Hollow Atoms Research Articles

Corrections to the image potential for ions

A short review is given of various corrections to the image potential felt by an ion incident on a solid. First of all I will give the general frameworks for calculating the real (image interaction) and imaginary (energy loss) parts of the interaction between an ion and a solid. I will then describe velocity dependent effects such as dynamical screening, trajectory description and retardation. After that I will discuss aspects that are connected to the extension of the ionic and solid charge distributions, as well as the effective charge (transfer) and the possible nonlinear screening effects. Finally, I address the concept of hollow atoms in the context of highly or multiply charged ions.

Multi-electron ejection of inner-shell electrons through multiphoton excitation of clusters

The existence of coherent electronic motions induced by the presence of a strong external optical field enables the process of direct multi-electron ejection from an inner-shell to occur with high probability. Experimental evidence for this mechanism is present in the observed spectra of five cases studied (Kr(M), Kr(L), Xe(N), Xe(M) and Xe(L)). Analysis shows that the multiphoton interaction can enter a regime of strong-coupling (Z2a>or=1) in which the rate of multiple electron ejection can become comparable to the removal of a single electron. The enhanced coupling arises from the coherent motion of the (Z) field-ionization electrons whose interactions constructively interfere, thereby behaving like a quasi-particle with a charge Ze. This mechanism consistently explains (1) the unusually high values of Xe M-shell ionization observed in both Xe(M) (Xe36+) and Xe(L) (Xe45+) emissions and (2) the anomalous double 2p vacancy production seen in the Xe(L) spectrum. Clusters of heavy atoms appear as exceptionally favourable systems for the copious production of multiple vacancy hollow atom configurations.

Charge exchange and electron emission in slow collisions of highly charged ions with C60

We simulated the population dynamics of target and projectile levels during the interaction of slow, highly charged ions and C60. Our investigation is based on a semiclassical over-barrier description of long-range charge exchange and a spherical model for the C60 cluster. It includes the transient formation of hollow atoms and relaxation of the projectile due to Auger cascades. As a first application, we consider slow Ar8+ projectiles and give estimates for the occupation changes of all coupled target and projectile levels, charge-state evolutions and energy-resolved spectra of emitted Auger electrons. Our results are in satisfactory agreement with recent experimental data.

Coster-Kronig transitions in hollow atoms created during highly charged ion-surface interactions.

We present high resolution KLL Auger spectra emerging from collisions of H-like and metastable He-like C, N, and O on a B-doped Si(100) surface. In combination with the high resolution electron spectroscopy, Hartree-Fock calculations allow us to succesfully identify KLL Auger electrons arising from specific states that form a part of the autoionization cascade as the multiply excited projectiles deexcite. We find strong evidence for Coster-Kronig transitions taking place in the hollow atoms which give rise to missing peaks in the Auger spectra.

Multiphoton-induced X-ray emission at 4–5 keV from Xe atoms with multiple core vacancies

SEVERAL recent experimental findings1–3 have pointed to a possible route for making an X-ray laser, which could in principle provide an imaging system capable of molecular resolution4. The method involves the multiphoton excitation of atoms in van der Waals clusters or in molecules to yield ions with core-electron vacancies1,2, which can then decay by emission of X-rays, in conjunction with a self-chanelling propagation mode of electromagnetic radiation3. The multiphoton excitation may be stimulated by ultrahigh-brightness, subpicosecond pulses of laser light5. We have previously observed2 emission of X-rays from L-shell transitions in core-excited krypton atoms using this approach. Here we report the multiphoton production of X-rays of wavelength 2–3 A from highly ionized xenon atoms which possess a large number of innershell vacancies while retaining several electrons in relatively weakly bound outer orbitals. Atoms with this ‘inverted’ electronic configuration are designated ‘hollow atoms’6,7. We find that generation of hollow atoms can become the dominant excitation mode for such systems, making their exploitation in an X-ray laser a real possibility.

Electron emission induced by slow highly charged ions on a clean metal surface

A review is presented on recent investigations concerning the interaction of slow highly charged ions (HCI) with a clean metal surface. Detailed information on the generation and decay of transiently developing multiply excited “hollow atoms” can be gained from the measurements of total electron yields, electron energy distributions and, in particular, electron emission statistics. Complementary information on physical processes relevant for electron emission are provided by the projectile scattering characteristics and the emitted soft X-rays. A complete scenario of slow HCI-metal surface interaction, based on a recently developed classical over-the-barrier model, is discussed in the light of available experimental evidence.

On the mechanism of formation of hollow atoms below a surface

The interaction of Ar 17+, Fe 25+ and Kr 35+ ions on surfaces has been studied by looking at the X-rays emitted in flight by the ions with a SiLi detector. A model has been developed to describe the behavior of the ions inside the solid and the formation of “hollow atoms” below the surface.

Electron emission following the interaction of highly charged ions with a Pt(110) target.

We measured secondary-electron energy spectra of the H-like ions N[sup 6+], O[sup 7+], and Ne[sup [ital q]+]([ital q]=3,7,8,9,10) incident on a solid Pt(110) surface at different angles (10[degree]--90[degree]) using impact energies of 10[ital q] and 15[ital q] keV. Clear evidence is found for subsurface electron emission of the [ital KLL] Auger electrons. The stepwise filling of both [ital K]-shell holes of the Ne[sup 10+] projectile is observed in the electron energy spectrum. The dynamics of the filling of the [ital L] and [ital K] shells in the solid is studied for the Ne[sup 10+] ion. It is found that hollow Ne atoms are formed inside the solid. With decreasing incident charge state of the Ne projectile the [ital LMM] Auger lines are shifted to lower energies due to the different screening of the nuclear charge, which is consistent with corresponding Hartree-Fock calculations.

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.

Interaction of Ar17+ ions on metallic surfaces at grazing incidence.

The interaction of Ar[sup 17+] ions on metallic surfaces has been studied at various incidence angles by looking at the x rays emitted in flight by the ions with a crystal spectrometer. A model has been developed to describe the behavior of the ions outside the surface, and Dirac-Fock calculations of the Auger rates for states with many electrons in high-[ital n] states have been performed. The formation of [ital hollow] [ital atoms] outside the surface, in high-[ital n] states, has been observed at grazing incidence, in contrast to the hollow atoms formed below the surface in lower-[ital n] states ([ital n]=3--4). The decay to the ground state of these very excited ions outside the surface represents less than 1% of the overall decays.

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.

Deexcitation of hollow atoms: autoionization of multiply excited N, O and Ne atoms at a Pt(110) surface

The K-Auger electron emission during the interaction of 60 keV N 6+, 70 keV O 7+ and 90 keV Ne 9+ with a Pt(110) surface occurs below the target surface for glancing angles in the range 10–90°. Therefore the projectile K-Auger electron spectra are distorted by solid state effects. These effects can be removed from the measured spectra by using a procedure originally developped for photoelectron spectroscopy. The resulting primary excitation spectra are compared with results of atomic structure calculations. The analysis reveals that at the instant of K-Auger electron emission on the average five electrons are present in the L-shells of the hydrogen-like N, O and Ne projectiles. Furthermore the ratio of the KLL- and KLM-Auger electron emission yields is determined. It is 3.4, 3.7 and 4.6 in case of the N 6+, O 7+ and Ne 9+ projectile, respectively.

Characterization of a helium discharge for hollow anode furnace atomization non-thermal excitation spectrometry

A discharge in He has been characterized for hollow anode furnace atomization non-thermal excitation spectrometry (HA-FANES). The small surface area of the cathode and high current densities make it possible to stabilize He discharges up to 1200 hPa using a d.c. power supply operating in the constant current mode. Current—voltage characteristics were determined for pressures up to 800 hPa. Characterization at higher pressures was not possible owing to limitations of the power supply. The release of thermionic electrons from the cathode surface at temperatures in excess of 1750 K caused a dramatic decrease in the discharge potential during the atomization cycle. Increased ohmic heating of the cathode with increased currents caused a time shift in the release of thermionic electrons. Excitation temperatures, obtained from Boltzmann plots of five He I lines, decreased slightly as a function of increasing pressure and increased as a function of increasing current. Dramatic decreases in the discharge potential were observed for high concentrations (250 mg ml–1) of NaCl. The potential decreases were too short-lived to permit the accurate measurement of excitation temperatures. Integrated analytical signals increased linearly with increasing pressure.

Above-surface neutralization of highly charged ions: The formation of hollow atoms

We investigate the interaction of multiply charged ions with metallic surfaces. Simulations are presented displaying the formation of hollow atoms when slow highly charged ions approach the surface. It is shown that above surface neutralization proceeds via hollow-atom formation. Relaxation of the multiply excited states to the ground state occurs only subsequent to the close encounter with the topmost atomic layer. Recent experimental data on K Auger emission and image acceleration confirm this scenario.

Image acceleration of multiply charged ions by metallic surfaces.

We present a simulation for the acceleration of multiply charged ions approaching metal surfaces by the interaction with its image charge, based on the classical over-barrier model. The image attraction leads to a gain in velocity perpendicular to the surface. The amount of energy gain is directly related to the distance of formation of hollow atoms. The role of a transient hollow negative ion is discussed. Comparison with recent data by Winter [Europhys. Lett. 18, 207 (1992)] shows good agreement.

Electron emission from slow hollow atoms at a clean metal surface.

For impact of slow multicharged N, Ne, Ar, and I ions on clean polycrystalline gold (impact velocity 1\ifmmode\times\else\texttimes\fi{}${10}^{4}$${\mathit{v}}_{\mathit{p}}$15\ifmmode\times\else\texttimes\fi{}${10}^{4}$ m/s, for the first time the statistics of the resulting electron emission have been determined, from which rather precise total electron yields have been derived. From the impact-velocity dependences of yields and the shapes of electron multiplicities two different contributions to electron emission are identified, one probably being due to autonization before surface impact and the other due to faster autoionization just at the surface or already inside the solid.

Auger decay of slow highly-ionised ions neutralised at surfaces

We present results of calculations of decay rates for hollow atoms which could be formed when an ion approaches a surface and we discuss the implications of these results for the interpretation of recent experiements concerned with the neutralisation of slow highly charged ions at surfaces. We find that multiply excited states in neutral atoms have lifetimes considerably shorter than those in the corresponding ions and point out that this property can explain the large number of low-energy electrons, which have been observed experimentally. In addition, the lifetimes appear to be of the right order of magnitude to explain the observed time development of the neutralisation process. We consider also the implications for the high-energy spectrum.

Determination of trace metals by laser-excited fluorescence with hot hollow cathode atomization

The possibilities of hot hollow cathode atomization combined with laser-excited fluorescence detection for analysis of solutions and powdered samples are discussed. Optimum analytical conditions (discharge gas, its pressure and cathode temperature) were chosen. Significant improvement of signal-to-background ratio was provided by switching off the discharge current with excitation of fluorescence in the resulting afterglow. Detection limits of Co and Ni in dry residues of aqueous solutions were 15 pg ml and 5 ng ml , respectively. The possibility of Co trace determination in powdered Al 2O 3 and SiO 2 was shown.

Study of the interaction of very slow hollow atoms with a solid

Study of the interaction of very slow hollow atoms with a solid

Emission characteristics of chromium in hollow anode-furnace atomization non-thermal excitation spectrometry

Emission parameters for the determination of Cr using hollow anode-furnace atomization non-thermal excitation spectrometry (HA-FANES) were investigated. The spectral background between 280 and 400 nm included strong emission bands from CN, NH and CH. The best signal-to-noise ratio for Cr was observed for the atomic line at 359.3 nm. Argon atomic emission (420.1 nm) showed no decrease in intensity in the presence of thermionic electrons while Ar ionic emission (434.8 nm) showed a dramatic decrease in intensity. The integrated Cr emission signal exhibited a broad maximum between atomization temperatures of 1800 and 2100°C. Peak areas increased with pressure with the best signal-to-noise ratio found at 160 Torr (21 300 Pa). The discharge current ranged from 10 to 90 mA with the optimum current dependent on the pressure. A limit of detection of 0.8 pg was obtained for an Ar pressure of 160 Torr, a current of 70 mA, and an atomization temperature of 2000°C. This limit of detection is comparable to conventional GFAAS and a factor of five better than hollow cathode FANES (HC-FANES). The accuracy of the technique was assessed by the determination of Cr in a standard reference water (SRM 1643b).