Changes In Secondary Emission Research Articles

Tracking hydroxyl adsorption on TiO 2 (1 1 0) through secondary emission changes

In surface science, rutile TiO 2 continues to be one of the most studied surfaces and in the catalysis field numerous groups study how adsorbates interact with this surface. All groups face the difficult problem of reproducibility due to surface preparation unknowns like defect concentration and the continuous aging of the crystals. Recent studies, using STM imaging, showed that hydroxyl adsorption takes place even in very good vacuum conditions. Upon adsorption, the surface electric field is reduced and the work function decreases. We found that this change may be readily detected in the onset energy of the secondary electrons. By following the onset region of secondary electron emission it is possible to track hydroxyl adsorption in quantities well below the detection level of XPS and LEIS. With this knowledge, we show that the time elapsed after surface preparation and water partial pressure should be accounted in the study of TiO 2 surfaces.

Electron beam induced secondary emission changes investigated by work function spectroscopy

The secondary emission coefficient, δ, of a given solid changes upon bombardment of the surface by an electron beam of high current density. Secondary emission and work function changes were measured simultaneously by work function spectroscopy during electron irradiation. The decrease of secondary emission and a successive increase of work function occurred on contaminated Si and W plates. The dependence of the work function- and the secondary emission change on primary energy was observed to be qualitatively the same. The change of δ as a function of primary electron energy has a maximum in the energy region where the mean free path of primaries and secondaries lies in the same range. From the results it can be assumed that the low energy secondaries have a great influence on Δδ.

Adsorption of potassium on Cu(100) and coadsorption with oxygen: A study by AES, work function and secondary emission changes

Evidence is found for a critical coverage of potassium, θ c, at which the mode of coadsorption of oxygen suddenly changes. This change occurs precisely at the coverage required for the work function minimum. In the high coverage region there are linear variations in work function; these point to the nucleation of a dense adsorbed potassium phase at θ c. The uptake of oxygen for high K coverages occurs effectively in two stages which can be related to the presence of adsorption sites which place negatively charged oxygen predominantly first below and then above the potassium layer.

Adsorption of lithium and oxygen on gallium arsenide

The adsorption of lithium on GaAs(100) and the coadsorption of lithium and oxygen were investigated by Auger electron spectroscopy, secondary emission changes and variations in the work function ϕ. Surface treatments produced an amorphous-like substrate structure which persisted throughout the study. The lithium layer grows by the Stranski-Krastanov mode and shows a work function minimum at about 0.75–0.85 monolayers. At low lithium coverages oxygen adsorbs preferentially in “on-top” sites and so increases ϕ; at higher coverages “underneath” sites appear to be favoured and this leads to decreases in the work function. Competition between the two types of site leads to unusual dynamic effects.

Oxidation of lithium monolayers on silver(111): A study by AES, work function and secondary emission changes

The uptake of oxygen by a silver(111) substrate pre-dosed with monolayer and submonolayer quantities of lithium was followed by Auger spectroscopy, work function (φ) variations and changes in secondary electron emission. Oxidation stabilizes the layers by preventing dissolution of lithium into the bulk. The maximum quantity of adsorbed oxygen is proportional to the original lithium coverage, θLi, up to the first lithium monolayer. The work function variations on oxygen adsorption exhibit remarkable changes as the coverage of pre-adsorbed lithium is increased. Initial decreases (positive-outward dipoles) are observed which indicate incorporation of oxygen in an underlayer. The magnitude of this decrease is proportional to θLi4. It is concluded that the underlayer oxygen atoms are “covered” by clusters of four lithium atoms. Long exposures lead to the formation of oxygen overlayers giving increases in φ (negative-outward dipoles). A new model and detailed calculations based on the dynamics of transitions from “on-top” to “underneath” sites, agree well with the observations. Secondary emission maxima are observed corresponding to the minima in φ. The growth mode of lithium is changed when deposition is made in a partial pressure of oxygen. Comparison is made with data for oxygen interactions with alkali metals on different substrates.

Oxidation of lithium monolayers on silver(111): A study by AES, work function and secondary emission changes

The uptake of oxygen by a silver(111) substrate pre-dosed with monolayer and submonolayer quantities of lithium was followed by Auger spectroscopy, work function (φ) variations and changes in secondary electron emission. Oxidation stabilizes the layers by preventing dissolution of lithium into the bulk. The maximum quantity of adsorbed oxygen is proportional to the original lithium coverage, θ Li, up to the first lithium monolayer. The work function variations on oxygen adsorption exhibit remarkable changes as the coverage of pre-adsorbed lithium is increased. Initial decreases (positive-outward dipoles) are observed which indicate incorporation of oxygen in an underlayer. The magnitude of this decrease is proportional to θ Li 4. It is concluded that the underlayer oxygen atoms are “covered” by clusters of four lithium atoms. Long exposures lead to the formation of oxygen overlayers giving increases in φ (negative-outward dipoles). A new model and detailed calculations based on the dynamics of transitions from “on-top” to “underneath” sites, agree well with the observations. Secondary emission maxima are observed corresponding to the minima in φ. The growth mode of lithium is changed when deposition is made in a partial pressure of oxygen. Comparison is made with data for oxygen interactions with alkali metals on different substrates.

Oxygen adsorption on polycrystalline titanium: Characterization of the first stages and Coadsorption with Lead

Clean polycrystalline titanium surfaces were exposed at near-ambient temperatures to oxygen at pressures near 10 −9 Torr and the oxidation process characterized by Auger electron spectroscopy, secondary emission changes, and by changes in the work function. A study was also made of the coadsorption of lead and oxygen. From the different behaviour of three different titanium Auger peaks during oxygen adsorption new information is obtained on the chemisorption of a subsurface monolayer of oxygen prior to the formation of thick oxide layers. Coadsorption with lead can produce an enhancement of the oxygen Auger signal which is ascribed to modifications in the location of the oxygen layer. The Auger spectra are also influenced by changes in electron transfer between the two metals and oxygen. Possible implications for the catalytic properties of metal particles on titanium oxides are pointed out.

Oxygen adsorption on polycrystalline titanium: Characterization of the first stages and coadsorption with lead

Clean polycrystalline titanium surfaces were exposed at near-ambient temperatures to oxygen at pressures near 10−9 Torr and the oxidation process characterized by Auger electron spectroscopy, secondary emission changes, and by changes in the work function. A study was also made of the coadsorption of lead and oxygen. From the different behaviour of three different titanium Auger peaks during oxygen adsorption new information is obtained on the chemisorption of a subsurface monolayer of oxygen prior to the formation of thick oxide layers. Coadsorption with lead can produce an enhancement of the oxygen Auger signal which is ascribed to modifications in the location of the oxygen layer. The Auger spectra are also influenced by changes in electron transfer between the two metals and oxygen. Possible implications for the catalytic properties of metal particles on titanium oxides are pointed out.

Secondary electron emission changes due to metal monolayer adsorption

Changes in secondary emission during the adsorption of lead on copper (111) and (100) surfaces, followed by monitoring the crystal current (SEECC technique), are compared with changes in the work function as well as with variations in the Auger signals. The work function variation for the (100) face exhibits a sudden change that may be due to a transition of the adsorbate from a 1D (atomic chain) type of structure to a 2D structure. The secondary emission changes are not explained entirely by the work function variations; there are also effects that are correlated with changes in adsorbate structure and which are probably due to diffraction at the surface.

Contactless electrical testing of large area specimens using electron beams

An e-beam test system has been developed to detect opens and shorts in three-dimensional networks of conductors embedded in a dielectric matrix. The specimens, which measure up to 100×100 mm, are tested electrically without making physical contact and without mechanical stepping. The system comprise two flood beams and a focused probe beam deflected over the full specimen area at a resolution of better than 3000 lines at 1–2 kV beam energy. One flood gun is located at the rear, the other at the front of the specimen. The change in secondary emission imposed by impinging electrons charging or discharging the surface is detected as voltage contrast which allows one to discriminate between uninterrupted and interrupted as well as shorted pairs of conductors. The physical concept, the contrast mechanism, and the electron optics of the system will be presented together with experimental results.

Feedback multiplier flat-panel television: III—Ion bombardment, dynode life, and ambient pressure

The gas-ion-feedback electron multiplier has been recognized for some time as capable of yielding a large-area electron source whose output can be spatially controlled on the scale of a picture element. Such a high-gain multiplier array can be conveniently formed using foils of oxidized Al-Mg alloy bonded to glass plates. During operation of the display the MgO dynodes of the electron multiplier are subject to intense ion and electron bombardment. This affects the device through three kinds of interaction. The ultimate life of a dynode is limited by sputtering. However, for the ion doses encountered during 104h of operation, the changes in secondary emission are not related primarily to sputtering damage but rather to the physisorption of the gas atoms on the MgO surface. This entrapment of gas also causes changes in the ambient pressure which must be compensated for. Finally, electron bombardment by itself can cause dissociation of MgO unless the MgO film has been stabilized by making the surface stoichiometric. Stabilized MgO dynodes can withstand a primary electron dose of 5000 C/cm2without damage and a dose of A+or He+ions of about 2 × 1017ions/cm2for an allowable decrease in secondary emission of 30 percent. An analysis of multiplier operation shows that a multiplier life of at least 104h can be expected in argon and helium; helium is the preferred choice because of the higher operating pressure and smaller sputtering damage to the electrodes. The sticking coefficient of 500-eV He ions on clean MgO at 23°C was measured to be 0.4 and it was found that at least six monolayers of He could be pumped into the dynodes. The characteristic time for gas release from the walls τ r is not constant but depends on the ion energy and on the degree of inward diffusion during the experiment. At 23°C, \tau_{r} > 100 h initially for 500-eV ions, and only 25 percent of the gas is recovered after three days. At 90° C, \tau_{r} \approx 1 h. These data were used to compute the pressure variations during various operating cycles. It is concluded that a gas supply is needed which is capable of delivering a total quantity of 20 1. torr of He on demand over hundreds of hours at 10-3torr, and which can reabsorb a comparable quantity of gas in about a minute. A pumping speed as small as 0.2 1/s would be sufficient to accomplish this.

A cathode-ray tube analogue-to-serial digital converter

Small fluctuations of the screen equilibrium potential of a cathode-ray tube cause proportionate small fluctuations in the steady secondary emission current arriving at a collector anode. The application of this fluctuation of secondary emission to an analogue-to-serial digital converter is discussed. An investigation of the parameters affecting changes of secondary emission using an electrode on the face of the tube, maintained at an alternating potential, to vary the screen potential is described. The secondary emission is observed as a signal, developed across a resistor between collector and earth, at screen electrode supply frequency, amplitude-modulated by the changes in secondary emission which occur when the screen is scanned by an electron beam.

Temperature Effects in Secondary Emission

Measurements have been made on the effects of temperature changes on the emission of secondary electrons from iron, nickel, cobalt, and molybdenum. Abrupt changes of one or two percent were observed to accompany the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\gamma}$ transition of iron, while the hexagonal to face-centered cubic transformation of cobalt was accompanied by a change in secondary emission of only about 0.4 percent. The magnetic transformation was found to alter the secondary emission coefficient of nickel by less than 0.3 percent. The temperature coefficient of secondary emission, in the case of nickel, cobalt, and molybdenum was found to be much less than the volume coefficient of expansion of the metal. The smallness of the temperature coefficient and the effect of the magnetic transformation are shown to lend support to the view that the secondary electrons are scattered or absorbed by an excitation process similar to that whereby they are originally produced.

Secondary-electron emission from nickel, cobalt and iron as a function of temperature

The evidence of other workers to the effect that the magnetic and structural changes which occur in ferromagnetic metals are accompanied by changes in the secondary-electron emission is critically discussed, and it is shown that this evidence is in many respects unsatisfactory. The authors have measured the secondary-emission coefficients for nickel, cobalt and iron at various primary voltages over a temperature range of about 400°C., and find in all cases either no change or a slight and gradual fall in the coefficient with rising temperature, the latter effect probably being due to surface gas. No evidence was obtained of any discontinuous changes in secondary emission at the transformation points, though a change of less than 5 per cent could readily have been detected.

Surface conditions and stability of characteristics in screened-grid tubes

The characteristics of screened-grid tubes are discussed and the influence of secondary emission is emphasized. The marked change in secondary emission with surface contamination causes instability of characteristics. Experiments with carefully evacuated ′24 tubes indicate that the evaporation of active material from the cathode considerably increases the secondary emission from the plate and that the liberation of “getter” in a completed tube limits the stability attainable.