J. Kemmler

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.

Secondary-electron velocity spectra and angular distributions from ions penetrating thin solids☆

Abstract We present velocity spectra and angular distributions of secondary electrons emitted from cleaned surfaces of thin carbon and gold foils penetrated by H+ (Ep = 0.8 and 1.2 MeV), H2+ (Ep/Mp = 0.8 MeV/u) and C+ (Vp = 2.30− and 2.85 VB). Prominent structures in doubly differential velocity spectra d 2 n/ d E d Ω of secondary electrons (SE) are (1) the “true SE” peak at low electron energies Ee

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.

Secondary electron emission from entrance and exit surfaces of thin clean foils bombarded with H+, C+, and O+

Abstract We have measured secondary electron emission from thin, clean foils of C, Al, Ti, Ni, and Cu bombarded with H+, C+, and O+, at energies between 0.3 and 1.2 MeV/amu for protons and 20 to 80 keV/amu for the heavy projectiles. We observe in some but not all cases a proportionality between the secondary electron yield and the stopping power, but such a proportionality seems to break down at the lower projectile energies used by us. The yields emitted in forward direction are generally somewhat larger than those emitted backwards. The results are discussed in the frame of an extended Sternglass model.

Study of convoy electrons from collisions of hydrogenic projectiles (Z = 1 and Z = 28) with solids

Abstract The target thickness dependent convoy electron yields Y e from hydrogenic projectile ions H 0 , H + (3 MeV) and Ni 27+ , Ni 28+ (15.6 MeV/u) are compared and discussed in the framework of an extended model for ELC and ECC in solids considering also contributions from excited states. It is shown that convoy electrons from light and heavy projectile ions in principle can be described with the same Ansatz, but discrepancies remain. To understand the target thickness dependent evolution in the case of incident Ni 27+ it must be assumed that convoy electrons produced by ELC arise mainly from excited states. The mean transport length λ c for convoy electrons for light projectile ions is equal to the attenuation length of isotachic free electrons λ e , whereas for the heavy ions an enhanced transport length λ e ⪢ λ e must be introduced.

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.