + thermal emission. The computed composition and pressure of the vapour phase over AIN indicate that sublimation is stoichiometric and that the equilibrium pressure is high, even at 1500 K. Moreover, the bulk electronic properties of AIN are such that work functions as high as 6 eV can be obtained, provided that surface states are not present in the band gap. The evolution of the work function vs temperature was measured on an n-type AIN single crystal, using the Shelton retarding field method. The work function is temperature independent, which infers Fermi level pinning at the surface by an empty intrinsic surface state band. With a value of 5.35 eV for the work function, AIN seems to be a practical alternative to iridium for surface ionization applications. Moreover, direct thermal Al + emission from AIN, expected because of the relatively low ionization energy of Al, has been observed by mass spectrometry. Finally, the mechanisms of thermal positive ion emission from solid sources are discussed in the light of experimental results.

Determination of the Vapor Pressures of Lithium Iodide and Potassium Chloride by Molecular Beam Surface Ionization

Abstract Combined techniques of molecular beam effusion from a Knudsen cell and of positive surface ionization on a heated polycrystalline rhenium filament were employed to determine the vapor pressures P (in atm) of LiI and KCl as a function of cell temperature T (in K), thereby yielding the equations: log10P(LiI)=(7.498±0.046)−(9254±30)⁄T for 583–717 K, log10P(KCl)=(7.210±0.058)−(11135±45)⁄T for 687–877 K. The present data on the pressure and on both entropy and enthalpy of sublimation are compared with others in literature.

Application of wide-gap semiconductors to surface ionization: Work functions of AlN and SiC single crystals

For surface ionization purposes, a study of the work functions of SiC and AlN, both refractory wide-gap semiconductors, has been undertaken. Work function measurements have been performed in the 300–1600-K range using the Shelton retarding field method. Surface cleaning was carried out by heating in uhv to a high temperature using of a cw CO2 laser. Both n- and p-type 6H SiC single crystals with extreme bulk doping levels were investigated. For each doping type, the work functions have been found to be temperature independent. They exhibit only a slight variation from the n to the p type (4.75 and 4.85 eV, respectively) giving evidence, as in the case of Ge and Si, of Fermi level pinning at the surface by the intrinsic surface states. For an n-type AlN single crystal, the work function, measured in the high temperature range, remains constant as the temperature varies. The value of 5.35 eV obtained for AlN makes it a most attractive material for positive surface ionization applications.

Periodicity of the degree of positive ionization of sputtered atoms in the conception of thermal spikes

This is the first calculation of the degree of positive surface ionization, α +, of atoms sputtered from pure elements for almost the Whole Periodic Table by 15 keV Xe + ion bombardment. The calculation is based on the thermodynamic model of non-equilibrium surface ionization (NESI). All the parameters of the calculation are defined and the known tabulated values are used. For the temperatures of sputtering centers, T sc, the values calculated from experimental results of M + and M − ion yields for 35 pure elements and 9 compounds are used. The temperatures of sputtering centers are defined by means of the non-equilibrium surface ionization equations. The results of this calculation confirm the previously experimentally observed periodic dependence of the degree of ionization of sputtered atoms. Good agreement between calculated and experimental results verifies the thermodynamic model for non-equilibrium surface ionization during sputtering of solid surfaces by ion and neutral beams.

Progress in ion source development for on-line separators

Abstract Design criteria for ion sources will be discussed in view of the principle requirements of the ISOL-process. The demands for high ionization efficiency (at a level above 1%), for versatility (the ionization of almost all elements) and for at least some selectivity (the ionization of one element at a time) can be met with present-day technology by a set of a high temperature discharge ion source, an ionizer with variable-temperature and variable-material options for positive and negative surface ionization, and a “high temperature cavity ion source”. The present yield limitation is related to the release efficiency rather than to the ionization efficiency. A number of examples will demonstrate to what extent the principal design aims have been realized in existing devices. The fundamental characteristics of thermal ion sources are also dealt with. The boundary conditions-quasi-neutrality and thermal equilibrium-are treated in detail, permitting the calculation of the ionization efficiency. The occurrence of “impossible” ion species in thermoionization mass spectra (e.g. noble gas and multiply charged ions) and “halos” of the mass lines will be explained in terms of the relevant ionizing processes.