Surface Ionization Technique Research Articles

The nature of the adsorption bond between graphite islands and iridium surface

To reveal the nature of adsorption bonds between two-dimensional graphite islands and iridium (111) and (100) faces, a study has been made of the adsorption of potassium and cesium atoms on the surface of these systems, using thermal desorption and Auger electron spectroscopy, as well as surface ionization and thermionic emission techniques. The graphite islands are shown to be weakly bound to the iridium substrate by Van der Waals forces. The unsaturated valence bonds at the periphery of the graphite islands are “lowered down” on to the metal. The recess between the graphite layer and the metal is filled by adsorbing particles through defects in the graphite layer. The atoms can penetrate into the recess in two ways: at T > 1000 K directly from the flux incident on the surface, and at T < 1000 K also by migration from the graphite island surface. The adsorption capacity of this state is ∼ (2−3) × 10 14 cm -2. Thermal destruction of the islands at T > 1900 K liberates the potassium and cesium atoms from under the graphite islands. Our study suggests that the reason for the “raised” position of the islands lies in the valence bonds of the graphite layer being saturated, the valence bonds of the metal and its crystallographic orientation being less significant. Therefore one may expect the graphite layer to be raised also above other metals as well. The filling by cesium of the recess between the graphite layer and iridium and of the adsorption phase on the graphite surface, does not change the general “graphitic” shape of the carbon Auger peak. This cesium results, however, in a pronounced splitting of the negative spike on the carbon peak (which provides information on its location relative to the graphite layer) indicating the appearance in the valence band of graphite near the Fermi level of two narrow (∼ 2−3 eV) regions with an enhanced density of states originating from the presence of the alkali metal.

Measurement of atomic densities using radiation trapping

The apparent lifetimes of the 2P1/2 and 2P3/2 states of Na have been measured as functions of n, the atomic density, and the results compared to theory. It is shown that these measurements made under equilibrium, steady-state conditions may be used to determine n under nonequilibrium, nonsteady-state conditions. This technique of measuring n has advantages over the surface ionization technique, especially for values of n greater than about 1010 cm−3. Extension of the technique to other atoms and molecules is discussed.

Aerosol characterization using molecular beam techniques

Molecular beam techniques are applied to measure fundamental properties of aerosol particles. An aerosol-laden gas is sampled with a microprobe or a capillary tube and drawn into a vacuum chamber, where gas molecules are pumped out and an aerosol beam is formed. A surface ionization technique is used to measure the particle number intensity in aerosol beams of alkali metal compounds (primarily potassium iodide). The particle velocity is measured by a time-offlight technique, and the particle mass is measured by electrically charging the particles and subsequently deflecting them in an electrostatic field. Velocity relaxation in a retarding gas is utilized to measure the particle size, and the electric polarizability of neutral aerosol particles is measured by deflecting them in a nonuniform electrostatic field. Particle velocities from 70 to 670 m/sec masses from 1 × 10 −20 to 1 × 10 −18 kg, and radii from 0.01 to 0.1 μm were measured. Anomalously low mass densities and high electric polarizabilities were observed for these particles. This work demonstrates that aerosol beam techniques are potentially valuable tools for fundamental studies of aerosol particles.

Absolute Isotopic Abundance and the Atomic Weight of a Reference Sample of Thallium.

The accepted atomic weight of thallium has remained at a value of 204.37 ± 0.03 since 1962. At this level of uncertainty, however, the atomic weight becomes a limiting factor to high accuracy analysis. The new mass spectrometric determination of the atomic weight of thallium has been completed. A high precision assay technique was developed so that accurately known quantities of the 203Tl and 205Tl separated isotopes could be mixed to produce standards for calibration of the mass spectrometer. This assay technique involved the gravimetric determination of 99.3 percent of the0020thallium as Tl2CrO4. The soluble thallium was then aliquoted and determined by isotope dilution mass spectrometry. Before making up the final solutions from which the assay and calibration samples would be withdrawn, the separated isotopes were purified by solvent extraction and electrodeposition. A tungsten filament surface ionization technique was developed for the determination of precise isotopic abundance measurements for thallium. This technique allowed isotopic analysis of the separated isotopes, calibration standards, and a natural thallium reference standard with precisions of better than 0.1 percent. The 205Tl/2Tl absolute isotopic abundance ratio of the reference sample was found to be 2.38714 ± 0.00101, yielding an atomic weight of 204.38333 + 0.00018.

Analysis of ion cyclotron resonance line shapes

Asymmetric cyclotron resonances resulting from the electric field gradients and spatial distribution of ions in an ICR cell are discussed. A comparison is made between ICR line-shapes obtained using surface ionization and electron impact techniques, and qualitative agreement is found with a previously proposed theory. The influence of electric field gradients on the mass resolution of the ICR device is investigated theoretically.

The electron affinity of tetracyanoethylene and other organic electron acceptors

Information on the molecular structure of tetracyanoethylene and its mononegative ion is used to derive the electron affinity of tetracyanoethylene from the experimentally determined threshold energy for photodetachment of electrons from the negative ions. The resultant electron affinity of 2.3(� 0.3) eV is 0.6(� 0.4) eV less than a value derived by the magnetron surface ionization technique. Relative electron affinities for the strong electron acceptor organic molecules, which are known from charge-transfer spectra, are placed on an absolute scale using the photodetachment result for tetracyanoethylene.

Thermal ion emission from some transition metal carbides heated on graphite filament

Thermal ion emissions from the carbides of Ti, Zr, Hf, V, Nb, and Mo heated on graphite filament have been studied by means of the mass spectrometric single filament surface ionization technique. With Ti–C, Zr–C, Hf–C, V–C, and Nb–C systems dicarbide ions and with Mo–C system monocarbide ions were identified, respectively, in addition to the corresponding metal ions. Abundance ratios MC2+(g)/M+(g) and heats of vaporization of some ionic species were determined, and heats of vaporization of neutral species were also derived.

Electron affinities of Cu and Ag

In a crossed-beam experiment, photodetachment of Cu− and Ag− ions by an Ar-ion laser beam (4880 Å) is studied by energy analyzing the detached electrons. The electron affinities are found to be: E.A. (Cu)=(1.226 ± 0.010) eV, E.A. (Ag)=(1.303−0.011+0.007) eV. These electron affinities are approximately 0.5 eV less than those determined by surface ionization studies. The surface ionization technique is discussed and possible reasons for the discrepancies are advanced. The new electron affinities are used to estimate those of the atoms in the long series of the periodic table. Measurements of the angular distribution of the photodetached electrons yield β=2.00 ± 0.03 for the anisotropy parameter β in both cases.

A Precise and Quick Measurement of Isotope Ratio 235U/ 238U Using a Varian MAT CH-5 Mass Spectrometer

A precise and quick measurement of the isotope ratio of uranium by means of surface ionization technique has been investigated using a Varian MAT CH-5mass spectrometer. Adopting a zero-point correction and a triple filament method, the relative standard deviation of the ratio 235U/ 238U was reduced to 0.1% and the measuring time was within an hour.

A theoretical study of the use of internal standards for precise isotopic analysis by the surface ionization technique Part II: Error relationships

Using an idealized model it is shown that, to minimize errors in double-spiked isotopic analyses, spike mass numbers should alternate with normal. The comprehensive error analysis which follows assumes that the individual ion currents are the independent varietes for statistical purposes: errors in these are obtained by combining (i) a coefficient of variation eP due to fluctuations in ion emission with (ii) a resistor noise index eR and (iii) a shot noise index eS: the noise indices are the ratios [error due to noise]/[error (i) due to emission fluctuation] for the largest ion beam in the spectrum. Methods of computing error multipliers (operating on eP) from eR, eS and the isotopic abundances, are developed. For a particular element each error multiplier can be regarded as a function of two variables, the ratio of the two spikes selected and the spike/normal ratio, which can then be optimized by numerical methods. Optimum values of spike composition and spike/normal ratio are given for observations of isotopic fractionation of lead, molybdenum and iron using various pairs of spike mass numbers. If both noise indices are taken to be zero the minimum error multipliers are not very much increased by deviations from the ideal model. When eR is given values corresponding to experience with a typical electrometer, it is shown that satisfactory results would be obtained on Pb and Mo with quite low ion currents, but that for iron, because of its difficult isotopic composition, the maximum ion current should be as large as possible. Some possible deviations from first-order fractionation theory are considered. A general algebraic analysis for the determination of radiogenic isotopes is given, and different approaches to fractionation computations are discussed.

Determination of Cosmic-Ray-Produced Potassium and Calcium in Iron Meteorites.

A Long lived radio-nuclide 40K and stable nuclides of calcium produced by cosmic ray in iron meteorites were determined by mass spectrometry using surface ionization technique with an electron multiplier. These nuclides were extracted simultaneously in a systematic wet chemical separation scheme with other stable nuclides of titanium, vanadium and chromium and long lived nuclide 53Mn. For the determination of absolute contents of the products, the dilution method was apllied using spikes, 39K and 42Ca. The contamination level of natural potassium and calcium was reduced as low as possible; the lowest, we obtained, was 0.2ppm potassium and 0.4ppm calcium. In such sample, the enrichment of 42Ca, 43Ca, 44Ca and 46Ca due to cosmic-ray products were observed at 70, 200, 20 and 200% respectively. It was not always possible to reach such low contamination level. On the other hand, the most of 40K extracted from iron meteorites was found to be the cosmic-ray products even at a higher contamination level. For the isotope fractionation and the isotope discrimination problem, long lived 40K seems not to be affected seriously because of the very high ratio of cosmic-ray product to natural 40K and of being placed between two stable nuclides of potassium. When we got the lowest contamination, cosmic-ray-produced 41K should be detected at the about 5% enrichment in mass 41 peak. However, we have no mean to correct the isotope fractionation and discrimination of potassium and could not determine the 41K in this work. According to the spallation systematics and also our earlier works, fortunately, 40Ca and 48Ca are produced by negligible amounts by cosmic-ray irradiation. The isotope fractionation and discrimination in the mass spectrometry were corrected by normalizing 40Ca and 48Ca peaks in samples to their reference values which were selected from literatures and our data of reagent calcium. When sample size was very small, mass 42 and 43 peaks were usually disturbed by tenacious hydrocarbon peaks. In order to eliminate such disturbances, the cavity of ion source in mass spectrometer and the filament had to be baked before the measurement in each run. Examples of results are as follows[unit: 1013atom/g of meteorite]. 40K: Grant A-350, 0.526, Grant Q-260. 0.541, Grant E-240, 0.470, Grant I-110, 0.42, Grant I-55, 0.458, Aroos, 0.715, Treysa, 0.546. 43Ca: Grant A-350, 1.77, Grant Q-260, 1.6, Grant I-110, 1.6, Grant I-55, 1.42, Aroos, 2.29, Treysa, 1.55. A dependency on the depth in the meteorite Grant was found to be about 20%, smoothly decreased from the surface to the center for both 40K and calcium. Our samples were not enough in number to determine the preatomospheric size of the meteorite yet. A fine structure in a cumulative spallation yield curve was examined in the isotopic ratios of cosmic-ray-produced calcium analyzed in a highly enriched sample. The yields of 42Ca and 44Ca were 10±5% higher than an expected yield of 43Ca. When we borrow the data by H. Voshage, the yield of 41K was also found to be apparently lower. Experimental procedures, especially the chemical procedure and the mass spectrometry of very small amount of these elements are described in detail.

Mass-Spectrometric and Theoretical Evidence for NH4 and H3O

The unusual chemical species H3O and NH4 have been studied using both experimental and theoretical methods. Experimentally the species were investigated by means of a mass spectrometer equipped with two different reactors designed to produce reactive species. The NH4 was produced by surface chemistry techniques, whereas H3O was produced by irradiating water vapor with ionizing electrons. A study of the ionization potential of NH4 by surface ionization techniques gave a value of 5.9 eV. The ionization potential of H3O was measured by conventional techniques and a value of 10.9 eV was obtained. In the theoretical investigation, the physical parameters are given including s-, p-, and d-type orbitals for H3O and s-, p-, and f-type orbitals for NH4. The calculations for NH4 predict a tetrahedral structure with internuclear distances of about 1.06 Å and they predict a planar H3O molecule with bond distances about 1.03 Å. The ionization potentials of H3O and NH4 were estimated to 3.9 and 3.0 eV.

A theoretical study of the use of internal standards for precise isotopic analysis by the surface ionization technique: Part I - General first-order algebraic solutions

Mass discrimination is a variable and unpredictable source of systematic error in mass spectrometric analysis using surface ionization techniques. A method of eliminating it through an internal standard technique is investigated. To simplify the algebra a method of describing mass discrimination has been developed (equations (11)) which is symmetrical with respect to all isotopes in a sample. A set of equations for mixtures of samples with different isotopic compositions is set up and solved for elements with three or four isotopes, four isotopes giving the simplest solutions. The solutions can be applied to quantitative analysis by the isotopic dilution technique, to the measurement of absolute isotopic abundances and to the measurement of relative abundances of samples of polyisotopic elements which have undergone isotopic fractionation. To make the most of these proposals the precision of measurement of ion current ratios from thermal ionization sources must be improved.