M. Burkhard

Secondary electron yields from the entrance and exit surfaces of thin carbon foils induced by penetration of H+, H0 and H+2 projectiles (1.2 MeV/u)☆

We report experimental work on secondary electron emission from both entrance (γb) and exit (γf) surfaces of thin carbon foils traversed by H+, H0 and H+2 projectiles (1.2 MeV/u). Secondary electron coefficients γb and γf, were measured simultaneously. The results are discussed in the framework of a semiempirical model for kinetic emission of target electrons from solid surfaces.

Convoy electron response to the charge pre-equilibrium of ions in solids☆

Abstract Absolute numbers of convoy electrons of H+ and H0 (1.2–3.0 MeV) passing through carbon foils (2 ⩽spx ⩽ S 20 μg/cm2) ha been measured using a calibrated magnetic 90°-sector electron analyzer. In the charge pre-equilibrium (px ⩽ 5 μg/cm2) the convoy electron yield is strongly dependent on the initial charge state and is dominated by the direct loss of the projectile electron (DELC) in case of initial charge state O. Qualitatively and quantitatively the experimental data can be described by a two-step model which includes the production of convoy electrons as a function of charge changing and excitation processes (referred to as direct and indirect ELC and direct and indirect ECC) and the projectile independent transport of convoy electrons through the solid.

Strong convoy electron yield dependence on surface properties

Abstract Velocity spectra of electrons ejected in the forward direction as a result of ion-solid collisions, are characterized by a cusp shaped structure, the convoy electron peak, if the velocity up of the projectile matches that of the electron, νe ∼- νp. production models assume either volume-multiple step processes or surface-last-layer processes. To probe these models convoy electrons from sputter-cleaned surfaces of C, Al, Ni, Cu, Pd, Ag, Sm, Gd, Au and Bi targets have been studied for the first time under UHV-conditions by proton or krypton ion impact (0.8 and 2.0 MeV respectively), it is found that the convoy electron yield depends strongly on the surface coverage B of adsorbed substances, on the target material and, for Kr+ ions, on the work function of the investigated elements. For C targets both yield and convoy electron energy are affected by the hydrogen content.

Ultrahigh vacuum analyzer for convoy electron measurements

Abstract A new parallel plate electrostatic electron energy analyzer for measurements under ultrahigh vacuum (UHV) conditions is described. The construction and characteristic data of the spectrometer are given and typical convoy electron spectra are shown.

Wake effects in the stopping power of molecular ions

Abstract The stopping power of diatomic molecular ions is calculated in the framework of the dielectric theory using a Monte Carlo procedure. Vicinage effects are included by application of wake-potential interference and Coulomb repulsion of both molecular fragments. Multiple scattering and energy straggling of the projectile ions are considered and are found to be significant for the stopping power values. The results are compared with stopping power measurements of N 2 + ions in carbon.

Surface characterisation of thin solid foil targets by ion impact

The characterise the surface condition of thin solid foils (x approximately=1000 AA) by ion impact, a novel combination of different independent methods of control has been used: (1) Auger electron spectroscopy (AES); (2) secondary electron spectroscopy (SES); (3) measurement of the total electron coefficient gamma ; and (4) detection of elastically scattered projectiles. Surfaces of various elemental foils (6<or=ZT<or=83) have been carefully prepared by heavy-ion- (Kr+-) induced sputtering. Coverage could be removed from the target surfaces (better than 0.3 remaining monolayers) and the roughness could be reduced considerably.

Target characterization by fast ion impact

Abstract Accelerator based, easily accessible methods are discussed to control and characterize targets and their surfaces by (MeV) ion impact. Independent, but simultaneously applicable techniques such as Rutherford backscattering (RBS), elastic recoil detection (ERD), secondary electron emission (SEE), ion induced Auger electron spectroscopy (IIAES), and particle induced X-ray emission (PIXE) will be shown as tools for bulk and surface analyses. We also present studies of the transformation of the surface conditions, e.g. surface cleaning and smoothing under UHV conditions by heavy-ion sputtering. Possibilities, limitations, and sensitivities of the techniques are discussed.

Molecular effects observed in auger transitions from H+ AND H2+ collisions with solid, molecular and atomic targets

Abstract The molecular effect is measured by the ratio R = Y A ( H 2 + ) Y A ( H + ) of H 2 + - and (1 MeV/u) H + -i For carbon foil surfaces R solid ⩽1.3 resulting from the high energy region of the Auger peak, is caused mostly by the projectile electron interacting with the target atoms. In collisions with CH 4 , C 4 H 10 and Ar targets, R gas ⩽ 0.75 is attributed to screeni under single collision conditions. The Auger line shapes and R values resulting from solid and gaseous targets are discussed.

Convoy electrons in coincidence with Hq+ and Heq+ outgoing projectiles traversing carbon foils

We have measured convoy electrons produced by the passage of light ions (H+ at 150, 200, 250 keV/u and He+ at 100, 150, 200 keV/u) through equilibrium thickness carbon foils (x = 5 μg/cm2) in coincidence with the charge states qf = 0, 1 resp. qf = 0, 1, 2 of the outgoing projectiles. All charge state fractions were registered simultaneously by a position sensitive parallel-plate-avalanche detector (PPAD). The yield of coincident convoy electrons Yc(qf) is normalized to the same number of outgoing projectiles and mirrors the fraction of the emergent ions of each charge state qf. The FWHM of the coincident convoy electron peak is found to be independent of qf. A transport length λc for convoy electrons in the range of the mean free path of free electrons is deduced from the data. This result is in contradiction with recent measurements for D+ (150 keV/u).

The influence of solid surfaces on low-energy convoy electron emission

The influence of solid surfaces on the emission of low-energy convoy electrons (8 eV<or=Ee<or=13 eV) from different target materials (C, Al, Cu, Ag, Sm, Gd and Au) after Kr+ impact (1.4 MeV<or=Ep<or=2.2 MeV) has been studied for the first time under ultrahigh-vacuum conditions. It is found that the convoy electron yield increases with decreasing residual surface coverage if the surface is cleaned by dynamic sputtering. The convoy electron yield reaches a saturation value which depends on the work function of the target; the convoy electron yield increases with decreasing work function. This indicates that the transmission of convoy electrons is influenced by both surface contamination and the surface potential barrier.