Positive Ion Emission Research Articles

Secondary Positive Ion Emission from a Tantalum Surface

The secondary positive ions ejected from a tantalum target by a mass-analyzed primary beam were detected and studied in a mass spectrometer. The secondary ion emission consisted of species characteristic of the metal (Ta+), metal compounds (oxides), and multiatomic ions of the metal (Ta2+, Ta3+). Reflection of normally incident gas ions from the Ta surface was not observed; the specular reflection coefficient of ions to ions is then less than 10−6. The relative secondary ion yields for tantalum, measured for normally incident Hg202 ions at energies between 0.3–14 keV, were independent of the incident ion charge. A dependence of the Ta+ secondary yield on mass of the incident ion was observed. The dependence of the secondary ion yield on mass and energy of the incident ions was very similar to that observed for total sputtering yields.

Mass Spectrometric Study of Positive-Ion Emission from Iron Oxide Catalysts Containing Alkali Metals

Samples of iron oxide catalysts containing small amounts of added alkali-metal oxides were mounted in the ionization chamber of a mass spectrometer equipped with a grid. The evaporation products obtained under various conditions were investigated. Alkali atom emission, the intensity of which decayed rapidly, and alkali ion emission were measured. The ratio of potassium ion emission to sodium ion emission from samples containing the two alkalis decreased markedly with heating time. The magnitude of the experimentally derived apparent work functions for evaporation of the alkali ions from the iron oxide catalysts agreed with those computed for the evaporation of the same ions from pure alkali metals.

Sources of positive ions for space charge neutralization at low temperatures

In order to operate a thermionic power converter from a heat source which utilizes fossil fuels burned in air, it is desired to have a converter which operates at a temperature of 1200°C or lower. At these low temperatures, it is difficult to obtain an adequate source of positive ions for space charge neutralization. Surface ionization of cesium, which is the most common source of positive ions in a thermionic converter, is not effective at these low temperatures. Two methods for providing positive ions have been investigated experimentally: 1. (1) The thermionic emission of positive ions from solid compounds or mixtures of compounds and 2. (2) The application of repeated short pulses of electric power to the converter to ionize the cesium vapor in the interelectrode space. Thermionic emission of positive ions has been studied by many investigators. One of the best ion emitters reported in the literature is beta eucryptite (Li 2O, Al 2O 3, 2SiO 2) which will emit significant current densities of lithium ions in the region from 1000°C to 1200°C. We have studied ion emission from the eucryptite analog of other alkali metals and from mixtures containing alkali metal compounds. Ion emission current densities of several milliamperes per cm 2 have been obtained, but the lifetime of the emitter has often been less than anticipated. Space charge neutralization electrons has been demonstrated in a preliminary experiment using a three element structure with the ion emitter located between the electron emitter and collector. Pulsed operation of a thermionic diode has been studied using a modified commercial diode to which cesium vapor was introduced at controlled pressures. During the application of a 10 to 20 V pulse, a discharge occurs in the cesium and ions are formed. After the pulse is terminated, the ions formed neutralize the space charge and allow a high electron current to flow. The experimental tube has a nickel matrix emitter and a cesium coated copper collector so that the relative work functions of the emitter and collector do not allow useful power output densities to be obtained. However, an external bias voltage was used to adjust the interelectrode potential difference to simulate a thermionic converter. The diode current after the pulse was terminated was typically about 300 mA compared to a steady state current before the pulse of about 0·3 mA. Preliminary results indicate a charge transfer between emitter and collector between pulses which is 30 times the charge transfer during the pulse. Further work to optimize this ratio and to provide the pulse feedback of a portion of the diode output power is in progress.

Heat- and radiation-tolerant vacuum tubes

In order to operate a thermionic power converter from a heat source which utilizes fossil fuels burned in air, it is desired to have a converter which operates at a temperature of 1200°C or lower. At these low temperatures, it is difficult to obtain an adequate source of positive ions for space charge neutralization. Surface ionization of cesium, which is the most common source of positive ions in a thermionic converter, is not effective at these low temperatures. Two methods for providing positive ions have been investigated experimentally: 1.(1) The thermionic emission of positive ions from solid compounds or mixtures of compounds and2.(2) The application of repeated short pulses of electric power to the converter to ionize the cesium vapor in the interelectrode space.Thermionic emission of positive ions has been studied by many investigators. One of the best ion emitters reported in the literature is beta eucryptite (Li2O, Al2O3, 2SiO2) which will emit significant current densities of lithium ions in the region from 1000°C to 1200°C. We have studied ion emission from the eucryptite analog of other alkali metals and from mixtures containing alkali metal compounds. Ion emission current densities of several milliamperes per cm2 have been obtained, but the lifetime of the emitter has often been less than anticipated. Space charge neutralization electrons has been demonstrated in a preliminary experiment using a three element structure with the ion emitter located between the electron emitter and collector.Pulsed operation of a thermionic diode has been studied using a modified commercial diode to which cesium vapor was introduced at controlled pressures. During the application of a 10 to 20 V pulse, a discharge occurs in the cesium and ions are formed. After the pulse is terminated, the ions formed neutralize the space charge and allow a high electron current to flow. The experimental tube has a nickel matrix emitter and a cesium coated copper collector so that the relative work functions of the emitter and collector do not allow useful power output densities to be obtained. However, an external bias voltage was used to adjust the interelectrode potential difference to simulate a thermionic converter. The diode current after the pulse was terminated was typically about 300 mA compared to a steady state current before the pulse of about 0·3 mA. Preliminary results indicate a charge transfer between emitter and collector between pulses which is 30 times the charge transfer during the pulse. Further work to optimize this ratio and to provide the pulse feedback of a portion of the diode output power is in progress.

Hot-Cathode Magnetron Ionization Gauge for the Measurement of Ultrahigh Vacua

In order to extend the low-pressure limit of the conventional hot-cathode ionization gauge it is necessary, at a given emission current, to increase the ratio of the ion current to the x-ray photo-current. This ratio may be increased by increasing the sensitivity of the ionization gauge. The gauge may be modified to permit the electrons to travel in long paths before they are collected by the positive grid or anode. Under these conditions the probability of the electrons' colliding with an ionizing a gas molecule will be enhanced, and the sensitivity of the gauge will be improved with no increase in x-ray photoemission. To accomplish this a magnetron with a cylindrical anode was selected for the ionization gauge and operated in a magnetic field with an intensity of 2.5 times the cutoff value. Two end plates maintained at a negative potential relative to the cathode collect the ion current generated in the magnetron and prevent the escape of electrons. Positive ion emission from the cathode is suppressed by mounting a hairpin filament on the axis of the cylindrical anode and well removed from the region of the negative ion collector. A very low level of electron emission is used to prevent instabilities in operation and to give a maximum ratio of ion current to x-ray photocurrent. Mearurements of sensitivity and x-ray photocurrent indicate that the magnetron gauge is linear down to a pressure of 4×10−14 mm Hg. Ability to measure low pressures with the gauge appears to be limited by the sensitivity of the circuit measuring the low ion currents. The ion pumping speed was found to be 0.003 liter/sec under normal operating conditions. Magnetron gauges have been built with ceramic metal envelopes.

Secondary Positive Ion-Emission from Platinum

Mass analysis of the secondary particles ejected from Pt surfaces by inert gas ion bombardment or by heating revealed species characteristic of the base metal, of certain bulk and surface impurities, and of the bombarding beam (so-called ``reflected'' ions). The Pt+ ions seemed to come directly from the substrate rather than from surface compounds. Their yield increased with temperature, bombarding ion energy, and bombarding ion mass. The ratio of sputtered Pt ions to sputtered neutrals is estimated to be of the order of 1 to 1000 at room temperature. The kinetics of the formation and desorption of a certain surface compound [possibly Pt(CO)2] was studied by secondary ion analysis. The compound formed spontaneously on the surface from the background gas in the instrument (CO at 10—8 mm Hg). Its formation rate was greatly enhanced by inert gas ion bombardment. It desorbed readily at 1000°C with an activation energy of 0.74±0.05 ev. The ``reflected'' ions appeared to be sputtered rather than reflected. Their energy was always extremely low (<1 ev) and the dependence of their yield on ambient gas pressure and target temperature is consistent with the notion that they originated from inert gas atoms trapped in the surface layers.

12. Secondary positive ion emission from metal structures: Richard C. Bradley, J. Appl. Phys., 30, 1–8, Jan. 1959

12. Secondary positive ion emission from metal structures: Richard C. Bradley, J. Appl. Phys., 30, 1–8, Jan. 1959

Secondary positive ion emission from metal surfaces

Secondary positive ion emission from metal surfaces

Secondary positive ion emission from metal surfaces.

Secondary positive ion emission from metal surfaces.

Thermal Emission of Alkali Ion Pulses from Clean and Oxygenated Tungsten

The spontaneous emission of positive ions from a heated tungsten filament occurs as pulses, a single pulse releasing as many as 106 ions in less than 100 μsec. The pulses are composed mainly of potassium ions, present as impurities in the tungsten lattice. It is suggested that the emission of positive ions in the form of bursts is dependent upon the existence of edge dislocations in the metal. At temperatures where the formation of an adsorbed layer is possible, oxygen has a very pronounced effect upon the rate of emission of positive ion pulses.

Secondary Positive Ion Emission from Metal Surfaces

Secondary positive ions ejected from surfaces of Mo, Ta, and Pt under bombardment by inert gas ions of low energy (<1000 ev) have been studied in high vacuum (≤10−8 mm of Hg of background gases) using a 60° sector field mass spectrometer with a 6-in. radius of curvature, the object being to explore this as a technique for investigating solid surfaces in general. Ion species characteristic of the base metal, of surface compounds, and of a few bulk impurities were found. Quantitative analyses based on abundance ratios of the ions are unreliable, however owing to the enormous and essentially unknown differences in ionization efficiencies for the various species. Application of the technique to the investigation of surface kinetics seems highly promising, and as an illustration certain observations on the formation of PtO3 on a Pt surface and its desorption at high temperatures are presented. Another possible application is to the study of sputtering, although some of the properties of secondary ions are shown to be quite different from those of sputtered particles in general. The secondary ions characteristic of the base metal comprise only a small fraction (0.01% for Pt) of the total sputtering yield, are principally singly charged and monatomic, have a low average energy (4–5 ev) and energy spread, and may result either from physical sputtering of the base metal or from the dissociation of surface compounds or both.

Positive Ion Emission from Metal Surfaces caused by Ion Bombardment

The phenomenon reported in this paper was first observed during an lnvestigation into the adsorption of gas on to metal surfaces. When tungsten targets were bombarded with positive ions of the noble gases, and subsequently degassed by heating, a positive ion emission was often observed in addition to a desorption of gas from the target. These ions have been analyzed with a mass spectrometer and shown to be sodium, potassium. rubidium, and cesium. This is reasonable, as these elements form significant impurities in tungsten and may be because of their low ionization petential, be expected to desorb mainly as ions. In view of the results it is certain that these ions also originate from impurities already present in the target before the bombandmeni took place, rather than directly from the adsorbed ions. (auth)

Chemical Studies on Ion Emission from Solids. IV. Positive-IonE mission from NaCl, KCl and Their Mixtures Mounted on Alumina-Coated Tungsten Filament.

一定量のアルカリハライド試料を塗付した長さ6cmの被覆線の中央2cmの部分からの放出陽イオン電流は時間とともに指数関数的に減ずる (実験条件: 試料溶液は単独塩, 混合塩いずれの場合もすべて同一mol濃度とし, 担体の加熱温度Tは640°～830℃,加速電圧γは8～115V)。試料塩化ナトリウム, 塩化カリウムにつき, 試料溶液1mgあたりの放出陽イオン総数NはT, Vと共に増大する。70V, 750℃におけるNの序列: KBr>KCl>NaCl>LiCl。NaCl-KCl, NaCl-LiCl, NaCl-KBr, KCl-KBr, KCi-CdCl2, NaCl-CdCl2等の2成分混合系につき, 組成のmol分率対Nの関係は加成的である。塩化カドミウム単独からの陽イオン放出はない。既述第3報の理論を考察基準にすればT, γ等の影響はかなりよく理解でき, またNの序列に対してもその理論的根きょが与えられる。

Chemical Studies on Ion Emission from Solids. III. Mechanism of Positive-Ion Emission from Heated Solids.

Chemical Studies on Ion Emission from Solids. III. Mechanism of Positive-Ion Emission from Heated Solids.

Measurement of Concentrations of Gaseous Halide Tracers in Air by Positive Ion Emission Techniques

Dichlorodifluoromethame, Freon 12, is a promising tracer for atmospheric diffusion studies. It is am odorless, colorless gas, noncorrosive and nontoxic in the concentrations thnt are required. This gas cam be measured in air in concentrations as low as 0.1 ppm by special techniques involving the use of a modified version of a halogen leak detector available commercially. The feasibility of this approach has been demonstrated by field tests illustrated herein and described more fully elsewhere. Current studies on the mechanism of operation are briefly outlined. (auth)

Field Ionization of Gases at a Metal Surface and the Resolution of the Field Ion Microscope

The mechanism of field emission of positive ions is considered as depending upon the supply of molecules and their ionization probability in a field up to 500 million volts/cm. Experimental current-field characteristics match the theoretical curves. The velocity distribution, measured in a retarding potential tube, shows that the ions originate some 5 to 100 A above the surface, depending upon the field. A simple mass spectrometer is described which uses the field emitter as a point ion source. The resolution of the field ion microscope is found to depend upon tip radius, polarizability and ionization potential of the gas, and possibly upon the accommodation coefficient and the temperature. Helium seems to give the best resolution of about 4 A. The possibility of operating a field desorption microscope with sufficient resolution to show the lattice structure of the surface is discussed.

On the Positive Ion Emission from Hot Platinum in Various Gases

On the Positive Ion Emission from Hot Platinum in Various Gases

電離真空計陰極の陽イオン放射について (較正上の一注意)

現在電離真空計のうちで最もよく使用されているものにFogel電離真空計がある。これは衝立型プレートと酸化物被覆ヘヤピン型陰極でできており, 構造が簡単で作り易い上に感度や直線性もよく, 規格にあわせて, 作れば感度も一定したものが得られ易く, 個々のものについて, いちいち較正をしなくてもすむので好んで用いられている。我国では酸化物陰極よりも純タングステン線を熱陰極として使用している所が多いようである。筆者は電離真空計の較正のときに。とくにFogel型ではタングステン陰極からの陽イオン放射が較正時における誤差の一原因となることを認めたので, その値, および対策につき報告する。

Effect of Vapours of Halogen Compounds on the Emission of Positive Ions from Platinum

It was observed by C. W. Rice in 1944 that the emission of positive ions from a platinum anode heated in air is increased very markedly when vapours of compounds containing halogens such as carbon tetrachloride, chloroform, ‘Freon’, sulphur hexafluoride etc., strike the anode surface1. We have studied this phenomenon, the Rice effect, and obtained the following results.

Thermionic emission of positive ions from alumina coated tungsten

Thermionic emission of positive ions from alumina coated tungsten