Barium Films Research Articles

Photoemission from thin barium films: V N Lyasnikov and A N Arseneva-Geyl, Uch Zap LGU, No 354, 1970, 30–33 ( in Russian)

Photoemission from thin barium films: V N Lyasnikov and A N Arseneva-Geyl, Uch Zap LGU, No 354, 1970, 30–33 ( in Russian)

538. Photoelectron energy distribution of barium films with different thickness

538. Photoelectron energy distribution of barium films with different thickness

Reflectance Studies of Ba, Sr, Eu, and Yb

In situ reflectance measurements have been made on films of the alkaline-earth metals barium (bcc) and strontium (fcc), and rare-earth metals europium (bcc) and ytterbium (fcc) from 1.0 to 11.6 eV. Extreme care was taken in all aspects of the work so that a Kramers-Kr\"onig analysis of the reflectance data yielded optical constants ${\ensuremath{\epsilon}}_{1}$, ${\ensuremath{\epsilon}}_{2}$, $\ensuremath{\sigma}$, and $\ensuremath{\alpha}$ which were accurate to better than \ifmmode\pm\else\textpm\fi{} 20%. The volume-loss functions calculated from these optical constants yielded an agreement with loss-function peak positions obtained in Kunz's high-energy electron-loss experiments which was not seen in previous optical studies, and served as verification of the improved accuracy of the present study. A comparison of the interband optical conductivities for the alkaline-earth and rare-earth metals failed to indicate strong structure or increased oscillator strength in the rare-earth metals which could be unambiguously attributed to transitions from $4f$ electrons found in europium and ytterbium. The filled $4f$-electron states in these two metals are known to be accessible to the spectral range of the present study, and an explanation, based on matrix elements, is proposed for this inability to excite such electrons. The structure which was observed in the interband conductivity of all four of these metals was correlated to crystal structure and showed much better agreement with the predictions of strong $d$-electron effects implied in recent band calculations than it did with the simple predictions of the nearly-free-electron theory.

Displacement Phenomena in the Barium-Nitrogen-Oxygen System

The displacement or replacement of adsorbed nitrogen from the surface of an evaporated barium film has been demonstrated by a series of flow experiments using a glass uhv system. The method consists of passing small controlled oxygen fluxes over a film that has previously sorbed nitrogen. The oxygen displaces nitrogen which is continually pumped away to be measured. The results show that for nitrogen sorptions by a barium film of less than 0.03 cm3 Torr/cm2 of actual film area, only a small fraction of the oxygen pumped (<3%) displaces nitrogen. This fraction increases as the film continues to take up nitrogen and decreases slowly with time once the film is isolated. The displacement reaction is very rapid and probably involves an exchange between oxygen and adsorbed molecular nitrogen.

Residual gases in electron tubes after storage

The composition of residual gases in electron tubes with impregnated cathodes was measured with the aid of an omegatron residual gas analyzer during operation after preparation and after storage. When stored electron tubes are switched on a considerable pressure increase occurs. It was found that hydrogen is the main component of the residual gases under these conditions. The partial pressure increase of hydrogen after switching on is about 14 times faster after storage than after preparation. It is supposed that hydrogen originates from an evaporated barium film from the cathode deposited on the inner surface of the anode. The barium film on the anode getters residual gases during storage and when the electron tube is switched on, hydrogen is released due to higher temperature and electron bombardment. The hydrogen was released at a rate of 1 × 10 −7 litre torr/sec. This value is compared with known parameters of the barium film.

The evolution of methane from barium films

The evolution of methane from barium films

975. The evolution of methane from barium films: J J Moscony,Nuovo Cimento Suppl, 5 (1), 1967, 7–17

975. The evolution of methane from barium films: J J Moscony,Nuovo Cimento Suppl, 5 (1), 1967, 7–17

Direct Comparison of the Kelvin and the Electron-Beam Methods of Contact-Potential-Difference Measurement

Both the Kelvin and the retarding potential, or electron beam, techniques have been used to measure the contact potential differences between (110) and (100) oriented tungsten single-crystal surfaces clean and covered with vapor deposited barium films. The evaporation of barium in an ultra-high vacuum is difficult and it was necessary to double evaporate the metal before the final evaporation could be made without a pressure increase inside the experimental tube above, at best, 3×10−10 Torr. Contrary to the result obtained from a previous similar experiment, the contact-potential-differences measured by the two methods differed by 130 mV for the (100) crystal face and by 150 mV for the (110) crystal face. Differences of this magnitude can be explained either in terms of a two-patch model, provided there is a work function difference of up to 1.0 eV between the patches, or by a change in the reflection coefficient for electrons. To account for the difference in terms of changes in the reflection coefficient only, a change of from zero for the clean crystal to about 0.6 for the barium-covered surface would be required.

Optical Constants of Evaporated Barium in the Vacuum Ultraviolet*

The optical properties of evaporated barium films have been investigated in the wavelength range from 1500 to 3000 Å. Reflectance measurements were made at angles of incidence of 17.5° and 72.5° and the complex index of refraction obtained from graphical solutions of the Fresnel reflection equations. The films were prepared in an ultrahigh-vacuum reflectometer having a base pressure of about 5 × 10−10 torr. Radiation from a hydrogen glow-discharge source was dispersed by a normal-incidence vacuum monochromator which was optically connected to the ultrahigh-vacuum system by means of a sapphire window. The energy-loss function Im(1/∊), as computed from the optical constants, is compared with published characteristic electron-loss (CEL) data. The complex index of refraction was fitted to a Drude model for which the plasma energy was fixed at 7.4 eV and the damping at 1.6 × 10−16 sec.

Gettering properties of barium films in television tubes

The method of measuring the sorption rates and capacities for barium getters in television tubes has been further refined to cover more adequately the very high sorption rates encountered. With this improved method it has been possible for the first time to investigate the true relationships existing between barium film distribution, as influenced by getter shape, and gettering properties in 19 in.-114° television tubes, both for endo- and exothermic alloys.

Barium and oxygen: incorporation characteristics at very low pressures

Anderson and Hunt have demonstrated that oxygen may be incorporated in barium films, leaving the contact potential difference with respect to some reference surface unchanged. A kinetic theory of the balance between adsorption and incorporation of gas is suggested. Thus it is deduced that there is a critical pressure below which there will be an increase in the amount of oxygen incorporated during the formation of a very thin protective layer of oxide. Such critical pressures have been found in experiments, for two different temperatures. Hence the heat of binding and the activation energy of mobility of oxygen on the surface have been estimated. Also, the sticking factor for oxygen on barium increases with pressure below 10-7 torr. These results have a practical importance: at the very low pressures obtained in sealed-off devices, with the aid of barium getters, the capacity for incorporating oxygen is greater than expected from experiments done at higher pressures (>10-7 torr).

Crystal Phases of Nickel as Related to Adhesion of Thermionic Coatings and to Emission Phenomena

For conventional oxide cathodes, the possibility of forming microseparations between the nickel base and the coating during the activation phase is demonstrated in a largescreen thermionic emission microscope. When using work-hardened nickel, shearing forces necessarily occurring during recrystallization owing to crystal growth and grain-boundary expansion must lead to disruption of the base-to-coating contact. Severe limitations to emission current by increased resistance, next to interface resistance, are imminent. A preventive measure of pre-heating all cathode sleeves at 1200 deg C is recommended. Artificial activation of the polycrystalline nickel base by a thin- film barium layer leads to the qualitative observation of anomalous emission phenomena as caused by the anisotropy of different crystal planes. Emission reversal with decreasing temperature, crystal memory shapes, preferential etchability of higher work function planes, and recovery after apparent barium exhaustion and after hot oxygen poisoning are among the observations. Theoretical efforts for reconciling apparent contradictions seem essential since thin-film activation appears to be the ultimate mechanism of electron emission in any thermionic cathode structure. (auth)

371. Hydrogen sorption by barium films in the pressure range 10 −3 to 10 −4 torr : G. Rauscher and G. Standt, Vakuum-Technik, 11 (1), Feb. 1962, 17–20

371. Hydrogen sorption by barium films in the pressure range 10 −3 to 10 −4 torr : G. Rauscher and G. Standt, Vakuum-Technik, 11 (1), Feb. 1962, 17–20

Retarding Field Technique for Measuring Sublimation from Dispenser and Oxide Cathodes

A retarding field technique for measuring the instantaneous evaporation rate of barium and barium oxide from dispenser and oxide cathodes is described. The method is based upon measuring the time rate of change of the anode work function caused by the deposition of sublimate from the cathode surface. A simple planar diode, employing a molybdenum anode, is used. In contrast to the typical curve for barium films on molybdenum, showing a minimum work function at approximately a monolayer coverage, the results for barium oxide films indicate a similar decrease in work function up to a monolayer coverage; further deposition leads to a still lower work function.

The sorption of hydrogen by barium getters

Sorption of hydrogen by barium films is shown to be made up of surface adsorption of the gas followed by its diffusion to the interior. The velovity of surface adsorption is studied at low pressures and its dependence on surface area, pressure and temperature is considered. The velocity of diffusion is examined for pressures and film thicknesses which are near to those in current industrial use. In this case too, particular attention is paid to the influence of temperature, as well as to the influence of parameters such as temperature of condensing walls, residual pressure at evaporation and time of flashing of the getter, all of which are of considerable practical interest.

248. The absorption of gases by barium getter films applied as a tool: J. J. B. Fransen and H. J. R. Perdijk, Vacuum, 10, 199–203, 1960

The present work describes the apparatus and techniques used at the S.A.E.S. Research Laboratories to study the gettering action of evaporated barium films on different gases. During the past few years the sorption of gases by getters (evaporated or not) has been increasingly studied from a kinetic approach, by techniques capable of giving a direct measurement of the velocity of sorption. Kinetic measurements can be effected either by measuring the fall of pressure with time in a sealed container of known volume, or by estimating the gas flow through a capillary of known conductance. The latter method can be applied by keeping the manifold pressure constant and allowing the getter pressure to vary with time, but more useful results can be obtained when the pressure on the getter is kept constant. Measurements at constant pm are by us usually confined to preliminary large scale investigations. For such measurements suitable apparatus and automatic pressure recordings are used, which enable us to carry out more than one test at the same time. For sorption studies at very low pressures, the normal apparatus is attached to an ultra high vacuum system. The experimental results obtained by measuring the gas flow at constant pg, when suitably interpreted, allow us to study the complex sorption mechanism, showing the existence of diffusion phenomena, giving useful indications about the mobility of molecules on the surface and within the film and permitting the evaluation of the activation energy of different processes. Suitable apparatus enables us to study the influence of hot filaments and ionizing electron currents in the vicinity of the getter. A knowledge of these effects is in fact necessary for a correct evaluation of the data obtained for sorption velocities by using ionization gauges and permits these effects to be kept at a minimum in the experimental procedure. Further suitable apparatus is employed to study other parameters which may effect the activity of barium getters, such as the time of flashing, the pressure immediately preceding the firing of the getter, the distance and temperature of the surface on which the film is formed. In particular such devices permit the study, by electron microscopy, of the structure of the barium films obtained under various conditions. Finally, the recent introduction of mass spectrometers of reduced dimensions and able to measure very low partial pressures (e.g. the omegatron), enable us to confirm the eventual chemical processes occurring on the barium film (catalytic reactions) and also it opens a way to the study of getter action on gaseous mixtures.

251. Nitrogen sorption by barium films: P. della Porta and E. Argano, Vacuum, 10, 223–226, 1960

251. Nitrogen sorption by barium films: P. della Porta and E. Argano, Vacuum, 10, 223–226, 1960

250. Carbon monoxide sorption by barium films: F. Ricca and P. della Porta, Vacuum, 10, 215–222, 1960

250. Carbon monoxide sorption by barium films: F. Ricca and P. della Porta, Vacuum, 10, 215–222, 1960

252. Hydrogen sorption by barium films: P. della Porta and S. Origlio, Vacuum, 10, 227–230, 1960

252. Hydrogen sorption by barium films: P. della Porta and S. Origlio, Vacuum, 10, 227–230, 1960

249. The adsorption of methane by barium films in the presence of a thermionic current: M. G. Charlton and F. H. Southam, Vacuum, 10, 212–214, 1960

249. The adsorption of methane by barium films in the presence of a thermionic current: M. G. Charlton and F. H. Southam, Vacuum, 10, 212–214, 1960