Charged Xenon Ions Research Articles

Benchmarking calculations of level energies, transition parameters and electron impact excitation cross sections of S-like Xe38+

Atomic structure parameters for levels corresponding to 2s22p63s23p4, 2s22p63s3p5, 2s22p63s23p23d2, 2s22p63s3p43d and 2s22p63s23p33d configurations of S-like Xe38+ are calculated using the fully relativistic multiconfiguration Dirac-Hatree-Fock (MCDHF) method followed by the subsequent relativistic configuration interaction (RCI) calculations. The parameters evaluated include energies of the lowest 75 levels and E1, M1, E2, and M2 transition parameters among these levels. The effect of the choice of virtual orbitals for generating the wavefunctions is discussed. The accuracy of our line strengths is established through the rigorous calculations of their associated uncertainties using three different methods. Another set of calculations using the many-body perturbation theory (MBPT) to validate the present MCDHF-RCI energies is carried out. Further, the electron impact excitation cross-sections using the relativistic distorted wave (RDW) theory are determined for all transitions from the ground and metastable states in the incident electron energy range from the excitation threshold to 10 keV. For plasma physics applications, the fitting parameters for these cross sections employing two distinct equations tailored for low and high-energy regimes are reported. Moreover, the rate coefficients are determined in the electron temperatures range of 15 eV to 100 eV by taking into account the Maxwellian electron energy distribution function and our computed cross-sections. In addition to addressing the atomic data gap in highly charged Xe ions, the present results are of good accuracy and can serve as benchmark tests for other theoretical evaluations. Moreover, they can also assist in line identification of the complex spectra of highly charged sulphur-like xenon ions.

Nanostructuring of sapphire by ion-induced plasma

Sapphire single crystals were irradiated with slow highly charged xenon ions from electron beam ion trap (EBIT) facility. Scanning electron microscopic measurements showed the ability of the used ions in creating surface nanohillocks by single ion impacts. A test charge approach is used to provide an explanation for the mechanism of nanostructure formation in the sapphire surface. The model shows an increase of the height of the created nanostructure by increasing of the ionic density and electron temperature. This is in agreement with the dependence of the size of the induced nanostructures as a function of the deposited ion energy. The influence of the test charge speed and the relevance of an attractive wakefield potential are discussed in terms of plasma parameters.

Velocity distributions of particles sputtered from supported two-dimensional MoS2 during highly charged ion irradiation

A key problem in ion-solid interaction is the lack of experimental access to the dynamics of the processes. While it is clear that the mechanisms of interaction and sputtering depend on the kinetic and potential energy (sum of ionization energies) of the projectile, the importance and interplay of the various interaction mechanisms are unknown. Here, we have irradiated substrate-supported (Au, ${\mathrm{SiO}}_{2}$) monolayers of ${\mathrm{MoS}}_{\text{2}}$ with highly charged xenon ions (HCIs; charge state: $17+$ to $40+$), extracted the emitted neutral postionized Mo particles in a time-of-flight mass spectrometer, and determined their velocity distributions. We find two main contributions, one at high velocities and a second at lower velocities, and assign them to kinetic and potential effects, respectively. We show that for slow HCIs (5 keV) the interaction mechanisms leading to particle emission by electronic excitation and momentum transfer, respectively, are independent of each other, which is consistent with our atomistic simulations. Our data suggest that the predominant mechanism for potential sputtering is related to electron-phonon coupling, while nonthermal processes do not play a significant role. We anticipate that our work will be a starting point for further experiments and simulations to better understand the interplay of processes arising from ${E}_{\text{pot}}$ and ${E}_{\text{kin}}$.

Energy deposition and formation of nanostructures in the interaction of highly charged xenon ions with gold nanolayers

The effect of the deposition of kinetic energy and neutralization energy of slow highly charged xenon ions on the process of the nanostructures creation at the surface of gold nanolayers is investigated. The nanolayers of thickness of 100 nm were prepared by e-beam evaporation of gold on crystalline silicon Si(100) substrate. The samples were irradiated at the Kielce EBIS facility of the Jan Kochanowski University (Kielce, Poland), under high vacuum conditions. The irradiations were performed for constant kinetic energy 280 keV and different ions charge states (Xeq+, q = 25, 30, 35, 36 and 40) and for constant charge state Xe35+ and different kinetic energies: 280 keV, 360 keV, 420 keV and 480 keV. The fluence of the ions was on the level of 1010 ions/cm2. Before and after irradiation the nanolayer surfaces were investigated using the atomic force microscope.As the result, well pronounced modifications of the nanolayer surfaces in the form of craters have been observed. A systematic analysis of the crater sizes (diameter on the surface and depth) allowed us to determine the influence of the deposited kinetic and the neutralization energy on the size of the obtained nanostructures. The results are theoretically interpreted within the micro-staircase model based on the quantum two-state vector model of the ionic Rydberg states population. The charge dependent ion–atom interaction potential inside the solid is used for the calculation of the nuclear stopping power. According to the model the formation of the nanostructures is governed by the processes of the ionic neutralization in front of the surface and the kinetic energy loss inside the solid. The interplay of these two types of processes in the surface structure creation is described by the critical velocity. Using the proposed theoretical model, the neutralization energy, deposited kinetic energy and critical velocities were calculated and compared qualitatively with the experimental results. The results are consistent (after normalization) with previous experimental data and molecular dynamics simulations for single ionized Xe and crystalline gold surface.

Energy-loss enhancement and charge-equilibration time for highly charged xenon ions at near-Bohr velocity in solids

We probe the energy-loss enhancement in preequilibration and the charge-equilibration time for highly charged Xe ions $({q}_{\mathrm{in}}=15--26)$ at near-Bohr velocity impinging on an Al target by measuring the dependence of the Xe $L$-shell x-ray yield on the initial charge state of the ions. The present results are found to be very consistent with those obtained in ion-transmission thin-foil experiments. The charge-equilibrium time is measured in the near-Bohr velocity region of highly charged ions in solids. Moreover, for the present collision system it is demonstrated by measuring the inner-shell x rays that the ionization of the inner-shell electrons of the projectile ions starts to contribute to ion energy loss at near-Bohr velocity.

Neutralization dynamics of slow highly charged ions passing through graphene nanoflakes: An embedding self‐energy approach

AbstractWe study the time‐dependent neutralization of a slow highly charged ion that penetrates a hexagonal hollow‐centred graphene nanoflake. To compute the ultrafast charge transfer dynamics, we apply an effective Hubbard nanocluster model and use the method of nonequilibrium Green functions in conjunction with an embedding self‐energy scheme, which allows one to follow the temporal changes of the number of electrons in the nanoflake. We perform extensive simulations of the charge transfer dynamics for a broad range of ion charge states and impact velocities. The results are used to put forward a simple semi‐analytical model of the neutralization dynamics that is in very good agreement with transmission experiments, in which highly charged xenon ions pass through sheets of single‐layer graphene.

Research on the keeper electrode’s sputtering mechanism of the ion thruster

Bombardment of the keeper electrode by high-energy ions limits the lifetime of the ion thruster. To investigate its sputtering mechanism, this paper established a two-dimensional simulation model and applied the Particle-In-Cell method to simulating the generation process of the high-energy ions and their sputtering process on the keeper electrode surface. The Monte Carlo Collision method was used to address the collisions between particles. The results showed that the plasma potential downstream of the keeper electrode oscillated; therefore, ions in this area were accelerated and their energy was increased from 0.085 to 35 eV, which was enough to erode the keeper electrode’s surface. double charged xenon (Xe++) ions and charge exchange xenon (CEX) ions were the two kinds of high-energy ions. The most severe position was the center of the keeper electrode surface, and its erosion rate was about 2.0 × 10−4 m/1000 h, while the average erosion rate was about 10−5 m/1000 h when the frequency and the amplitude of the plasma potential were 50 Hz and 35 eV, respectively. Plasma potential oscillation greatly affected the sputtering mechanism of the keeper electrode.

Simulation study on electron extraction mechanism of an ECR neutralizer

To investigate electron extraction through the orifice of an electron cyclotron resonance (ECR) neutralizer, two-dimensional axisymmetric particle-in-cell simulations with a Monte Carlo collision algorithm (PIC-MCC) have been conducted. The diagnostic experiment of Langmuir probe verified that the plasma distribution in the ECR neutralizer with an L-shaped antenna was circumferentially uniform, which provided the experimental basis for two-dimensional axisymmetric simulations. The distribution of plasma parameters and extracted electron current of the neutralizer were obtained by tracking the motion of electrons and singly charged xenon ions, simulating the elasticity, excitation, ionization and Coulomb collisions. Moreover, computational results and analysis demonstrated that there were two kinds of electronic extraction channel in the ECR neutralizer. One was affected by the potential well between magnetic well and anode, another was affected by the valley potential of electronic double sheath. The property of these two electronic extraction channels determined the variation of extracted electron current and mode conversion voltage.

Multiple ionisation and Coulomb explosion in terawatt photoexcitation of Xenon clusters: An experimental and theoretical study

Multiple ionisation and Coulomb explosion following photoexcitation of xenon clusters has been studied at ∼1012 W/cm2 at three different wavelengths in UV and visible region. Multiply charged xenon ions (Xen+) up to +4, +6 and +10 states were observed in time-of-flight mass spectra at 266, 355 and 532 nm respectively. Energetic electrons were also detected and electron energy distribution was measured at all the three wavelengths. The maximum electron energy was found to be dependent on the ionising wavelength. It has been established that multiphoton ionisation creates Xe+ ions and the energised electrons cause secondary ionization of singly charged xenon ion (Xe+) to give rise to higher charged states of xenon. An analytical model has been developed to understand the multiple ionization of xenon clusters at terawatt laser intensity. It reveals that lowering of ionization potential and excitation to Rydberg states of ions are important factors to generate high charge states like Xe10+commensurate with the experimental results.

Vanishing influence of the band gap on the charge exchange of slow highly charged ions in freestanding single-layer MoS2

Charge exchange and kinetic energy loss of slow highly charged xenon ions transmitted through freestanding monolayer ${\mathrm{MoS}}_{2}$ are studied. Two distinct exit charge state distributions, characterized by high and low charge states, are observed. They are accompanied by smaller and larger kinetic energy losses, as well as scattering angles, respectively. High charge exchange is attributed to two-center neutralization processes, which take place in close impact collisions with the target atoms. Experimental findings are compared to graphene as a target material and simulations based on a time-dependent scattering potential model. Independent of the target material, experimentally observed charge exchange can be modeled by the same electron capture and de-excitation rates for ${\mathrm{MoS}}_{2}$ and graphene. A common dependence of the kinetic energy loss on the charge exchange for ${\mathrm{MoS}}_{2}$ as well as graphene is also observed. Considering the similarities of the zero band-gap material graphene and the 1.9 eV band-gap material ${\mathrm{MoS}}_{2}$, we suggest that electron transport on the femtosecond timescale is dominated by the strong influence of the ion's Coulomb potential in contrast to the dispersion defined by the material's band structure.

Electron impact excitation of Ge-like to Cu-like xenon ions in the extreme ultraviolet

Electron impact excitation of Ge-like (Xe22+) to Cu-like (Xe25+) highly charged xenon ions are studied and dipole allowed fine-structure transitions in the wavelength range 9–25 nm are considered. We use relativistic distorted wave method to calculate the electron excitation cross sections for the different transitions of the four ions in the scattered electron energy range from excitation threshold to 1500 eV. The ionic bound state wavefunctions have been obtained using multi-configuration Dirac–Fock method. To ascertain the accuracy of the wavefunctions, the wavelengths and oscillator strengths of the considered transitions in these ions are calculated and compared with the previously reported measurements and other theoretical calculations, where available. For plasma diagnostic applications, the obtained cross sections are further used to calculate the rate coefficients of the transitions as well as fitted with an analytical expression which can be used in plasma model conveniently.

Electron impact excitation of highly charged Ge-Like Xe ion

SynopsisA systematic study of electron impact excitation of Ge-like highly charged xenon ions have been carried out in Extreme Ultraviolet (EUV) range within fully relativistic multiconfiguration Dirac-Fock framework. The cross section calculation reported here are performed for dipole allowed transitions for Xe22+ ions from ground state 4s24p2 to 4s24p3 and 4s24p4d excited fine structure levels. The analytic fitting for the cross section data for incident electron energy range within 50 to 1500 eV have been provided for further applications in various plasma diagnostic techniques.

Formation of nanocraters on the surface of gold nanolayer by an impact of highly charged xenon ions

SynopsisIn this work we have systematically studied process of energy deposition and formation of nanostructures on metallic (Au nanolayer) surfaces as a result of the irradiation with slow highly charged xenon ions (HCI). Well pronounced modifications of the surfaces in form of craters were observed. We found that, in the systems considered, the kinetic energy influences the crater depth, while the potential energy affects the crater size at the surface. This finding showed that for gold nanolayers the potential energy plays important role in the nanostructures creation.

First experimental results from a new high-power electron gun: Electron-impact ionisation of multiply charged xenon ions

SynopsisDuring recent years we have developed a new high-current electron gun for the Giessen electron-ion crossed-beams experiment aiming at an extension of the previously available collision-energy range of 1–1000 eV to 3500 eV. Here, we report on the first measurements of absolute cross sections for electron-impact ionisation of multiply charged xenon ions employing the new electron gun. Comparisons with available experimental and theoretical data from the literature provide convincing evidence that the new electron gun can be operated reliably over the entire design energy-range.

Energy deposition of highly charged ions transmitted through single layer MoS2

SynopsisHighly charged ions (HCIs) are an efficient tool for the perforation of suspended 2D materials. Only a fraction of their potential energy is transferred to the atomically thin target during the very short interaction time and is available for pore formation. Charge exchange spectra were measured for highly charged xenon ions transmitted through suspended, single layer MoS2 in order to determine the deposited potential energy available for pore formation. Additionally, charge exchange dependent ion stopping responsible for kinetic sputtering was measured simultaneously.

Equivalent two-dimensional numerical simulation of an ECR neutralizer

The electron cyclotron resonance (ECR) neutralizer is a component of the 10-cm electron cyclotron resonance ion thruster (ECRIT) and its performance has a great effect on the overall performance of the thruster. To clarify the mechanisms of electron extraction of the ECR neutralizer, a particle-in-cell/Monte Carlo collision (PIC/MCC) solver was developed to simulate the plasma in the ECR neutralizer. This paper equivalently simplified the three-dimensional model of the ECR neutralizer into a two-dimensional axisymmetric one according to experimental results and the circumferential electric drift of charged particles. Hence, a two-dimensional three-velocity component full-particle calculation model was used in the solver. Through analyzing ECR heating mechanism, the validity and accuracy of the equivalent ring antenna were discussed. The distribution of plasma parameters and extracted electron current were obtained by tracking the motion of electrons and singly charged xenon ions, simulating the elasticity, excitation, ionization and Coulomb collisions. The results showed that the density of neutral particles can be adjusted to coincide simulation current with experiment. It was also found that due to the magnetic field gradient, plasma will migrate to the magnetic well, where the ion was in dominant position to form a potential well and hinder the extraction of electron.

Creation of surface nanostructures in lanthanum fluoride single crystals by irradiation with slow highly charged ions

Slow highly charged ions (HCI) were utilized successfully for the formation of various nanostructures in the surfaces of different materials. The creation mechanism of HCI-induced nanostructures was intensively studied in alkali- and alkaline-earth fluorides. Here, we are investigating another type of fluorine-containing ionic crystals of different crystalline and electronic structure, namely lanthanum fluoride, LaF3. The single crystals were irradiated with slow (eV-keV) highly charged xenon ions from an Electron Beam Ion Trap. After irradiation, the crystals surfaces were investigated by scanning force microscopy. The measured topographic images show nanohillocks emerging from the surface. These nanostructures were observed only after exceeding a well-defined threshold in the potential energy. The role of ion parameters for nanohillocks formation as well as a comparison with results for swift heavy ion irradiations of LaF3 single crystals are presented. Furthermore, the similarities and differences between LaF3 and other ionic fluoride crystals, in the creation of surface nanostructures, are discussed.

Observation of extreme ultraviolet light emission from an expanding plasma jet with multiply charged argon or xenon ions

We report on the first direct demonstration of the possibility to generate extreme ultraviolet (EUV) radiation with a freely expanding jet of dense plasma with multiply charged ions supported by high-power microwaves. The detected emission power is about 20 W at 18–50 nm for argon and xenon and 0.3 W at 13–17 nm for xenon. The discharge with a peak electron density of up to 3×1016 cm−3 and a characteristic size of 150 μm is supported by the focused radiation of a recently developed gyrotron with unique characteristics, having a 250 kW output power at 250 GHz and operated in a relatively long (50 μs) pulse mode. Up-scaling of these experimental results gives grounds for the development of a point-like kilowatt-level EUV source for high-resolution lithography, which is able to meet the requirements of the microelectronics industry.

Three-dimensional particle simulation of ion thruster plume flows with EX-PWS

Ion thruster plumes from a multi-thruster array of different working configurations are simulated by a hybrid fluid-particle software. The particle in cell method is employed to model the transports of ions. The direct simulation Monte Carlo method is used to model momentum and charge exchange (CEX) collisions. The software is based on unstructured grids which make it easy to handle with complex geometry. The results of chamber simulation are compared with experimental data in ion current density and number density, which show good agreements. The maximum difference of current density along the thruster centerline is less than 9.30%. The interaction effects of plumes when multiple thrusters are operating in vacuum are predicted. Distributions of single charged xenon ions are significantly different in the near-field plume flow, however, merge into one in the far downstream region. Moreover, the interaction effect on the spatial distribution of CEX xenon ions is displayed as well.

The role of radiative de-excitation in the neutralization process of highly charged ions interacting with a single layer of graphene

X-ray emission of slow (<1 a.u.) highly charged Argon and Xenon ions is measured for transmission through a freestanding single layer of graphene. To discriminate against X-ray emission originating from the graphene's support grid a coincidence technique is used. X-ray emission of 75 keV Ar17+ and Ar18+ ions with either one or two K-shell vacancies is recorded. Using a windowless Bruker XFlash detector allows us to measure additionally Ar KLL and KLM Auger electrons and determine the branching ratio of radiative vs. non-radiative decay of Ar K-shell holes. Furthermore, X-ray spectra for 100 keV Xe22+-Xe35+ ions are compared, showing a broad M-line peak for all cases, where M-shell vacancies are present. All these peaks are accompanied by emission lines at still higher energies indicating the presence of a hollow atom during X-ray decay. We report a linear shift of the main M-line peak to higher energies for increasing incident charge state, i.e. increasing number of M-shell holes.