Hollow Atoms Research Articles

X-ray emission in lithium-like and helium-like argon ions colliding with molybdenum surfaces

We have measured the spectra and yields for Ar K and Mo L x-rays from Ar 15+ and Ar16+ ions colliding with molybdenum (Mo) surfaces . The spec tra and yields are measured as a function of the projectile kinetic energy. In t he case of Ar15+ -Mo, both Ar K x-rays and Mo L x-rays are observe d, and the yields of Ar K x-rays are related to the projectile kinetic energ y, the potential energy of hollow atom formed in the interaction, the competitio n of the x-rays emitting from the projectile and the target etc. But in the case of Ar15+-Mo, Mo L x-rays are observed only. The yields of Mo L x- rays induced in both cases increase with the projectile kinetic energy.

Theoretical study of the application of hollow atom production to the intensity measurement of short-pulse high-intensity x-ray sources

We perform a theoretical study for the measurement of short-pulse high-intensity x-ray sources through the x-ray emission processes from multi-inner-shell excited states (1s22s22pk3s23p2, k = 1–4) and hollow atoms (1s22s23s23p2) of Si. We discuss the effect of weak-intensity long-pulse x-rays mixed with high-intensity short-pulse x-rays and the ratio of the x-ray emission from multi-inner-shell excited states with that from hollow atoms.

Relativistic Energy of Highly Triply Excited 3l3l′nl′′ 2 P o (m) (m = 1–5)States of Lithium

The high-lying triply excited 3l3l′nl′′ 2 P o (n⩾3) states ofthe double hollow lithium atom are studied by using the saddle-pointvariational method. Relativistic corrections are evaluated with thefirst-order perturbation theory, and the mass polarization effect isalso included. The dominant configuration mixtures of these triplyexcited states are given. The results are compared with othertheoretical and experimental data in the literature.

Decay of hollow atoms inNe10+−C60collisions

We have studied the decay of hollow atoms formed in ${\text{Ne}}^{10+}{\text{\ensuremath{-}}\mathrm{C}}_{60}$ collisions at low energy $(E=100\phantom{\rule{0.3em}{0ex}}\text{keV})$. The decay products of hollow atoms, i.e., the scattered projectiles and the number of ejected electrons from autoionization cascades, have been measured. The projectile energy loss has been analyzed as a function of the final charge state, in order to differentiate collisions at large distance, near the ${\mathrm{C}}_{60}$ through the electronic clouds and inside the ${\mathrm{C}}_{60}$ cage. For scattered projectiles ${\text{Ne}}^{8+}$ where two electrons are stabilized $(s=2)$, up to 16 ejected electrons have been observed, and lower energy loss compared to projectiles with more stabilized electrons $(sg2)$ has been measured. These results are interpreted by the formation and relaxation of completely neutralized compact hollow atoms passing very close to the ${\mathrm{C}}_{60}$ molecule (into the electron clouds). The observed large number of ejected electrons is explained by a direct filling of the $N$ shell during the interaction time and a fast decay via quasicomplete autoionization cascades.

The initiation of EBIS/T-based HCIresearch in Japan

This article reviews early EBIS/T-based HCIresearch in Japan designed to understand atomic processes of highly charged impurity ions in plasmas. In particular the work of the NICE (Naked Ion Collision Experiments) project is discussed. The NICE experiment showed for the first time that translational energy spectroscopy would give useful information about the intermediate multiple-excitedstates in the transfer ionization processes, and therefore it would be a powerful methodfor spectroscopy of so-called hollow atoms.

Statistical energy distribution model for the decay of hollow atoms formed in collisions of slow(v<1a.u.)highly charged ions andC60

It is now well known that electron-capture processes in high impact parameter collisions between slow highly charged ions and clusters (as ${\mathrm{C}}_{60}$ for instance) or surfaces populate high Rydberg levels of the projectile forming a so-called ``hollow atom.'' We present in this paper a statistical energy distribution (SED) model for the analysis of the decay of such hollow atoms. The overbarrier model is employed to estimate the initial binding energy of captured electrons and total geometrical cross sections. We show that although the model firstly derived by Russek was incomplete, a simple modification to it that accounts for hollow atom specifications leads to a better agreement with experimental cross sections over a wide range of projectile ions of different atomic numbers (Ar, Kr, Xe) and different charge states $(q=16--30).$ We present also a quantum-mechanics-based formulation of a SED model that allows us to reproduce rather nicely experimental cross sections too. The presented model applied better for high projectile charge states allowing the capture of a great number of electrons that populate a quasicontinuum of Rydberg states for which we do not have to take into account individual Auger transitions.

200 nm?1000 nm spectra of light emitted in the impact of 40Ar10+ upon Al and Si solid surfaces

This paper reports the measured results of the 200 nm–1000 nm characteristic spectral lines of Al, Si and Ar atoms when highly charged ions 40Ar10+ are incident upon Al and P-type Si surfaces. The ion 40Ar10+ is provided by the ECR ion source of the National Laboratory of the Heavy Ion Accelerator in Lanzhou. The results show that when the low-speed ions in the highly charged state interact with the solid surfaces, the characteristic spectral lines of the target atoms and ions spurted from the surfaces can be effectively excited. Moreover, because of the competition of the non-radiation de-excitation of the hollow atom by emitting secondary electrons with the de-excitation process by radiating photons, the spectral intensity of the characteristic spectral lines of Ar atoms on the P-type Si surface is, as a whole, greater than that of Ar atoms on the Al surface.

Electron emission from below the surface induced by highly charged ions: Effect of depth distribution of electron excitation and ion transport on the emission properties

Electron emission induced by the impact of highly charged ions on metal surfaces is due to different mechanisms. During the interaction of the ion with the metal surface, hollow atoms are formed above and below the surface. At low impact energies, autoionization processes in front of the surface mostly determine the emission properties. With increasing impact energy, excitation processes from below the surface gain in importance. When the hollow atom moves inside the metal, empty inner orbitals are successively filled by Auger transitions and collisional charge transfers. The filling dynamics of the hollow atom determines the strength of the excitation by Auger processes as well as the depth distribution of this excitation. The ion transport process in the first layers of the metal also influences this depth distribution. In this work, electron emission due to the release of K- and L-Auger electrons is calculated by Monte Carlo simulation for Ne9+ impact on Al. The transport of incident ions and of the excited electrons as well as the filling dynamics of the hollow atom are taken into account.

Excitation of triply excited Be+ (2lnl′n′l″) states in 300 and 500 keV-Be+ + CH4 collisions

We have studied high resolution beryllium Auger electron spectra in the 120–160 eV projectile electron energy range following intermediate energies (1.2 and 1.5 a.u. impact velocities) in Be++CH4 ion–molecule single collisions [Comments At. Mol. Phys. D 14 (1984) 117]. The major focus of this study is the double K-shell excitation of the Be+ projectile forming triply excited BeII*** states (hollow atoms) and BeIII** K shell ionization plus excitation of BeII doubly excited states with two inner shell vacancies. In such intermediate velocity ion-molecular collisions, the CH4 target plays an important role in the enhancement of the population of BeII*** triply excited states when compared to a more tightly bound He (1s2) 1S ground state target. Due to the pioneering theoretical work of Gou and Chung [Phys. Rev. A 29 (1996) 6103] most line structures can now be unambiguously identified. The main triply excited states are associated with the 2s22p, 2s2p2 and 2p3 and 2l2l′n′l″ (n=3 and 4) configurations decaying to different final BeIII (1snl) ionic states. The line identification method used here is based on Auger energy levels, decay channels and branching ratios.

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

Very fast hollow-atom decay processes in Xe30+-C60 collisions.

In Xe30+-C60 collisions at low velocity (0.2 a.u.<v<0.4 a.u.), very fast electron ejection is observed not only for frontal collisions but also for near C60 cage collisions. In frontal collisions, the exit-charge distributions obtained with three projectile velocities are reproduced using a multistep exponential decay model. A mean Auger rate of about 0.4 a.u. is estimated for the decay of hollow atoms during the very short interaction time (<3 fs).

Statistical properties of hollow atoms

We investigate the statistical properties of a prototype of a ``hollow atom,'' i.e., an atom having a large number of empty inner shells. In particular, we have carried out ab initio calculations of the positions and widths of the ${1s}^{2}4{\mathcal{l}}^{5}$ states of nitrogen. These states give rise to a dense spectrum of strongly overlapping resonances. Due to the large number of open channels, the statistical description of the system simplifies considerably. We find that the distribution of the nearest-neighbor energy-level spacings follows a Wigner distribution, while the widths of the states are narrowly distributed about the average perturbative width.

A microcapillary target as a metastable hollow ion source

Interaction of highly charged ions (HCIs) with solid surfaces has been studied with microcapillary metallic foils, which have many straight holes of ∼100 nm in diameter and ∼1 μm in thickness distributed regularly over the foil. It was found that some considerable fraction of ions can pass through the small capillary without suffering violent collisions with the capillary wall but at the same time capturing multiple electrons in their highly excited Rydberg states, i.e. hollow atoms (ions) are effectively extracted in vacuum. The study of ion–capillary interaction now gets even more promising because of new developments of nano-lithographic techniques, which allow preparing highly ordered microcapillaries. The interaction of HCIs with microcapillaries has been studied through (1) visible light measurements, which provide information on the very beginning of the charge transfer processes, (2) soft X-ray measurements, which reveal the electronic configuration at the moment of inner shell filling processes in a later stage and also (3) angular distribution measurements of each charge-changed component, which provide insight on the position of the ion from the inner wall when the charge transfer takes place. It was found that the ion trajectory is not very much deformed during electron transfers, i.e. one can prepare a well-collimated monoenergetic beam of metastable hollow ions (atoms), which is a unique probe carrying high potential energy at relatively low charge states.

Auger transitions and plasmon decay produced by hollow atoms at an Al(1 1 1) surface

Low-energy electron emission induced by 1–4 keV Ne 4+ ion impact on an Al surface was analyzed. Distinct lines observed near 22 eV are attributed to Auger electrons ejected from doubly excited Ne 2* decaying well above the surface. Spectral structures near 11 eV are primarily associated with the decay of bulk plasmons. After instrumental effort of extending the capability of the spectrometer to measure electrons with energies as low as 2 eV, we made observations of intense spectral structures near 6.5 eV, which are likely to be due to the decay of surface plasmons. Absolute electron yields were studied as a function of the incidence angle of the Ne 4+ projectile and were interpreted by means of model calculations.

X-ray emission from hollow atoms produced by collisions of multiply charged ions with a solid

The X-ray emission from hollow atoms produced by collisions of multiply charged ions accelerated by a short pulse laser with a solid or foil is studied theoretically. The possibility of obtaining a high conversion efficiency X-ray source in an ultrafast atomic process (∼1 fs) is demonstrated using the multistep-capture-and-loss (MSCL) model. Such an X-ray source has a clear advantage for the spectral range around a few kiloelectron volts over the conventional Kα X-ray source. Namely, the number of X-ray photons increases as the laser energy becomes larger and could reach 3 × 1011 photons for a laser energy of about 10 J.

Correlated electrons in lithiumlike hollow atoms.

We present a scheme for the explicit construction of highly correlated triply excited hollow states in Coulombic three-electron systems. Our analytical ansatz for the three-electron state is physically well justified and it produces configuration mixing coefficients in good agreement with data obtained from ab initio calculations.

Spin-polarized Dirac-Slater calculations on the energies of hollow argon atom

In this work, the multiplet splitting in terms of a spin-dependent model is analyzed. The spin-polarized and unpolarized single configuration Dirac-Fock-Slater wavefunctions have been used in the evaluation of the total energies of highly ionized argon with different L shell population The transition energies of hollow argon atom with initial configurations 1s 0 1/22s m 1/22p n 1/22p l 3/2 with m = 0 to 2 and n + l varying from 6 to 1 are reported in this work. The calculations have been carried out by taking into account a relativistic exchange potential in the Dirac-Slater potential. To account for the correlation effects, a correction term has also been considered perturbatively. The present calculations show that the spin-polarized technique which is mainly applied to the ground states of atoms may also be applied to atoms ionized in the inner shells with a good degree of accuracy.

Mechanisms for plasmon production by hollow atoms above and below an Al surface

Low-energy electrons ejected by 1–4 keV Ne 4+ ion impact on an Al surface were measured to study spectral structures near 11 eV attributed to the decay of plasmons. Absolute electron yields from the plasmon decay were primarily studied as a function of the incidence angle of the Ne 4+ projectile. Model calculations were performed to separate contributions from ions penetrating into the solid and ions reflected at the surface. Opposite energy dependencies were obtained for the two contributions suggesting that they are governed by potential and kinetic energy effects, respectively.

Metastable states produced with beam–capillary interaction

The formation and relaxation processes of hollow atoms and related excited states of ions produced with a Ni microcapillary thin foil have been studied employing various experimental techniques. The first stage of the charge transfer from the surface to the ions was studied by using visible light measurements. On the other hand, X-ray measurements revealed the core electronic configurations of ions at the last moment of the hollow atom evolution. The overall feature of the electron capture processes in the capillary was given by the charge state distribution measurements.

Density Functional Theory with Optimized Effective Potential: An Application to Hollow Atoms in the Bulk of Metallic Materials

AbstractThe local spin‐density functional theory with an optimized effective potential and self‐interaction correction is used to study the energy structure of hollow atoms or ions in the bulk of metallic materials. The energy structure of conduction electrons in the bulk of metallic material is treated by the jellium model. Based on this method, we have studied the emit ted x‐ray spectra of Arq+ hollow atoms and ions in the bulk of Ag as well as in the vacuum. By comparing the energy spectra of Arq+ hollow atoms or ions in the vacuum and in the bulk of Ag, we can illustrate the conduction electron screening effect. Our calculated x‐ray spectra of Arq+ hollow atoms or ions in the bulk of Ag is in reasonable agreement with the experiment.