Secondary Electron Yield Measurements Research Articles

The scattering of doubly charged ions from surfaces

The scattering of doubly charged ions from clean surfaces provides the opportunity to study electron exchange processes between states of higher binding energy than in the case of the scattering of singly charged ions. Experiments on the scattering of such doubly charged ions have been carried out in the incident energy range 0–10 keV and at 200 keV. At the lower energies, no doubly charged particles are observed to survive the scattering event in a doubly charged state. Substantial neutralisation of the doubly charged incident ion occurs, but the yield of singly charged particles due to a single electron capture by the doubly charged incident ion is greater than the yield of singly charged particles surviving the collision when a beam of initially singly charged particles with the same velocity as the doubly charged particles is incident. Measurements of secondary electron yield, though crude, indicate there is a strong potential emission associated with the scattering of the doubly charged ions. At the higher energy, a beam of doubly charged ions incident on the surface gives rise to neutral, singly charged and doubly charged components in the scattered beam. A low resolution electrostatic analysis system has been used to study the proportions of scattered particles in the various charge states. Preferential neutralisation to the singly charged ion is observed, with the fraction of scattered neutral and singly charged particles being about equal and both fractions being an order of magnitude greater than the fraction of doubly charged particles in the scattered flux. The energy spectra of single and double charged particles show no preferential charge exchange at the surface. The results suggest the ion charge state is determined at the transition through the surface layer.

Monolayer and multilayer surface diffusion, growth mode and thermal stability of indium on W {100}

Surface diffusion parameters, the thin film growth mode, and thermal stability of indium deposited on a W {100} single crystal have been studied using LEED, TDS and scanning AES, the surface diffusion studies being of central importance as part of a programme to examine the effect of lateral adatom-adatom interactions on diffusion parameters. A correlation of LEED, AES and secondary electron yield (SEY) measurements during stepwise deposition of indium point to completion of a pseudomorphic indium monolayer before subsequent growth occurs. Following layers grow in a long-range disordered fashion. First layer growth initially takes place via random occupation of surface sites, with two-dimensional islands forming at higher coverages. TDS indicates that evaporation from a thick film occurs down to two monolayers (ML) over the relatively narrow temperature range 700 to 800 K, the desorption energy, 240 kJ mol −1, being close to the reported sublimation energy for indium. Desorption of the second and first monolayers occurs between 800 and 1200 K, indicating a strongly coverage-dependent activation energy for desorption. Up to a monolayer coverage both the activation energy for diffusion ( E m) and the diffusivity ( D 0) are strongly coverage-dependent, indicating net repulsive lateral adatom-adatom interactions (for θ = 0.25 ML, E m∗ = 106 kJ mol −1 while for θ = 0.75 ML, E m∗ = 64 kJ mol −1) . The activation energy for diffusion is roughly a constant fraction of that for desorption (≈ 30%) in the monolayer region. Second and subsequent layers of indium diffused much more rapidly than the first; in this case E m∗ and D 0∗ are insensitive to coverage, and the diffusion energy barrier is only 10% of the desorption energy.

Secondary electron yield and Auger electron spectroscopy measurements on oxides, carbide, and nitride of niobium

Secondary electron yield measurements before and after Ar ion sputter cleaning were made on Nb and Nb compounds of interest for rf superconducting cavities. Total secondary electron yields (σ) for primary energies 20–1500 eV were measured for solid Nb (σmax=1.3 at 300 eV), anodized Nb2O5 (σmax=1.2 at 300 eV), and powders of Nb(σmax=1.0 at 400 eV), NbO (σmax=0.9 at 400 eV), NbO2 (σmax=1.0 at 400 eV), Nb2O5 (σmax=0.95 at 400 eV), NbC(σmax=0.8 at 400 eV), and NbN (σmax=0.8 at 500 eV). Determinations were made for Auger elemental sensitivities, and the relationship between Auger peak heights and oxide stoichiometry is discussed. The sputter etch rate of anodized Nb2O5 was measured by depth profiling anodic coatings of known thickness.

Secondary electron yield measurements from limiter materials in the joint European torus

Secondary electron yield measurements are presented for a graphite limiter section removed after exposure from the Joint European Torus (JET) and for unexposed pocographite. The primary electron beam energy was in the range 20 to 2200 eV and was incident in the range 0° to 60° to the surface normal. Yields were measured before and after ion bombardment and in both cases were greater for the used graphite than for virgin pocographite. Under certain conditions the yields were greater than one for the JET limiter, whereas for pocographite they were below one.

Secondary electron yield measurements from limiter materials in the Joint European Torus

Secondary electron yield measurements from limiter materials in the Joint European Torus

Experimental evidence for the influence of the atomic shell structure on the energy loss of high energy heavy ions in matter

Secondary electron yield measurements for 252Cf fission products in carbon foils reveal significant deviations for the electronic stopping cross section from semiempirical predictions. This effect may correspond to the Z 1 oscillations observed previously at much lower ion velocities.