Electron Reflection Coefficient Research Articles

Influence of surface parameters on anode losses in arcjets

Surface parameters influence on energy transfer to arc jet anode, discussing work function, accommodation coefficient and diffuse reflection coefficient of electrons

Determination of the specular reflection coefficient of conduction electrons in Zn whiskers atT=4.2�K

The influence was studied of the transverse dimensions of Zn single crystals in the form of whiskers and thin platelets on their specific resistance atT=4.2°K. The thickness of the specimens measured ranged from 10 µm to 0.2 µm. With a decreasing thickness of the specimens the specific resistance increased considerably. Under the assumption that for a pure diffuse reflection of electrons on the specimen boundary the product (ρ∞ ·d∞) in a bulk specimen is equal to 1.8×10−11 Ωcm2 for a crystallographic orientation corresponding to whiskers, and 2.2×10−11 Ωcm2 for a crystallographic orientation corresponding to platelets, a specular reflection coefficientp approximately equal top=0.6 was calculated for both whiskers and platelets.

An investigation into the instabilities of the hot cathode ionization gauge caused by operation in hydrogen

This paper describes a series of measurements made over a long period of time of the sensitivity of Bayard Alpert ionization gauges for both hydrogen and deuterium. Short term variations in sensitivity from 5 to more than 10% were found in gauges with cathode operating temperatures of the order of 2000°K and above. Corresponding instabilities for gauges with cathode operating temperatures below 1400°K were less than 2%. The large instabilities were shown to be related closely to the production of atomic hydrogen, and the consequent accelerated rate of adsorption, in the gauge envelope. Evidence is presented which suggests that it is hydrogen contamination of the electron collecting grid which is the most important factor affecting the sensitivity. It is further postulated that the changes occur because of the strong dependence of the reflection coefficient of electrons at the grid on hydrogen adsorption.

Work-Function Measurements on Monolayer Films of Uranium on (100), (110), and (113) Oriented Faces of Tungsten Single Crystals by Photoelectric and Contact Potential Difference Techniques

Uranium was evaporated on to (100), (110), and (113) oriented faces of tungsten single crystals at pressures of less than 5×10−10 Torr. The work function of each of the tungsten surfaces was monitored by photoelectric and contact potential difference (Kelvin and electron-beam) techniques as uranium was adsorbed on to it. The curves of work function versus coverage were fitted to the Gyftopoulos and Levine theory of monolayer adsorption. The constants of the curves gave a value for the density of uranium atoms in a monolayer on each tungsten surface: For the (100) surface one uranium atom is adsorbed for each unit cell in the two-dimensional lattice of surface tungsten atoms, on the (110) surface there is one uranium atom for every two unit cells, and on the (113) surface there are four uranium atoms to each unit cell. For film thicknesses of about two monolayers, the photoelectric technique gave work function values in excellent agreement with those from the Kelvin method, the values being 3.73±0.02 eV for the U/W100 system, 3.90±0.03 eV for U/W110, and 3.67±0.03 eV for U/W113. The electron-beam values were higher than those obtained by the other two methods, probably due to an increase in the electron reflection coefficient as uranium is adsorbed on to tungsten.

Erratum: Quantum-Mechanical Reflection of Electrons at Metal-Semiconductor Barriers: Electron Transport in Semiconductor-Metal-Semiconductor Structures

Numerical methods have been used to solve the Schrodinger equation for the quantum‐mechanical reflection coefficient (QMR) of electrons at metal‐semiconductor barriers. The calculations were carried out for a ``MacColl'' type barrier and a ``Thomas‐Fermi'' type barrier. It was found that for both barrier models the QMR≳50% for an electron incident on a barrier with energy less than 0.05 eV with respect to the barrier maximum. The QMR decreases slowly with increasing energy. On the basis of these calculations and calculations of the effects of electron‐phonon scattering in the collector and emitter semiconductors of semiconductor‐metal‐semiconductor structures, the predicted net current transfer ratio in such structure is less than 0.5 exclusive of base‐transport losses.

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.

Electron Reflection Coefficient at Zero Energy. I. Experiments

Abstract : The zero energy reflection coefficient of electrons at a freshly evaporated silver surface has been measured to be 7% + or 1%. The coefficient rises linearly with 39% e/V over more than 0.5 eV. Particular care is taken that only those electrons which have actually interacted with the surface are counted; those which have been reflected by the applied field without interaction with the surface are not counted. This is accomplished by (1) use of a total energy spectrometer and (2) simultaneous recording of the energy spectrum and target current. Therefore, the reflection coefficient determined is essentially that for orthogonal incidence. This report is intended as a critical discussion of the experimental method; the measured coefficient is thought to be caused by reflection in the patches, and agrees with the value to be reported in a subsequent paper. (Author)

Electron Reflection Coefficient at Zero Energy. II. Computer Experiments on the Reflection of Slow Electrons in the Electrostatic Field of Surface Patches

As reported in paper I, the zero-energy reflection coefficient of electrons at a polycrystalline silver surface has been measured to be (7\ifmmode\pm\else\textpm\fi{}1)%. A theory consistent with this experimental result is presented below. This theory shows that the zero-energy reflection coefficient may be fully explained by the effect of patches of different work functions. The different potentials presented at the surface of a metal by the differently oriented crystal surfaces converge exponentially to the average potential as one moves away from the surface. The potential variation with the largest spatial period extends farthest from the surface. It will be shown that this component alone determines the reflection behavior at the threshold of energy. Because the surface presents a two-dimensional distribution, two independent periods must be considered which range in their aspect ratio from a square checkerboard to long stripes. The reflection coefficient is found to be independent of patch size and patch potential amplitude, and is (7.73\ifmmode\pm\else\textpm\fi{}0.03)% for the stripe-type and (5.4\ifmmode\pm\else\textpm\fi{}0.2)% for the square-checkerboard-type patches. The reflection coefficient jumps abruptly from 100% below the threshold of energy to some 6% at an energy immediately above threshold; therefore, it is not surprising that recent energy analyses with a simple patchy counterfield electrode of several-hundred-millivolt patch amplitude can resolve fine structure in the energy of less than 10 meV, as exhibited in the energy spectrum of electrons field-emitted from a liquid-helium-cooled tip.

The Effects of the Electrodes on an Electron Avalanche†

ABSTRACT The influence of the electrodes of a uniform field discharge gap on an electron avalanche ore discussed in. this paper. The presence of both electrodes reduces the number of electron/atom collisions in their vicinity thereby increasing the mean electron energy above the equilibrium valuo (∊). There is an associated decrease in the transit time and an increase in the amplification of the electrons across the gap. The spatial variation of the drift velocity, diffusion coefficient and amplification coefficient have been calculated. It has been shown that for a tube voltage greater than lOe that the electrons are no longer influenced by the electrodes when they reach the centre of the gap, and the energy and associated parameters attain equilibrium conditions. The cathode boundary condition has been analysed in detail, and has yielded an integral expression for the loss of electron current by ‘back diffusion’ which is more general than either the Thomson (1028) or Varney (1962) equation. The electron current source is shown to consist of both pole and dipole sources, which whilst escaping from the cathode surface undergoes a change in size, mean energy and energy distribution governed by the electron reflection coefficient from the cathode surface and the initial electron energy distribution.

Quantum-Mechanical Reflection of Electrons at Metal-Semiconductor Barriers: Electron Transport in Semiconductor-Metal-Semiconductor Structures

Numerical methods have been used to solve the Schrödinger equation for the quantum-mechanical reflection coefficient (QMR) of electrons at metal-semiconductor barriers. The calculations were carried out for a ``MacColl'' type barrier and a ``Thomas-Fermi'' type barrier. It was found that for both barrier models the QMR≳50% for an electron incident on a barrier with energy less than 0.05 eV with respect to the barrier maximum. The QMR decreases slowly with increasing energy. On the basis of these calculations and calculations of the effects of electron-phonon scattering in the collector and emitter semiconductors of semiconductor-metal-semiconductor structures, the predicted net current transfer ratio in such structure is less than 0.5 exclusive of base-transport losses.

Reflection and Diffraction of Slow Electrons from Single Crystals of Tungsten

Measurements have been made of the elastic reflection coefficient and total secondary electron coefficient for electrons with energies from 1-206 eV incident normally on single crystals of tungsten (112), (100), and (110). The apparatus permitted visual observation of the full diffraction pattern in the same energy range. Ultra-high vacuum techniques were used. The major features of the elastic reflection coefficient for incident energies below 20 eV were found experimentally to be associated with the specularly reflected or 0-0 diffraction beam. A model based on superposition of atomic scattering was found to give a better over-all description of the observations than a model based on a potential varying only along the normal to the crystal.

Schottky Effect for SrO Films on Molybdenum

The Schottky effect and the periodic deviations about the Schottky line have been simultaneously investigated for thin films of SrO on molybdenum. The experimentally determined Schottky slope is in good agreement with the theoretical value of ${e}^{\frac{3}{2}}\mathrm{kT}$ for deposits less than a monolayer; but becomes larger than theory for more than a monolayer. The periodic deviations obtained for a clean molybdenum surface wash out when as little as $\frac{1}{60}$ monolayer of SrO is deposited on the surface. The implication is that the electron reflection coefficient near a Mo-SrO surface cannot be associated with the simple $\ensuremath{\mu}$ reflection found for clean metals.

Potential Drop and Ionization at Mercury Arc Cathode

By means of a movable Langmuir collector, the potential, ion concentration and electron temperature were measured at various distances from a stationary mercury cathode spot, at various arc currents. The results indicated a cathode drop of 10.0 volts, and a small negative potential gradient beyond the fall space which was more pronounced at the larger currents. The ion concentration varied between 2${(10)}^{13}$ and 3${(10)}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ for distances between 0.4 cm and 1.7 cm and arc currents between 11 amp and 4.2 amp. The concentrations evidently greatly exceed these values very close to the cathode. The mean electron energies were about 1.4 volts near the cathode and fell to a little less than a volt at the greater distances. From the lack of saturation of currents to the collector at space potential, the coefficient of electron reflection at the amalgamated tungsten collector surface was found to be close to 0.5. It is shown that the thickness of the cathode fall space must be less than 1.76${(10)}^{\ensuremath{-}4}$ cm and that the field at the cathode surface must exceed 7.6${(10)}^{4}$ volts\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{cm}}^{\ensuremath{-}1}$.

Distinction between Contact-Potential Effects and True Reflection Coefficients for Low-Velocity Electrons

Distinction between Contact-Potential Effects and True Reflection Coefficients for Low-Velocity Electrons