Slow Multicharged Ions Research Articles

Interaction of slow multicharged ions with solid surfaces: current concepts and new information on slow electron emission

Slow multicharged ions (MCI) approaching a solid surface are transiently converted into highly excited complexes, involving a potential energy possibly much larger than the kinetic projectile energy. De-excitation of such complexes proceeds within less than one pico-second via a manifold of interrelated reaction channels, giving rise to characteristic signatures as slow and fast electron emission, and X-ray radiation. For impact on LiF, also potential-energy dependent ejection of neutral and ionized target particles has been observed. Before their close contact with a metal surface, MCI are converted by multiple capture of target electrons into the so-called hollow atoms which rapidly decay by autoionization, at the surface loose outer electrons due to shielding, and eventually become de-excited inside the solid. At insulator surfaces, MCI neutralization and de-excitation is less well understood. Recently measured electron number statistics for MCI impact on clean metal (Au) as well as insulator (LiF) surfaces deliver interesting new information.

Charge exchange, energy loss and related electronic processes of ions in matter

The charge state approach to the energy loss of ions in matter is presented. The stopping of He projectiles in Al targets and the phase effect for the HZn system is explained with this model. Recent results obtained within Density Functional Theory to treat the nonlinear screening of slow multicharged ions in metals are also shown.

Ion-induced electron emission from solid surfaces: information content of the electron number statistics

Measured electron number statistics resulting from the impact of singly and multiply charged atomic, cluster and fullerene ions on atomically clean metal (polycrystalline gold) and insulator (lithium fluoride) surfaces depend in a characteristic manner on the given projectile-target combinations and projectile impact energies. Comparison with model statistics delivers, apart from the respective total electron yields, further information on the total numbers and single-electron emission probabilities of the complete electron ensemble taking part in the emission process. For potential emission induced by slow multicharged ions on gold, the present analysis provides new insights on hollow atom formation and decay in front of the surface, in excellent agreement with recently calculated hollow atom autoionization rates.

Electron emission from polycrystalline lithium fluoride bombarded by slow multicharged ions

Total electron yields have been determined from electron emission statistics measured for impact of H +, N q+ ( q = 1, 5, 6) and Ar q+ ( q = 1, 3, 6, 9) on clean, polycrystalline lithium fluoride. Ion impact energies have been varied from almost zero up to 10 × q keV. The obtained total electron yields deviate considerably from available data derived via ion- and electron current measurements for LiF single crystal targets. Our results are explained by comparison with a recent model for MCI induced potential electron emission from clean metal surfaces, which has been properly adapted, available theory for kinetic electron emission from alkalihalide surfaces, and by considering also measured secondary electron yields for LiF. Dependences of the electron emission statistics and -yields on projectile impact energy and -charge differ strongly from corresponding properties for clean metal surfaces, which can be explained from the different roles of potential- and kinetic emission and, in particular, a relatively stronger contribution from secondary electron emission induced by fast electrons from finally neutralising projectiles inside the LiF bulk.

Nonlinear screening effects in the interaction of slow multicharged ions with metal surfaces

Density functional theory is used to treat the problem of screening of highly charged ions in an electron gas when inner shell vacancies are present. Results are presented for N and Ne ions in various electronic configurations. These include information about screening charge densities, induced densities of states in the conduction band, bound energy levels and KLL Auger lines.

Secondary ion emission from lithium fluoride under impact of slow multicharged ions

Secondary ion emission has been investigated for bombardment of polycrystalline lithium fluoride by slow multicharged Ar ions (charge state q ≤ 9, impact energy E k ≤ 500 eV). The F − ions originate from collisional energy transfer, almost independent of the primary ion charge, whereas the F + yield strongly increases with q. The F + ions are produced via interatomic Auger transitions from the F − 2p valence band into projectile states, and their desorption from LiF is controlled by Coulomb interaction of F + with Li + and F − surface ions, and LiF lattice relaxation. At high impact energy, emission of Li + is also mainly due to collisional energy transfer, but toward lower E k the primary ion charge plays an increasingly important role. The present measurements demonstrate that secondary ions account for less than 0.1% of our earlier measured total sputter yields from LiF.

Rotating crucible electron beam evaporation technique for the deposition of multicomponent oxide films

Slow multicharged ions (MCI) approaching a solid surface are transiently converted into highly excited complexes, involving a potential energy possibly much larger than the kinetic projectile energy. De-excitation of such complexes proceeds within less than one pico-second via a manifold of interrelated reaction channels, giving rise to characteristic signatures as slow and fast electron emission, and X-ray radiation. For impact on LiF, also potential-energy dependent ejection of neutral and ionized target particles has been observed. Before their close contact with a metal surface, MCI are converted by multiple capture of target electrons into the so-called hollow atoms which rapidly decay by autoionization, at the surface loose outer electrons due to shielding, and eventually become de-excited inside the solid. At insulator surfaces, MCI neutralization and de-excitation is less well understood. Recently measured electron number statistics for MCI impact on clean metal (Au) as well as insulator (LiF) surfaces deliver interesting new information.

Slow electrons produced by slow multicharged ions on a clean metal surface

In this progress report we discuss total electron yields for impact of slow ( ν p ≤ 2 × 10 5 m s −1) multiply charged ions (e.g. fully stripped Ne 10+ or Ar 18+) on clean polycrystalline gold, which have recently been determined from the related slow electron emission statistics. Within the classical over-barrier model for ion-surface interactions, these data are utilized to unfold a fairly complete scenario of the processes initiated by impact of slow multicharged ions on a clean metal surface, on which basis the roles of slow and fast electrons and X-ray photons as different messengers for the involved electronic transitions will be discussed.

Neutralization of slow multicharged ions at a clean gold surface: Total electron yields.

Total slow-electron (${\mathit{E}}_{\mathit{e}}$\ensuremath{\le}60 eV) yields as derived from measured electron-emission statistics are presented for impact of various slow (${\mathit{v}}_{\mathit{p}}$\ensuremath{\le}${10}^{5}$ m/s) multicharged ions (${\mathrm{N}}^{5+}$,${\mathrm{N}}^{6+}$,${\mathrm{Ne}}^{5+}$--${\mathrm{Ne}}^{10+}$, ${\mathrm{Ar}}^{5+}$--${\mathrm{Ar}}^{16+}$, ${\mathrm{Kr}}^{5+}$--${\mathrm{Kr}}^{10+}$, ${\mathrm{Xe}}^{6+}$,${\mathrm{Xe}}^{8+}$,${\mathrm{Xe}}^{10+}$, and ${\mathrm{I}}^{16+}$,${\mathrm{I}}^{20+}$,${\mathrm{I}}^{23+}$,${\mathrm{I}}^{25+}$) on atomically clean polycrystalline gold. The experimental data can successfully be modeled by classical over-barrier-type calculations, in which way three different electron-emission processes have been identified, i.e., autoionization of the transiently neutralized projectile (``hollow atom'') on its approach toward the surface, promotion into vacuum of electrons captured by the projectile, and ``peeling off'' of electrons remaining bound in highly excited states until projectile impact on the surface. The relative importance of these three processes depends strongly both on projectile species and impact velocity.

Neutralization of slow multicharged ions at a clean gold surface: Electron-emission statistics.

Emission of slow electrons (${\mathit{E}}_{\mathit{e}}$\ensuremath{\le}60 eV) induced by impact of slow multicharged ions (impact velocity ${\mathit{v}}_{\mathit{p}}$\ensuremath{\le}2\ifmmode\times\else\texttimes\fi{}${10}^{5}$ m/s) onto an atomically clean, polycrystalline gold surface has been studied both experimentally and by numerical simulation, based on the resulting electron-emission statistics. The projectile ions (${\mathrm{N}}^{\mathit{q}+}$, q=5,6; ${\mathrm{Ne}}^{\mathit{q}+}$, q=5--10; ${\mathrm{Ar}}^{\mathit{q}+}$, q=5--16; ${\mathrm{Kr}}^{\mathit{q}+}$, q=5--10; ${\mathrm{Xe}}^{\mathit{q}+}$, q=6,8,10; ${\mathrm{I}}^{\mathit{q}+}$, q=16,20,23,25) have been extracted from a recoil ion source pumped by the GSI UNILAC heavy-ion accelerator in Darmstadt, Germany. We discuss the shape of the experimentally obtained electron-emission statistics and, by means of numerical simulation based on the classical over-the-barrier model put forward recently by Burgd\"orfer, Lerner, and Meyer [Phys. Rev. A 44, 5674 (1991)], identify the various processes contributing to the ``above-surface'' electron emission, i.e., taking place until projectile impact on the surface. In particular, for impact of slow (E\ensuremath{\ge}50 eV) ${\mathrm{Ar}}^{12+}$ we show that most of the emitted electrons have energies below 50 eV, with the above-surface-produced fast Auger electrons being a small minority of less than 1%.

X-ray emission from multicharged ions interacting with solids

Systematic studies of X-ray emission in the interaction of various slow multicharged ions (MCI) with metallic surfaces were performed at various energies. A low resolution/high efficiency Si(Li) detector and a high resolution X-ray spectrometer have been used to detect the X-ray emitted by the ions. The results allow one to differentiate between X-rays emitted from the bulk (ion bulk interaction (IBI)) and from ions experiencing an interaction above the surface (ion surface interaction (ISI)). Another example for IBI at “high” vertical velocity is the observation of two electron-one photon double K α (or K αα) emission with low energy MCI.

Slow electron emission from slow hollow atoms produced near a clean metal surface

Statistics of electron emission have been measured for bombardment of a clean polycrystalline gold surface by slow (impact velocities 1–15 × 10 4 m/s) highly charged recoil ions N q +( q≤ 6) a, Ne q +( q≤10), Ar q +( q≤16), Kr q +( q≤10) and I q +( q≤ 25) produced with the GSI UNILAC heavy ion accelerator, from which precise absolute total electron yields have been derived. As an example, with 150 eV I 25+ a total electron yield of 70 with about 1% probability for emission of ≥ 85 electrons/ion was observed. A semi-classical over-barrier approach recently introduced by Burgdörfer et al. 1991 has been further extended and successfully applied for modelling the impact-velocity dependences of measured total electron yields. In this way, new insights could be achieved on the neutralization and electron emission taking place during conversion of slow multicharged ions into hollow atoms and their subsequent rapid de-excitation in front of a metal surface.

Remnant charge of slow multicharged ions scattered from a gold surface.

A time-of-flight technique has been used to measure the remnant charge of laser-produced pulses of multicharged C k+ (k=1-4), Al m+ (m=3-6), and Pb n+ (n=2-6) ions incidenton a clean amorphous Au surface under UHV conditions. Remnant charge states were investigated for 15 o , 30 o , and 45 o scattering angles for C and Al ions and 15 o only for Pb. For all angles considered, C and Pb were found to be completely neutralized after scattering from the Au surface

Status of the ECR ion source at the Uppsala cyclotron

The room-temperature ECR ion source at The Svedberg Laboratory, Uppsala, is being used as injector for the Gustaf Werner cylcotron as well as for atomic-physics experiments. In November 1990 the first heavy-ion beams from the ECR ion source were accelerated in the cyclotron. Nuclear-physics experiments have now taken data using beams of He2+, O5+, and N6+. The transmission through the cyclotron is typically 5%. The present energy record is 450 MeV N6+. The energy of the heavy-ion beams will be increased further by acceleration in the CELSIUS storage ring. Examples of atomic-physics experiments in preparation are emission spectroscopy of highly ionized atoms, in particular, to study the transitions between Rydberg states, and studies of electron transfer processes in collisions of slow multicharged ions with atoms. The studies of yrast transitions are of interest in the field of x-ray lasers and one motivation for the electron transfer studies is the importance of these phenomena in astrophysics and fusion research.

What ions experience on their way into a metal surface

We discuss interaction of slow multicharged ions with metal surfaces, by considering recent experimental results for emission of electrons and soft x-ray photons as well as charge states of scattered particles. Simple concepts are developed to explain the main processes which take place before, during and after projectile impact on the surface.

Atomic physics with ECR ion source in Uppsala

An ECR (Electron Cyclotron Resonance) ion source has recently been installed in Uppsala at The Svedberg Laboratory. The ion source will be used both as an injector for a K = 200 synchrocyclotron and to provide ion beams for atomic physics experiments. So far three different research programs in atomic physic are planned and a laboratory in connection to the source is presently under construction. After a short description of the ion source and the atomic physics laboratory, the three programs • Charge transfer mechanisms in slow collisions (J Nordgren) • Optical spectroscopy and laser excitation of doubly and multiply ionized atoms (0 Vogel) • Sputtering of organic solids with slow multi-charged ions (B Sundquist) are briefly presented.

Charge exchange at molecular-orbital pseudocrossings as an important mechanism for nonkinetic-electron emission in slow-multicharged-ion (v

Kinetic energy distributions of electrons emitted in the interaction of slow (10q-keV) multicharged ions (N,O,Ne,Ar) with metal surfaces (Cu,Au) are presented. We observe secondary-electron emission, quasielastic-scattering processes, and, superimposed upon this, one-center Auger decay of both projectile and target vacancies. Two classes of target Auger-decay features are observed. Long-lived target vacancies decay, leading to discrete Auger line features, similar to those arising from electron-impact excitation. Short-lived target vacancies appear to decay in the field of the projectile, leading to broadened Auger line features. Observation of the decay of target vacancies, and their correlation with the presence of particular projectile vacancies, shows that a measurable fraction of projectile vacancies survive until small-impact-parameter collisions with surface or subsurface atoms. The data imply that projectile neutralization proceeds via two channels, viz., capture of valence-band electrons to projectile excited states, followed by one-center Auger decay, and at pseudocrossings of molecular orbitals correlating with discrete inner-shell levels of the target and projectile.

Electron capture by slow multicharged ions: core effect on final 1 distributions

The authors have calculated sigma nl partial cross sections for collisions Iq++H to I(q-1)+(nl)+H+ in the keV amu -1 impact energy range (Iq+=C4+, N5+, O6+, C6+, O8+, Ne8+ and Ar8+). The molecular expansion has been used with a basis set of OEDM orbitals and a specific form of the common translation factor. They have investigated the competition between the Stark and core-electron interactions by comparing the results obtained for bare and non-bare projectiles with the same charge q. The most spectacular result concerns the increase of the population of the ns sublevels dominantly populated by capture at low impact energies (v<0.4 au). A very good agreement has been found between the present calculations and the experimental data. In the velocity range 0.2<v<0.7 au the agreement is very good with other theoretical calculations when available.

Two- and more-electron transitions in slow multicharged ion-He collisions

Cross sections for double-electron capture into bound states and for simultaneous single-electron capture and target ion excitation have been measured for impact of slow (0.5–6 keV/amu) multicharged ions C 4+, N 4+, N 5+, O 5+ on He. Results are discussed in connection with a recent extended classical over-barrier model for multiple-electron capture, and by consideration of relevant quasimolecular potential energy curve crossings.

Observation of fractional neutralization of slow multicharged ions by impact on a metal surface

We report the first observation of energy losses and fractional neutralization of slow multicharged ions incident on a metal surface. Data were obtained on 374 eV per charge aluminum 3≤z≤6 ions incident on an uncharacterized gold surface at a grazing angle of 7° and scattered through 14°. The resulting atoms consisted of about 60% neutral and 40% singly charged ions in the case of incident z=4 ions. The scattering process did not appreciably broaden the ion linewidths. The apparatus makes use of a laser generated heavy ion source.