Image Charge Acceleration Research Articles

Deposition and re-emission of potential energy - extended dynamical COB simulation

We present results of a numerical computer simulation based on the extended dynamical classical over the barrier model (EDCOM). To get an energy balance for the transferred potential energy of the highly charged ions (HCIs) to the solid surface, we apply an additional extension of the model evaluating the energy loss of Auger electrons which are generated during the relaxation of the HCI. From the results of the simulation we can distinguish between the potential energy deposited in the nuclear system due to image charge acceleration of the incoming HCI and the potential energy which is released over Auger processes. Further, we break down this energy into that which is deposited in the solid and that which is re-emitted due to escaping secondary electrons.

Angular dependence of energy loss in scattering of slow highly charged Ar ions from a Au surface

We have studied the energy loss of slow (velocity v≈0.06 a. u. ) highly charged Ar 9+ ions scattered at large angles from a clean Au(1 1 1) single crystal surface. The influence of the scattering geometry on the energy loss has been investigated for different incidence angles ( 5°⩽ψ⩽37.5°) and scattering angles ( 30°⩽θ⩽75°). At large angles ( ψ= 37.5°, θ= 75°), the main peak in the measured energy spectrum agrees well with the position expected for binary elastic scattering. With decreasing incidence angle a shift of the “binary peak” to lower energy is observed, and for decreasing scattering angles, a shift of the binary peak to higher energy than calculated by kinematics is found. The additional energy loss seen at small ψ ( ψ= 5°) is interpreted by inelastic energy loss. A model calculation which combines s rate-equation description of Auger cascades, recapture to the surface with a Firsov type model and image charge acceleration during ion–surface interaction is used to describe the data.

Energy loss in large-angle scattering of slow, highly charged Ar ions from a Au surface

Energy loss of slow ($v\ensuremath{\approx}0.06 \mathrm{a}.\mathrm{u}.$), highly charged ${\mathrm{Ar}}^{9+}$ ions scattered from a Au(111) single-crystal surface has been investigated for different incident angles ($\ensuremath{\psi}=5\ifmmode^\circ\else\textdegree\fi{}$ and $37.5\ifmmode^\circ\else\textdegree\fi{}$) and a fixed scattering angle of $75\ifmmode^\circ\else\textdegree\fi{}.$ The energy position of the elastic peak agrees well with the value expected for the binary elastic scattering. At small incidence angle $(5\ifmmode^\circ\else\textdegree\fi{})$ an additional inelastic energy loss was identified in spite of the large scattering angle. This additional energy loss is found to compare well with a combination of a modified Firsov model with processes of electron capture, recapture, Auger transitions, and image charge acceleration during the ion-surface interactions.

Energy loss of highly charged argon ions at grazing incidence on a graphite surface

We have used the time-of-flight technique to measure the energy loss for 51-keV argon ions scattered grazingly from a smooth highly oriented pyrolytic graphite surface. The energy loss has been measured for several impact angles and two charge states, q=2+ and q=16+. Our data indicate that the image charge acceleration in the front of the surface, which increases the effective impact angle of the projectile, affects the energy loss. After correcting for this effect, the energy loss does not depend on the initial charge state of the projectile but increases with increasing the angle of incidence. This indicates a full neutralization of the projectile before it reaches the distance of the closest approach. The experimental results agree approximately with predictions of a modified Firsov model.

Charge-state dependence of image-charge acceleration of convoy electrons in fast, grazing collisions of carbon ions with a silicon (100) surface.

Electron emission distributions, triply differential in emission energy and angles, are presented for collisions of singly through quadruply charged C ions impinging on a Si(100) surface, with ion energies between 3.6 and 6.0 MeV and grazing angles of incidence between 0.1{degree} and 1.0{degree}, respectively. The electron emission spectra measured under these conditions confirm previous reports of a shift of the convoy electron peak toward higher energy due to surface wake or image acceleration. Detailed angular distributions demonstrate a shift of convoy emission towards larger angles with respect to the surface plane, which is further evidence of the acceleration of emitted electrons by the surface wake potential. In addition, we observe the peak energy shift to depend upon the incident ion charge state for the higher projectile velocity ({approximately}4.5 a.u.). This dependence may be understood as a consequence of incomplete projectile charge equilibration at the time of convoy electron production. The charge-state dependence is not observed at the lower collision velocity ({approximately}3.5 a.u.). {copyright} {ital 1996 The American Physical Society.}

Image charge acceleration of multicharged ions in front of the surface of an insulator.

Multicharged xenon ions are scattered with keV energies from a LiF(100) surface under a glancing incidence. From the angular distributions of scattered projectiles, we deduce the existence of an attractive force acting on the projectiles on the incident path. We interpret this as the dielectric response of the insulator due to the presence of an ion. The interaction energies gained on the incident trajectory are reproduced by an ``overbarrier'' model of stepwise capture of electrons from the fluorine $2p$ band of LiF into Rydberg levels of the projectile.

Projectile image acceleration, neutralization and electron emission during grazing interactions of multicharged ions with Au(110)

Recent Oak Ridge work is summarized on projectile energy gain by image charge acceleration, scattered ion charge distributions, and K-Auger electron emission during low energy grazing interactions of highly charged Pb, I, O and Ar ions with a Au(110) surface.

Theoretical study of image charge acceleration of highly charged ions in front of a metal surface

We show how a simple model for electron transfer from a metal surface to a slow highly charged ion can be used to calculate the image charge interaction energy. In the theoretical model the attractive force, caused by the image charge, is described with a classical image potential. The velocity of the ion enters explicitly and the electron transition rate has an exponential decay with the distance from the surface. The distance where the rate of change of the ion charge has a maximum is required to fulfil a classical barrier condition. The results compare reasonably well with recent experiments for ions with charges up to q ≈ 80.

Ion/surface interaction studies with 1-3 keV/amu ions up to Th80+

Abstract First results from a series of experiments aimed at the study of the interaction of slow very highly charged ions with conductor and insulator surfaces are reported. Charge state dependences of secondary electron, x-ray and secondary ion emission are measured. In addition, microscopic studies are performed using an ‘Atomic Force Microscope’ to investigate surface defects, produced through the stored electrostatic potential of the incident ion. The ions that have been used for these studies range from O7+ to Th80+, with low kinetic energies (generally 1–3 keV/amu). Overall, enhancement of the low energy secondary electron, sputter ion and photon emission are observed with increasing charge states. Saturation in the electron emission yield at very low velocities confirm present models for image charge acceleration effects. X-ray emission spectra are found to be consistent with the formation of so called ‘hollow’ atoms near the surface during the neutralization processes. The microscopic studies rev...

Emission of electrons from a clean gold surface induced by slow, very highly charged ions at the image charge acceleration limit.

Total low-energy electron yields for the normal incidence interaction of slow, very highly charged ions ${(}^{136}$${\mathrm{Xe}}^{\mathit{q}+}$, 21\ensuremath{\le}q\ensuremath{\le}51; $^{232}\mathrm{Th}^{\mathit{q}+}$, 51\ensuremath{\le}q\ensuremath{\le}80) with a clean gold surface have been determined from the related measured electron emission statistics. The projectile impact energies could be reduced down to the image charge acceleration limit. The electron emission yield was found to increase proportionally with the increasing projectile charge state in all cases studied, suggesting no saturation in the ability of the Au target to provide necessary electrons within the above surface interaction time (e.g., about 280 electrons/projectile for ${\mathrm{Th}}^{79+}$ in less than ${10}^{\mathrm{\ensuremath{-}}13}$ s). Because of the relatively narrow energy distribution of the incident ions, as well as their very high charge states, the first clear measurements of the image charge acceleration from electron emission yields could be performed. Results of a quantitative study of this acceleration are in good agreement with those of a theoretical model based on classical over the barrier transitions (recently developed by J. Burgd\"orfer et al.).