Slow Multicharged Ions Research Articles

Energy dependence of multi-electron transfer reactions between slow multi-charged ions and neutral atoms

Using an ion-spectroscopy method, charge transfer reactions between slow multi-charged Arq+(q=3~5) ions and neutral Ar atoms have been studied in the laboratory collision energies from 3.1 to 4.6keV. The energy gain spectra of single-electron capture processes show no strong energy dependence. On the contrary, those of double-electron capture processes depend on the collision energy.

Desorption and sputtering on solid surfaces by low-energy multicharged ions

Desorption and sputtering on solid surfaces interacting with slow multicharged ions were studied by two different experimental methods: time-of-flight secondary ion mass spectrometry (TOF-SIMS) coupled with low-energy ion scattering spectroscopy (LEIS), and optical emission spectroscopy (OES) of secondary particles. Spectra reveal the anisotropic angular dependence of proton emission on an Al surface interacting with Arq+ (q = 8 and 14) ions at the incidence angle of ∼0.5° (tangential angle to the surface). Simultaneous spectra of a GaN surface, scattering Ar+ ions and scattering protons reveal the kinetic energy distribution of Ar6+ (15 keV, ∼10°) and the dependence of the protons on the kinetic energy of Ar+. Spectra of secondary particles, conducted on a Ti surface irradiated with Ar3+ ions (30 keV) at normal-incidence angle, reveal the mean velocity < v⊥ > of sputtered Ti* substrate atoms perpendicular to the surface, measured by analyzing intensity decay as a function of distance from the surface.

Isomeric effects in ion-induced fragmentation of α- and β-alanine

We have investigated the dissociation of α- and β- alanine following impact of slow multicharged ions, namely He+, He2+, O5+ and Xe20+ at 10 keV per charge unit. The collision products were analyzed using a reflectron-type time-of-flight mass spectrometer. In general, for a given projectile, ionization of both alanine-isomers gives rise to similar fragmentation patterns. However, significantly different peak intensities are observed for fragments with mass over charge ratios (m/q) of 30 and 44 amu. These differences can be explained in terms of isomer geometry and fragment stability. By analyzing the fragment kinetic energies, we found that protons as well as ionic oxygen fragments are sufficiently energetic (up to 200 eV for Xe20+) to induce further damage in a biological environment.

Compact 14.5 GHz all-permanent magnet ECRIS for experiments with slow multicharged ions

A compact 14.5 GHz electron cyclotron resonance ion source (ECRIS) for production of multiply charged ions (MCI) with a plasma-confining magnetic field generated by permanent magnets has been constructed. Microwave power with frequency between 12.75 and 14.5 GHz is transmitted from ground potential via an insulating window into the watercooled plasma chamber fitted with an aluminium liner. The HF coupling system serves as biased electrode. Operation in the gas-mixing mode is achieved with two remotely controlled gas inlet valves. The triode ion extraction system has been optimized for low acceleration voltages between 1 and 10 kV. The ECRIS is fully computer-controlled and can be remotely operated via Ethernet. Finally, we refer to recent experimental work on collisions of slow MCI generated by the new ECRIS with neutral gas particles and surfaces.

Multiple Charge Transfer by Slow Multi-Charged Xe Ions

Electron capture processes between low energy multi-charged ions and atoms are studied by an energy loss spectroscopy technique. Energy-loss spectra of charge-state-changed projectile ions scattered in the forward direction in collisions of Xeq+(up to q = 11) ions with He and Ar atoms have been measured at a collisional energy of 9 keV. The results are discussed using the classical over-barrier model.

Interaction of slow multicharged ions with surfaces

In recent years the interaction of slow multiply charged ions with solid surfaces has gained renewed interest. Basic understanding of charge transfer processes involved in the neutralization sequence has emerged in the 1990s. In this manuscript we present a short overview over refined experimental and theoretical methods which have been developed recently to study subtle details of the interactions.

Inelastic interactions of slow ions and atoms with surfaces

Interaction of slow atoms, singly- or multiply charged ions with a solid surface pumps – via nuclear and electronic stopping processes – part of the projectile energy into the target system. In this way ejection of slow electrons and target particles (sputtering) can be initiated. We discuss the relevant stopping processes and their interrelation with the projectile charge. Recently developed coincidence techniques are used for detailed studies of slow particle induced kinetic (KE) and potential electron emission (PE) from the selvage of mono-crystalline metal and insulator surfaces. We also present a model study on target-structure dependent KE from highly-oriented pyrolythic graphite (HOPG), and shortly review slow multicharged ion induced desorption from insulator surfaces (potential sputtering – PS).

A cavitation mechanism of material sputtering by slow multicharged ions

A thermodynamic approach to the sputtering of materials by slow multicharged ions is developed based on the cavitation mechanism of fracture of the surface layer of a target. It is shown that a strong electric field of a slow multicharge ion approaching the surface of a dielectric target leads to the formation of an extended metastable subsurface region. Cavities spontaneously appearing in this region form a percolation cluster leading to the fracture (cavitational electroexplosive erosion) of the target material. Universal relationships established between the volume of the region of fracture, on the one hand, and the ion charge and the target surface properties, on the other hand, qualitatively agree with experimental data on the sputtering of LiF and SiO2 by slow argon and xenon ions.

Slow multicharged ions hitting a solid surface: From hollow atoms to novel applications

Slow multicharged ions hitting a solid surface: From hollow atoms to novel applications

Ultra-thin SiO2 layers formation by ultra slow single and multicharged ions

Ultra-thin SiO2 layers formation by ultra slow single and multicharged ions

Comparative study of surface-lattice-site resolved neutralization of slow multicharged ions during large-angle quasi-binary collisions with Au( [formula omitted]): Simulation and experiment

In this article we extend our earlier studies of the azimuthal dependences of low energy projectiles scattered in large angle quasi-binary collisions (BCs) from Au(1 1 0). Measurements are presented for 20 keV Ar 9+ at normal incidence, which are compared with our earlier measurements for this ion at 5 keV and 10° incidence angle. A deconvolution procedure based on MARLOWE simulation results carried out at both energies provides information about the energy dependence of projectile neutralization during interactions just with the atoms along the top ridge of the reconstructed Au(1 1 0) surface corrugation, in comparison to, e.g. interactions with atoms lying on the sidewalls. To test the sensitivity of the agreement between the MARLOWE results and the experimental measurements, we show simulation results obtained for a non-reconstructed Au(1 1 0) surface with 20 keV Ar projectiles, and for different scattering potentials that are intended to simulate the effects on scattering trajectory of a projectile inner shell vacancy surviving the BC. In addition, simulation results are shown for a number of different total scattering angles, to illustrate their utility in finding optimum values for this parameter prior to the actual measurements.

A reflection lithography using multicharged ions

A new approach is proposed for lithography with an objective of high throughput and minimum feature size 50 nm and below — the Ion Reflection Lithography (IRL) using multicharged ions. The concept is based on a recent fundamental discovery of a special characteristic of multicharged ion interaction with solid surface: Very Slow Multicharged Ion (VSMCI) beams, i.e. in the eV/q energy range, can be selectively reflected on a nanometric scale from semiconductor and metal substrates. We present in this paper the basic processes of the interaction between a VSMCI and surfaces, and discuss the property of VSMCI to be reflected selectively in dependence on the conductive character of the substrate. The patterned ion beam formation which relies on an innovating reflective reticle is described and a first outline of an IRL system is presented.

HCI-Induced Molecule Fragmentation: non-Coulombic Explosion and Three-Body Effects

The study of Highly Charged Ion-induced diatomic (CO) and triatomic (CO2) molecules fragmentation by the coincident measurement of the fragment momenta is presented. It is shown that the experimental results together with ab initio calculations including a large number of potential energy curves evidence the limitation of the Coulomb Explosion model to reproduce the dynamics of the fragmentation. The geometry modification of the CO2 molecule during the breakup is explored and the concerted or sequential nature of the dissociation is discussed for two fragmentation channels. Finally, the fragmentation of the H2 molecules following collisions with slow multicharged ions is analysed for various projectiles and energies. From the energy spectra, the effect of the projectile on the H+ fragments is evidenced. This three-body effect is discussed in connection with the results of two model calculations including or not the role of the projectile.

Interaction of Slow HCI with Solid Surfaces: What Do We Know, What Should We Know?

A slow multicharged ion (MCI) which approaches a solid surface catches electrons from the latter into highly excited states and transiently becomes a neutral “hollow atom” (HA). Rapid autoionisation of this HA will be balanced by further ongoing electron capture until close surface contact where shielding sets in, and inner-shell vacancy recombination at and below the surface produces further slow electrons and characteristic fast electrons and soft X-rays. Until its complete neutralization the projectile continues to be attracted toward the surface by its decreasing image charge which causes a minimum impact velocity. Many aspect of this HA scenario have been studied by a number of groups for different ion species and - charge states at metal-, semiconductor- and insulator surfaces with a number of experimental methods. The most detailed comparison with available theory is possible if electron emission and projectile energy losses and -charge changes are studied in mutual coincidence for scattering along well defined trajectories at smooth mono-crystalline target surfaces. For insulator surfaces so called “potential sputtering” (PS) and strong secondary ion emission can be induced by the multi-electron capturing MCI. Our current understanding of the above-surface part of MCI-surface interactions is satisfactory and can also explain how “free” hollow atoms and -ions are produced by funneling MCI through thin capillaries or colliding them on clusters or fullerenes. Because of the approximately additive influence of potential- and kinetic ion energies on the resulting electron emission, for this part a straighforward definition of the term “highly charged ion” (HCI) can be given. On the other hand, processes occuring at and below the surface are so far less well understaood since they depend on the potential- and kinetic ion energies in a clearly non-additive way. This is particularly true for PS and excitation of target plasmons due to the potential MCI energy. PS and related phenomena are also of practical interest for new surface-analytical and nano-technological applications.

Oxide nanodots and ultrathin layers fabricated on silicon using nonfocused multicharged ion beams

We have used slow multicharged argon ions in ultrahigh vacuum with a partial pressure of oxygen to form ultrathin (subnanometer) oxide dots of a few tens of nanometer diameter on a silicon surface. The main characteristic of this technique is that the incident multicharged ion does not penetrate below the surface, so there is no implantation-induced modifications of the substrate. Thus, we utilized a unique possibility provided by the multicharged ions interaction with surfaces of solids, the noncontact trampoline effect, to open the bonds of hydrogenated silicon, and replace it with oxygen. This article presents the fundamentals of slow multicharged ion interaction with a surface, the experimental multicharged ion beam line build at X-ion laboratory, and obtained results of oxide dot formation on silicon, using them as a mask in a reactive ion etching process to grow three-dimensional crystalline silicon structures. Potential applications are in nanoflash–multidot type nonvolatile memories.

Potential sputtering: desorption from insulator surfaces by impact of slow multicharged ions

For certain target species, sputtering by slow multiply charged ions strongly depends on the primary ion charge state. In contrast to the usual ballistic sputtering it is not the kinetic but rather the potential energy stored in multiply charged ions that leads to this novel form of ion-induced sputtering (“potential sputtering”). A series of recent careful experiments is summarized where potential sputtering has been investigated for hyperthermal highly charged ions impact on Au, Si, GaAs, SiO 2, MgO, LiF, and NaCl. Only for alkali halides (LiF, NaCl) and to some extent for SiO 2 an enhancement of the total sputter yield, which is measured by a quartz crystal microbalance technique, with increasing charge state of the primary ion could be observed. All other targets showed only the common (collision induced) kinetic sputtering. With a defect mediated desorption model as known from electron stimulated desorption we can explain why potential sputtering is exclusively found for insulators with strong electron–phonon coupling.

Electron emission from a clean gold surface bombarded by slow multiply charged fullerenes

Total electron emission yields have been measured for impact of carbon cluster ions C i q+ on atomically clean polycrystalline gold as a function of projectile size ( i≤ 60), charge ( q ≤5) and kinetic energy ( E≤ 165 keV). Electron emission arises already far below the classical threshold for common kinetic emission, probably because of quasi-molecular autoionization in close collisions of projectile constituents with individual target particles at the surface. At a given impact energy, there is a linear dependence of electron yield on carbon cluster size. However, there is no influence of the projectile charge and, thus, apparently no potential electron emission, in striking contrast to the impact of slow multi-charged atomic ions on solid surfaces. We accredit this suppression of potential emission to a rapid transfer of the fullerene electronic excitation being gained during surface neutralization into projectile vibrational degrees of freedom and subsequent fast fragmentation. Molecular dynamics calculations show that this fragmentation takes place within picoseconds after surface impact, which is much faster than the established rates for thermionic electron emission from collisionally heated fullerenes.

Auger and radiative filling rates of highly charged ions below metal surfaces

We evaluate quantitatively the role of Auger and radiative recombination processes in the neutralization of slow multicharged ions in metals. The strong perturbation of the valence-band electrons by the ion requires a nonlinear treatment which is carried out using density functional theory. We find that this perturbation creates an electronic cloud around the ion which has an atomiclike character. We show that it is this phenomenon which determines the order of magnitude of the transition rates, as illustrated by the evaluation of Auger and radiative rates for Ne and Ar ions.

Hollow atom spectroscopy

A slow multicharged ion (MCI) approaching a solid surface will be converted by multiple electron capture from the latter into a transiently multiply excited, dynamically neutral atom. The resulting "hollow atom" decays rapidly in autoionisation cascades which give rise to abundant emission of slow electrons. Upon its arrival at the surface, the hollow atom will be rapidly squeezed to a much smaller size because of shielding by the quasi-free target electrons, then further de-excited and eventually fully relaxed below the surface. During these later de-excitation phases further slow electrons are emitted in conjunction with fast Auger electrons and soft x-ray photons, which result from recombination of projectile inner shell vacancies. All these processes, which take place during the rather short lifetime (typically picoseconds) of the hollow atom, can be elucidated by spectroscopy of the emitted electrons and soft x-ray photons, and from the kinematics (scattering angles) and dynamics (charge states and energy losses) of projectiles scattered on atomically flat target surfaces. Based on presently available experimental evidence, generation and decay of these hollow atoms can be reasonably well described within a semi-classical "over-the-barrier" model.

Interaction of slow multicharged ions with solid surfaces

The present report deals with the main aspects of the interaction of slow (impact velocity typically below 1 a.u.) multicharged ions (MCI) with atomically clean solid surfaces of metals, semiconductors and insulators. It is based to a large extent on the results obtained by the authors and their affiliates within the Human Capital and Mobility Network of the European Union on “Interaction of Slow Highly Charged Ions with Solid Surfaces”, which has been carried out during the last three years. After briefly reviewing the pertinent historical developments, the experimental and theoretical techniques applied nowadays in the field of MCI-surface interaction studies are explained in detail, discussing especially the transient formation and relaxation of “hollow atoms” formed in such collisions. Further on, the status of the field is exemplified by numerous results from recent studies on MCI-induced emission of slow and fast electrons (yields and energy distributions), projectile soft X-ray spectroscopy, charge-changing and energy loss of scattered and surface-channelled projectiles, MCI-induced sputtering and secondary ion emission, and coincidence measurements involving different signatures from the above processes. The presented theoretical and experimental work has greatly contributed to an improved understanding of the strongly inter-related electronic transitions taking place for MCI above, at and below a solid surface.