Electron and ion emission from liquid metal surfaces

The application of positive or negative electric fields to small-area liquid metal surfaces leads to very high brightness DC ion or pulsed electron emission. The stabilization of a cone-shaped structure by the balance of electrostatic and surface tensions forces is described. Electron and ion emission occurs by field emission and field evaporation mechanisms     

Secondary ion emission from Si under nitrogen ion bombardment

The yield and energy distribution of positive secondary ions emitted from Si under N₂⁺ ion bombardment were measured. The obtained mass peaks correspond to three types of secondary ion species, that is, physically sputtered ions (Si⁺, Si₂⁺), chemically sputtered ions (SiN⁺ Si₂N⁺) and doubly charged ions (Si²⁺). The dependence of secondary ion emission on the primary ion energy was studied in a range of 2.0–20.0 keV. The yields of physically and chemically sputtered ions were almost independent of the primary ion energy. The yield of the doubly charged ion strongly depended on the primary ion energy. The energy distribution of secondary ions of the three types showed the same dependence on the primary ion energy. The most probable energy of the distribution increased with the primary ion energy. On the other hand, for the energy distribution curves of sputtered ions, the tail factors N in E^?N were constant and showed a m/e dependence.     

Secondary ion emission from silicon and silicon oxide

The emission of Si⁺, Si²⁺, Si³⁺, Si₂⁺, SiO⁺ and B⁺ from boron doped silicon has been studied at oxygen partial pressures between 2 × 10^?10 and 2 × 10^?5 Torr. Sputtering was done with 2 to 15 keV argon ions at current densities between 3 and $\text{40}\text{μ}\text{A}\text{cm}{}^2$. The relative importance of the different ionization processes could be deduced from a detailed study of the yield variation at varying bombardment conditions. Comparison with secondary ion emission from silicon dioxide allows a rough determination of the composition of oxygen saturated silicon surfaces.     

Slow highly charged ion O4+ induced electron emission from clean solid surfaces

The total electron emission yields following the interaction of slow highly charged ions (SHCI) O⁴⁺ with different material surfaces (W, Au, Si and SiO₂) have been measured. It is found that the electron emission yield γ increases proportionally with the projectile velocity v ranging from 5.36×10⁵m/s to 10.7× 10⁵m/s. The total emission yield is dependent on the target materials, and it turns out to follow the relationship γ(Au)>γ(Si)>γ(W). The result shows that the electron emission yields are mainly determined by the electron stopping power of the target when the projectile potential energy is taken as a constant, which is in good agreement with the former studies.     

Secondary ion emission by nonadiabatic dissociation of nascent ion molecules with energies depending on solid composition

A new model of the emission of positive secondary ions Me⁺ from metal oxides is proposed to reproduce the strong mass dependence of their yield. The mass dependent aspects of this model are discussed and mathematically formulated. The energy transport in the collision cascades depends strongly on the mass ratio of the atoms involved. An approximation of the energy distribution of surface atoms is made which includes this mass effect. Surface molecule collisions lead to the emission of ions. They again depend on the mass ratio of the molecule's constituents. It is made plausible that particle complexes leave the surface only neutrally. In a nonadiabatic dissociation process free ions can be created outside the surface potential barrier. There is only a very low probability for neutralization depending on the mass of the ions involved. The model calculation reported in Part II of this paper reproduces the strong mass dependence experimentally observed. In Part I the new emission conception is additionally confirmed by the qualitative explanation of other Me⁺ emission effects: difference in the energy distribution of Me⁰ and Me⁺, change of the energy distribution of Me⁺ ions during oxygen exposure experiments, difference in the Me⁺ emission of metals with high (Ti) and low (Co) affinity to oxygen during oxygen exposure experiments.     

Secondary emission from small dust grains at high electron energies

A previous model for secondary electron emission from small grains is modified to calculate yields for micron sized grains, both spherical and cylindrical, when the primary electrons constitute a high energy parallel beam. It is found that, in general, the secondary electron yield is significantly higher than for the case of normal incidence. Moreover, the equilibrium potentials of the grain are always positive due to this enhanced secondary emission. These results are compared with experimental data recently available for micron sized glass particles, and equilibrium potentials, calculated based on the model presented here, and are found to be in reasonably good agreement with their measured potentials     

反冲原子对低速离子轰击Si表面时电子发射产额的影响

在兰州重离子加速器国家实验室电子回旋共振离子源高电荷态原子物理实验平台上,用低能(0.75keV/u≤EP/MP≤10.5keV/u,即3.8×105m/s≤vP≤1.42×106m/s)He2+,O2+和Ne2+离子束正入射到自清洁Si表面时二次电子发射产额的实验结果.结果表明电子发射产额γ近似正比于入射离子动能EP/MP.在相同动能下,γ(O)γ(Ne)γ(He),对于原子序数ZP比较大的O2+和Ne2+离子,ZP大者反而γ小,这与较高入射能量时的结果截然不同.通过计算不同入射能量下入射离子的阻止能损S,发现反冲原子对激发二次电子的作用随入射离子能量的降低显著增大,这正是导致在较低能量范围内二次电子发射产额与较高入射能量时存在差异的主要原因.     

Infrared emission from surfaces under low-energy electron impact

The infrared photon emission from metal surfaces stimulated by the impact of low-energy electrons of kinetic energies between 0–10 eV has been measured. The results are presented as isochromat spectra from clean Ag and Na surfaces under different temperatures. Some IR emission features have been associated tentatively with inverse photoemission processes.     

Characteristic photon emission from ion bombarded silicon and silica surfaces

Ion bombardment with 50 keV inert gas and reactive gas ions has been used to study the photon emission from excited radiating atoms and ions in the immediate vicinity of the surface of both silicon and silica glass targets (SiO₂).     

Field emission energy distribution (clean surfaces)

The emission of electrons from a cold cathode upon application of a strong electric field is called field emission. Since the electrons must tunnel through the classically forbidden barrier outside the solid, field emission was one of the first confirmations of the new quantum theory of the 1920's.¹ A field of tens of million volts per centimeter is required to obtain a reasonable current. In order to achieve such hgh fields at reasonable voltages, the cathode or emitter is usually etched to a very sharp point (～1,000 Å in radius). Therefore, several thousands of volts applied to the anode will produce the desired field. In 1937, Miiller2 developed what is known as the field emission microscope. The success of Miiller's microscope was a consequence of his realization that, if he produced a small hemispherically shaped tip that was thermally smoothed and cleaned, he would project a greatly magnified image of the spatial distribution of electrons tunneling from the emitter onto a fluorescent screen. Such a field emission pattern for clean tungsten is shown in Figure 1. The image on the screen is a nearly stereographic projection of the hemispherical end of the emitter. Because of its small size, the emitter is usually part of a single crystal and thus exposes all crystallographic orientations, so that individual crystal planes can be located and identified in the field emission pattern. The changes in the field emission pattern with exposure to adsorbed atoms or molecules have been used very successfully to study surface processes, such as diffusion, adsorption and desorption kinetics, or work function changes.³     

Mechanism of negative ion emission from surfaces of ferroelectrics

Experimental studies of the mechanism of negative ion and cluster ion emission from surfaces of ferroelectrics are described. The emission was produced by negative voltage pulses with the amplitude of about 400 V, with a rapid rise-time (below 10 ns) and a slow decay‐time (several μs). The pulses were applied between the back side of the ferroelectric sample and the metal tip touching the front emitting side. The surface of the ferroelectrics could be cleaned in situ by 1 keV Ar⁺ bombardment. The morphologic changes around the tip were observed with an atomic force microscope (AFM). Mostly negative ions and cluster ions were emitted and studied in our experiments. Positive ions were detected with much lower probability and are produced by an entirely different microscopic process than negative ions. Masses as well as energies of emitted ions were measured with a time-of-flight (TOF) spectrometer and compared with available spontaneous desorption (SD) spectra and Cs‐SIMS spectra in order to clarify the mechanism of the emission. The trajectory of ions emitted from the sample was studied by computer simulation. The conclusion of these studies is that the negative ion emission is caused by the Coulomb explosion of a polarization cloud rapidly formed at the front edge of the pulse. The explosion takes place in the vicinity of the tip-sample contact at distances several tens of μm from the contact where the stabilizing effect of the positively charged tip is already small.     

Secondary-ion emission from clean and oxygen-covered beryllium surfaces: II. Energy dependence

The secondary-ion energy distribution obtained by sputtering clean and oxygen-covered Be has been analyzed in terms of competing processes in secondary ion emission. The ion energy distribution N⁺(E) has been separated into an ionization coefficient R⁺(E) and a total energy distribution, N(E), i.e. N⁺(E) = R⁺(E) N(E). Experimentally, the dependence of R⁺(E) on both energy and oxygen coverage indicated a linear superposition of adiabatic tunneling and resonanance ionization processes from clean and oxygen-covered portions of the surface with no contributions to the secondary-ion yield from regions of intermediate coverage. Total energy distributions of sputtered Be atoms have been deduced and the principal features agree with the predictions of the collision cascade sputtering model. Variations of the energy distributions with oxygen coverage are in accord with small changes expected in the surface binding energy as a result of surface oxidation.     

Angular resolved Auger emission from clean and sulfur covered Ni(110) surface

The angular dependence of the nickel M₂₃VV and of the sulfur L₂₃VV Auger transitions are studied in detail, on clean and sulfur covered Ni(110) surfaces. New experimental data are presented for the anisotropy of both transitions as a function of polar and azimuthai angles of emission. Our model, which incorporates at the same time the multiple scattering effects in the final state wave function and the intrinsic anisotropy of the Auger emitter, is found to give a satisfactory account of the observed auger anisotropy. We find a large sensitivity to the position of the sulfur adsorbed atoms. The best agreement is obtained for the hollow site. slightly less than 0.9 Å above the top nickel layer. This conclusion is consistent with previous LEED and MEIS studies, but does not agree with the long bridge site obtained from quantum chemistry calculations. Moreover the sulfur emitter on this particular Ni(110) face appears to have an intrinsic anisotropy.     

Photon emission from rare gas ion bombardment of metal surfaces

Metal surfaces (Mg, Cu, Zr, Mo) are bombarded with He⁺, Ne⁺ and Ar⁺ in the energy range of 400 eV to 8 keV. Radiation from scattered projectiles and sputtered target particles is observed between 200 and 700 nm. It is shown that most of the radiating particles originate from surface collisions. Auger neutralization, resonance tunneling and direct electron transitions are the important electronic processes involved.     

Bulk plasmon induced ion neutralization near metal surfaces

A novel mechanism is proposed for ion neutralization near metal surfaces, whereby a bulk plasmon is emitted during the electron capture, induced by the presence of the external ion which does not penetrate the metal. In a semiclassical picture of this mechanism the electrons increase their velocity in the field of the ion until they surpass the threshold velocity for collective excitation, emitting the plasmon and getting bound to the ion.Primary evaluations of bulk plasmon transition rates for He⁺ interacting with Al surfaces indicate that very close to the image plane the bulk collective channel might become more efficient than the surface plasmon mode to neutralize the ion.     

Electron energy-loss spectra of clean and gas-covered Ni(100) surfaces

Electron energy-loss spectra have been measured on Ni(100) surfaces, clean and following oxygen and carbon monoxide adsorption, at primary energies of 40–300 eV. The observed peaks at 9.1, 14 and 19 eV in the clean-surface spectrum are ascribed to the bulk plasmon of the 4s electrons, the surface plasmon, and the bulk plasmon of the coupled 3d + 4s electron, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Assignments of the loss structures on the gas-covered surfaces have been attempted.     

LEED intensities from clean and hydrogen covered Ni(100) and Pd(111) surfaces

Hydrogen adsorbs on Ni(100) and Pd(111) surfaces without the formation of additional diffraction spots in the LEED patterns. Measurements of LEED intensities revealed that adsorbed hydrogen layers cause considerable changes even in such cases where displacements of surface atoms (“reconstructive adsorption”) may be excluded. After hydrogen adsorption on Ni(100) the intensities of Bragg beams are uniformly lowered whereas the background intensity increases which is attributed to the formation of a disordered adsorbed layer. With Pd(111) adsorbed hydrogen causes a slight decrease of the background intensity and characteristic modifications of the intensity/voltage curve of the (0,0) beam, suggesting the formation of an ordered 1 × 1 structure. In the latter case energy shifts of the primary Bragg maxima were observed and are interpreted as being caused by an expansion of the layer spacing in the surface region by about 2% owing the partial dissolution of the hydrogen.