Surface analysis of thermionic dispenser cathodes

In this thesis a combination of low-energy ion scattering (LEIS) and Auger electron spectroscopy (AES) was used to study the surfaces of thermionic dispenser cathodes, which are used as electron sources in cathode-ray tubes. The objective of the surface analysis was to investigate the relation between the surface properties and the electron emission capabilities of various types of dispenser cathodes. Furthermore, the dynamic behaviour of the dispenser cathodes during bombardment by noble gas ions was studied, thereby simulating the bombardment by ionised residual gases occurring in real cathode-ray tubes. To be able to perform the ion scattering analysis of the cathode surfaces with work functions of the order of 2 eV, a systematic investigation of the neutralisation mechanisms of noble gas ions at low-work function surfaces was carried out prior to the analysis of the cathodes. These investigations were required because reports about LEIS studies of low-work function surfaces are sporadic, and the influence of the work function on the neutralisation probability of noble gas ions at low work functions was not yet completely understood. Here, the neutralisation studies were performed using cathode model systems consisting of submonolayers of Ba adsorbed on W and Re single crystals. The crystals used as substrates for these model systems were especially prepared by the group of prof.dr. Vadim Glebovsky (Chernogolovka, Russia) using the electron-beam floating zone melting (EFBZM) technique, which resulted in crystals with a near-perfect crystallographic structure. Using these cathode model systems, the neutralisation probabilities of noble gas ions during the interaction with the surface were determined over a work function range from 6 eV to just below 2 eV. It was found that at high work functions (above approximately 3 eV) the neutralisation probability of the noble gas ions is independent of the work function, but does depend on the ion-target combination. For noble gas ions scattered from alkali and earth-alkali atoms, like Ba, the collision-induced neutralisation mechanism is the dominant mechanism at high work functions. The results of the investigations also show that Auger neutralisation is much less dominant as is commonly assumed. At work functions below approximately 3 eV the noble gas ions are additionally neutralised by resonant electron transfer from the levels near the Fermi level of the metal substrate to the first excited level of the ions. This resonant mechanism is not available at high work functions because the first excited level is situated above the Fermi level at the distance where the neutralisation takes place. The neutralisation probability due to the resonant neutralisation is governed by the macroscopic work function and increases exponentially with decreasing work function. In addition to the investigation of the neutralisation mechanisms, the consequences of the resonant neutralisation for the quantification of the composition of low-work function surfaces were discussed. The correction for the influence of the work function on the neutralisation probability was performed using the characteristic velocity method through investigating the dependence of the ion scattering signal on the energy of the ions. It was demonstrated that an additional correction is required for the influence of the ion velocity on the work function threshold of the resonant neutralisation, since otherwise the surface density of the species under investigation is overestimated. The insights gained from the fundamental neutralisation studies are of relevance for all LEIS investigations of low-work function systems. These newly gained neutralisation insights were applied for a quantitative comparison of the surface composition of various types of cathodes with different work functions. The composition of the Ba-O complex for top-layer cathodes with W, Re, Ir, and Os/Ru substrates was investigated, and it was for the first time established what the exact influence of the substrate material is on the work function of the cathodes during operation. The substrate material of course determines the work function of the clean non-covered surface and, most important, also determines the Ba-O coverage during cathode operation. The Ba-O coverage for the various cathodes depends on the bond between the O in the Ba-O complex and the substrate atoms. For a large coverage, and thus a low work function, a strong bond is required to overcome the electrostatic repulsion between the Ba-O dipoles. If only a weak bond exists, part of the Ba-O dipoles are desorbed due to this repulsion, resulting in a low coverage. The bond strength is determined by the orientation and the number of unoccupied d-orbitals of the substrate atoms and increases from the W, Re, Ir to the Os/Ru cathodes. Therefore, the coverage increases and the work function increases in that order of substrates. Thus far, it was commonly believed that the higher the work function of the non-covered substrate, the lower the work function of the cathodes during operation. However, here it has been shown that the bond between the O atoms in the Ba-O complex and the substrate atoms, is more important than the work function of the substrate. Despite the differences in absolute coverages among the cathodes, the Ba-O complex is similar on all cathodes. Because the Ba atoms can only reside on the cathode surface when bound to O atoms, the atomic Ba/O ratio is close to unity for all cathodes with the Ba atoms situated above the O atoms. During operation of the dispenser cathodes a dynamic equilibrium exists between the processes that remove the Ba-O compex, i.e. thermal desorption and ion bombardment, and the resupply process of the Ba and O atoms from the pores of the matrix. The influence of the ion bombardment on the surface composition and the electron emission at the dynamic equilibrium was investigated. It was shown that the ion bombardment removes the Ba and O atoms in a step-by step manner, first the outermost Ba atoms and subsequently the underlying O atoms, if the Ba has not yet been resupplied. Furthermore, we have shown that in the resupply process the Ba and O atoms arrive at the surface as individual atoms, and not in the form of BaO as is often suggested. The Ba resupply is approximately 4 times faster than the O resupply, but the residence time of the Ba atoms on the clean substrate is too short to result in a large enough diffusion length to cover the surface. However, when the Ba atoms are bound to O atoms, the residence time is sufficient to result in complete coverage. The resupply of the Ba-O complex is thus limited by the slow O resupply from the pores. The activation energy for the O resupply is determined by the production of free O in the pores, and is not limited by the O diffusion rate. Using the insight from these investigations a model was derived that describes the surface coverage and electron emission during the ion bombardment, and includes the influence of the cathode temperature and the incident ion flux. In conclusion, these studies have revealed new insights into the operating mechanism of thermionic dispenser cathodes. From the surface analysis, the difference in electron emission capabilities of various dispenser cathodes could be explained. Furthermore, an accurate model for the description of the dynamic behaviour of the cathode during ion bombardment was derived. To a large extend these new insights are related to a better understanding of the role of oxygen in the operation of the cathodes

Work function dependent neutralization of low-energy noble gas ions

The work function dependence of the neutralization of low-energy He+, Ne+, and Ar+ ions was studied by determining the neutralization probability of ions scattered from submonolayer coverages of Ba on W(110) and Re(0001) substrates. At high work functions (>3.5 eV), it was found that the dominant neutralization mechanism for noble gas ions with initial energy between 2 and 5 keV scattering from Ba is collision-induced neutralization. The neutralization probability for this mechanism was found to be insensitive to work function changes. We argue that collision-induced neutralization is also the dominant charge transfer process for scattering from other earth-alkali and alkali elements in this energy range, although at lower energies it is expected that Auger neutralization will become important. At work functions below roughly 3.5 eV, resonant neutralization to the first excited level of the noble gas ions occurs in addition to the charge transfer processes operating at high work functions. We show that the additional neutralization at low work functions can be described using resonant charge exchange theory. Due to resonant neutralization, the neutralization probability for noble gas ions increases exponentially with decreasing work function

Work function dependent neutralization of low-energy noble gas ions

The work function dependence of the neutralization of low-energy He+, Ne+, and Ar+ ions was studied by determining the neutralization probability of ions scattered from submonolayer coverages of Ba on W(110) and Re(0001) substrates. At high work functions (>3.5 eV), it was found that the dominant neutralization mechanism for noble gas ions with initial energy between 2 and 5 keV scattering from Ba is collision-induced neutralization. The neutralization probability for this mechanism was found to be insensitive to work function changes. We argue that collision-induced neutralization is also the dominant charge transfer process for scattering from other earth-alkali and alkali elements in this energy range, although at lower energies it is expected that Auger neutralization will become important. At work functions below roughly 3.5 eV, resonant neutralization to the first excited level of the noble gas ions occurs in addition to the charge transfer processes operating at high work functions. We show that the additional neutralization at low work functions can be described using resonant charge exchange theory. Due to resonant neutralization, the neutralization probability for noble gas ions increases exponentially with decreasing work function

Influence of Aerobic Exercise Induced Arousal on Neutral Word List Retrieval in Young Adults

Previous studies have shown that arousal during the encoding and consolidation phases facilitates memory performance, and that arousing stimuli are better remembered. The current study shifts attention to physical arousal in the retrieval phase. This study was designed to test whether physically induced arousal can enhance memory for neutral words. Participants (N = 48) were randomized over a control and an aerobic condition. On the first day they memorized a list of 30 words, on the second day they either watched a documentary or executed an aerobic cycling exercise before performing a memory recall and recognition test for the words memorized at day one. Prior to the main analyses a manipulation check on subjective and physiological arousal was conducted and a successful condition manipulation was confirmed. However, the analyses showed no significant difference in memory performance between groups. Implications and limitations are discussed

Influence of band width on the scattered ion yield spectra of a He + Ion by resonant or quasi-resonant charge exchange neutralization

The influence of the band structure, especially the bandwidth, on the scattered ion yield spectra of a He+ ion by the resonant or quasi-resonant neutralization was theoretically examined using quantum rate equations. When calculating the scattered ion yield spectra of He+ to simulate the experimental data, we observed that the band structure, especially the bandwidth, had a strong influence on the spectra at relatively low incident He+ ion energies of less than several hundred eV. Through many simulations, it was determined that theoretical calculations that include bandwidth calculation can simulate or reproduce the experimentally observed spectra of He+-In, He+-Ga, and He+-Sn systems. In contrast, simulations not including bandwidth simulation could neither reproduce nor account for such spectra. Furthermore, the calculated ion survival probability (ISP) at low incident ion energies tended to decrease with increasing bandwidth. This decrease in ISP probably corresponds to the relatively small scattered ion yield usually observed at low incident ion energies. Theoretically, such a decrease indicates that a He+ ion with a low incident energy can be easily neutralized on the surface when the bandwidth is large