Ne Ions Research Articles

Angular Distributions of Scattered Ne and Ar Ions at the Grazing Incidence on the InGaP (001) 110 Surface

Angular Distributions of Scattered Ne and Ar Ions at the Grazing Incidence on the InGaP (001) 110 Surface

Polarization phenomena in bichromatic ionization of Ne and Ar induced by circularly polarized VUV femtosecond pulses

SynopsisInterference effects resulting from direct single-photon and resonant two-photon ionization of neon and argon by bichromatic circularly polarized radiation were investigated in the framework of the perturbation theory. Analytical formulas are obtained for the differential cross section of the process in multielectron noble gas atoms. The possibility of coherent control of the photoelectron angular distribution and spin polarization will be discussed.

Распыление вольфрама ионами бериллия и неона

Sputtering yields for tungsten under Be and Ne ion bombardment as well as angular dependencies of the sputtering yields were calculated using molecular dynamics simulation. The results for Ne are in good agreement with experimental data. The obtained results for Be-W sputtering are needed to calculate the impurity influx into the plasma when the divertor material - tungsten - is bombarded with Be ions in ITER tokamak plasma. A model that explains the universal behavior of near-threshold sputtering yields for light ion-induced sputtering of tungsten was proposed.

Temporal electron collisions and adjunct ionization mechanisms in the positive column of a flowing N2 DC discharge

We studied a flowing nitrogen DC discharge by Langmuir double probes and optical emission spectroscopy (OES). We measured the reduced electric field (E/N) and electron density (ne) with the double probes. The gas (Tg) and N2(X1Σ+g) vibrational temperature (θ) were measured by OES from the nitrogen first and second positive systems transitions. We constructed a kinetic numerical model accounting for N2(X1Σ+g, 0 ≤ v ≤ 45), N2(A3Σ+u), N2(B3Πg), N2(a1Πg), N2(a′1Σ−u), N2(w1Δu), N2(a″1Σ+g), N2(W3Δu), N2(E3Σ+g), N2(C3Πu), N2+(X2Σ+g), N2+(B2Σ+u), N+, N3+, N4+, N(4S), N(2D), and N(2P) states. The species densities are calculated as functions of the discharge gas residence time in pure nitrogen for our experimental conditions. The kinetic model assumes as input parameters the experimental discharge gas pressure (p), tube radius (R), electron density (ne), gas temperature (Tg), reduced electric field (E/N). In this Letter, the kinetic model is applied to calculations of the fractional contribution of each ionization mechanism to the total ionization generated in the discharge. We compute 24 different ionization mechanisms to the five ions species. We have observed that ionization is generated by different physical-chemical reactions depending on the discharge gas residence time. It changes from dominant electron impact ionization process in the earlier times of the discharge to adjunct ionization mechanisms depending on the pooling of N2(a′1Σ−u) and N2(A3Σ+u) metastable molecules and associative ionization of N2(a′1Σ−u), N(2D) and N(2P) metastable atoms and molecules. Moreover, we have observed the occurrence of approximate charge neutrality for our studied experimental condition: discharge current of 30 mA, gas flow rate of 1 Slm−1, and gas pressure of 230 Pa. Also, the kinetic model fits quite well the measured N2(X1Σ+g) vibrational temperature at the end of the positive column.

Solvation of Silver Ions in Noble Gases He, Ne, Ar, Kr, and Xe.

We use a novel technique to solvate silver cations in small clusters of noble gases. The technique involves the formation of large, superfluid helium nanodroplets that are subsequently electron ionized, mass-selected by deflection in an electric field, and doped with silver atoms and noble gases (Ng) in pickup cells. Excess helium is then stripped from the doped nanodroplets by multiple collisions with helium gas at room temperature, producing cluster ions that contain no more than a few dozen noble gas atoms and just a few (or no) silver atoms. Under gentle stripping conditions, helium atoms remain attached to the cluster ions, demonstrating their low vibrational temperature. Under harsher stripping conditions, some of the heavier noble gas atoms will be evaporated as well, thus enriching stable clusters of NgnAgm+ at the expense of less stable ones. This results in local anomalies in the cluster ion abundance, which is measured in a high-resolution time-of-flight mass spectrometer. On the basis of these data, we identify specific "magic" sizes n of particularly stable ions. There is no evidence, however, for enhanced stability of Ng2Ag+, in contrast to the high stability of Ng2Au+ that derives from the covalent nature of the bond for heavy noble gases. "Magic" sizes are also identified for Ag2+ dimer ions complexed with He or Kr. Structural models will be tentatively proposed. A sequence of magic numbers n = 12, 32, and 44, indicative of three concentric solvation shells of icosahedral symmetry, is observed for HenH2O+.

Positive ion chemistry in an N2-CH4 plasma discharge: Key precursors to the growth of Titan tholins

Titan is unique in the solar system as it hosts a dense atmosphere mainly made of molecular nitrogen N2 and methane CH4. The Cassini-Huygens Mission revealed the presence of an intense atmospheric photochemistry initiated by the photo-dissociation and ionization of N2 and CH4. In the upper atmosphere, Cassini detected signatures compatible with the presence of heavily charged molecules which are precursors for the solid core of the aerosols. These observations have indicated that ion chemistry has an important role for organic growth. However, the processes coupling ion chemistry and aerosol formation and growth are still mostly unknown. In this study, we investigated the cation chemistry responsible for an efficient organic growth that we observe in Titan's upper atmosphere, simulated using the PAMPRE plasma reactor. Positive ion precursors were measured by in situ ion mass spectrometry in a cold plasma and compared with INMS observations taken during the T40 flyby. A series of positive ion measurements were performed in three CH4 mixing ratios (1%, 5% and 10%) showing a variability in ion population. Low methane concentrations result in an abundance of amine cations such as NH4+ whereas aliphatic compounds dominate at higher methane concentrations. In conditions of favored tholin production, the presence of C2 compounds such as HCNH+ and C2H5+ is found to be consistent with copolymeric growth structures seen in tholin material. The observed abundance of these two ions particularly in conditions with lower CH4 amounts is consistent with modeling work simulating aerosol growth in Titan's ionosphere, which includes mass exchange primarily between HCNH+ and C2H5+ and negatively charged particles. These results also confirm the prevalent role of C2 cations as precursors to molecular growth and subsequent mass transfer to the charged aerosol particles as the CH4 abundance decreases towards lower altitudes.

Do Chemical Effects Affect the Accumulation of Structural Damage during the Implantation of Fluorine Ions into GaN?

The accumulation of structural damage in GaN under irradiation with accelerated F and Ne ions with energies of 1.3 and 3.2 keV/amu is investigated. It is shown that chemical effects during implantation of fluorine ions within the doses under consideration do not noticeably affect the formation of stable structural damage both in the bulk and on the surface of GaN.

Investigation of the interaction of ion beams and X-ray quanta with deuterated crystal structures at the HELIS facility

The results of studies of the interaction of ion beams and X-ray quanta with deuterated crystal structures at the HELIS facility (LPI) are presented. Results on research of DD-reactions in deuterated crystal structures at deuteron energies 10 - 25 keV show significant enhancement effect. It is shown that the effect of the beams of ions Ne+ and H+ at energies in the range of 10 - 25 keV and a beam of X-radiation of 20 - 30 keV for deuterated target leads to stimulation of DD-reaction. For the target of CVD-diamond it is showed that the orientation of the sample with respect to the deuteron beam affects the neutron yield. Targets (deuterated CVD diamond, palladium, zirconium and titanium) were irradiated with both ion beams and X-ray quanta using an X-ray tube with an energy of up to 30 keV. Analysis of X-ray fluorescence spectra from deuterated targets of CVD diamond and palladium revealed "additional" peaks that are not identified by any of the characteristic radiation lines. Their appearance cannot be connected with any known element, as well as with diffraction processes.

The Effect of Cathodic Arc Guiding Magnetic Field on the Growth of (Ti0.36Al0.64)N Coatings

We use a modified cathodic arc deposition technique, including an electromagnetic coil that introduces a magnetic field in the vicinity of the source, to study its influence on the growth of (Ti0.36Al0.64)N coatings. By increasing the strength of the magnetic field produced by the coil, the cathode arc spots are steered toward the edge of the cathode, and the electrons are guided to an annular anode surrounding the cathode. As a result, the plasma density between the cathode and substrate decreased, which was observed as a lateral spread of the plasma plume, and a reduction of the deposition rate. Optical emission spectroscopy shows reduced intensities of all recorded plasma species when the magnetic field is increased due to a lower number of collisions resulting in excitation. We note a charge-to-mass ratio decrease of 12% when the magnetic field is increased, which is likely caused by a reduced degree of gas phase ionization, mainly through a decrease in N2 ionization. (Ti0.36Al0.64)N coatings grown at different plasma densities show considerable variations in grain size and phase composition. Two growth modes were identified, resulting in coatings with (i) a fine-grained glassy cubic and wurtzite phase mixture when deposited with a weak magnetic field, and (ii) a coarse-grained columnar cubic phase with a strong magnetic field. The latter conditions result in lower energy flux to the coating’s growth front, which suppresses surface diffusion and favors the formation of c-(Ti,Al)N solid solutions over phase segregated c-TiN and w-AlN.

Tunnel ionization of diatomic atmospheric gases (N2, O2) by laser radiation

We present the results of calculation of the rates of tunnel ionization of N2 and O2 molecules from the ground and excited vibrational states by linearly polarized laser radiation. These data are necessary for the theoretical description of filamentation. For N2, we consider ionization from the and shells, which give comparable contributions. For O2, we consider ionization from both the ground and excited electronic terms. We demonstrate that for N2, the ionization rate is maximum when the electric field vector of laser radiation is directed along the molecular axis (for shell) or perpendicular to it (for shell), whereas for the O2 molecule, for both electronic states considered, the rates have maxima when the angle between the electric field and molecular axis is equal to 45°.

Simulation study of the impurity radiation in the quasi-snowflake divertor with Ar seeding for CFETR

The snowflake divertor is considered as a possible candidate to exhaust the huge amount of power from core plasma for future fusion reactors. In this work, the radiative quasi-snowflake (QSF) divertor with argon seeding was studied by SOLPS5.0 simulation for the China Fusion Engineering Test Reactor. The relation between radiation power, Prad, electron density, ne, and effective ion charge, Zeff, was established based on the density and radiation fraction scan for both QSF and the ITER-like divertor (ILD). The comparison of radiative efficiency shows that QSF divertor is able to radiate more power with much lower impurity concentration than the ILD, when upstream density is relatively high. Further analyses are performed for the low (ne,sep ~ 2.3 × 1019 m−3) and high density cases (ne,sep ~ 4.5 × 1019 m−3). For low density cases, outer divertor plasma of QSF is firstly detached due to a much pronounced increase of outer divertor flux expansion, while the inner divertor detaches at first for the ILD configuration. Furthermore, the degree of detachment is much higher for the outer divertor in the QSF configuration under similar Prad, which can be beneficial for impurity screening in the outer divertor. For high density cases, both divertor targets of QSF are detached with relatively lower radiated power while the outer target of ILD is still attached. Zeff at outer mid-plane for QSF is obviously lower than that for ILD with similar Prad, which means better impurity screening in QSF.

Angular dependence of strong field ionization of N2 by time-dependent configuration interaction using density functional theory and the Tamm-Dancoff approximation

The ionization of N2 serves as an important test case for computational methods for strong field ionization. Because Koopmans’s theorem fails for Hartree-Fock calculations of N2, corrections for electron correlation are needed to obtain the proper ordering of ionization energies of N2. Lopata and co-workers found that real-time integration of time-dependent Hartree-Fock (rt-TD-HF) gave a ratio for strong field ionization parallel and perpendicular to the molecular axis that was too small compared to experiment, but real-time integration of time-dependent density functional theory (rt-TD-DFT) with an appropriately tuned long-range corrected functional, lc-ωPBE*, was in good agreement with experiment. The present study finds that time-dependent configuration interaction (TDCI) with single excitations based on a Hartree-Fock reference determinant (TD-CIS) has the same problems as rt-TD-HF. These problems can be overcome within the TDCI framework by calculating the excitation energies and transition dipole moments with density functional theory using linear response TD-DFT in the Tamm-Dancoff approximation (TDA) with suitably tuned long-range corrected functionals (TD-TDA). The correct angular dependence of the total ionization rate is obtained with TD-TDA using tuned lc-ωPBE*, lc-BLYP*, and ωB97XD* functionals. Partitioning of the total ionization rate into orbital components confirms that the larger ionization rate perpendicular to the molecular axis found for TD-CIS is due to greater π orbital contributions than those seen in TD-TDA. The use of density functional theory corrects this problem. At higher fields, both the TD-CIS and TD-TDA simulations show an increased ionization rate perpendicular to the molecular axis because of increased ionization from the π orbitals.

Ionization dynamics of Ne-doped helium clusters at low temperature: Ring-polymer molecular dynamics simulations including electronically nonadiabatic transitions

Ionization dynamics of NeHen (n ≤ 100) clusters has been studied by path-integral molecular dynamics (PIMD) and ring-polymer molecular dynamics (RPMD) simulations that can effectively describe nuclear quantum effects. The pairwise-additive interatomic potential model was used for the neutral NeHen clusters while the diatomics-in-molecule (DIM) model including spin-orbit coupling in Ne+(2P) was employed to describe the electronic structures of the cationic NeHen+ clusters. We found that the calculated ionization spectra of NeHen were very similar to those of Hen except for Ne ionization transitions. This indicates that helium density distributions are similar for both the Ne-doped and undoped clusters. The RPMD simulations have been performed to understand the initial charge migration dynamics and its associated nuclear dynamics through the instantaneous ionization of NeHe100. We found that, when the Ne atom is preferentially ionized, the He atoms in the first solvation shell are readily attracted by Ne+ to form a snowball structure.

Effect of plasma and beam parameters on focal dimensions in micrometer charged particle optics: Enhanced nonlinear demagnification below the Debye length

Detailed experiments and computer simulations employing two beam computational tools commonly used for benchmarking ion beam optics, AXCEL-INP and SIMION, are carried out to investigate the dependence of focal dimensions FD (length and image size) of ion beams extracted from a compact electromagnetic wave driven plasma source. The effect of plasma parameters, such as space potential VP, Bohm velocity vB, and the associated initial kinetic energy Ei, electron Te and ion temperatures Ti, ion mass M, ion current density Ji, and beam related parameters such as beam energy EB, plasma and beam limiter (BL) electrode aperture sizes, and potential applied to lenses (EL1 and EL2), on the FD are investigated. Variation of the focal length fl with vB, VP, and Ei indicates that fl increases with the increase in these parameters; however, it decreases with EB. fl is found to decrease with M and is independent of the BL aperture size; however, it is found to increase with extraction voltage V1 (EL1). Two different theoretical models to estimate the focal length of the combined lens system are developed, and the estimated values of fl are found to agree reasonably well with the experimental and simulation results. The sharpness of focus characterized by the beam angle at the focal point is determined for Ar, Kr, and Ne ion beams. A remarkable feature of nonlinear demagnification in plasma based beams is observed with significant enhancement when the plasma electrode aperture size is reduced to below the Debye length.

Target dependence of post-collision effects in ionization by proton impact

We have measured and calculated double differential cross sections for ionization of Ne and Ar by 75 keV proton impact for a broad range of fixed projectile energy losses as a function of scattering angle. Along with data obtained previously for lighter targets this made possible a systematic analysis of post-collision effects between the scattered projectile and the continuum electron in the exit channel as a function of the target ionization potential. The data are consistent with an increasing strength of such effects with increasing ionization potential. However, second-order effects involving the projectile—residual target ion interaction also play an important role.

Single-photon photoionization of oxygen-like Ne III

We offer a theoretical and experimental study of the single-photon photoionization of Ne III. The high photon flux and the high-resolution capabilities of the Advanced Light Source at the LBNL were employed to measure absolute photoionization cross sections. The resulting spectrum has been benchmarked against high accuracy relativistic Breit-Pauli R-matrix calculations. A large close-coupling wave function expansion which comprises up to 58 fine-structure levels of the residual ion Ne IV of configurations 2s22p3, 2s2p4, 2p5, 2s22p23s, 2s22p23p and 2s22p23d was included. A complete identification of the measured features was achieved by considering seven low-lying levels of Ne III. We found that the photoionization cross-section (σPI) exhibits the presence of prominent resonances in the low-energy region near the ionization thresholds that correspond to low-lying levels. These include high-peak narrow resonances with almost zero background introduced by relativistic effects. However, there does not exist a significant contribution to σPI from relativistic effects at the high-energy interval of the present study.

Prediction of ion-induced nanopattern formation using Monte Carlo simulations and comparison to experiments

Ion induced nanopattern formation has been experimentally investigated for many different ion-target combinations and different ion irradiation conditions. Several theories and models have been developed throughout the past few years to explain the observed boundary conditions for pattern formation as well as features of the patterns like wavelengths, growth rates, shapes, and amplitudes. To compare specific experiments with the predictions of analytical theories, it is necessary to calculate the linear and non-linear coefficients of the respective equation of motion of a surface profile. Monte Carlo simulations of ion–solid interactions based on the binary collision approximation provide a very fast, rather universal, and accurate way to calculate these coefficients. The universality expresses the broad range of ion species, ion energies, and target compositions accessible by the simulations. The coefficients are obtained from the moments of calculated crater functions, describing ion erosion, mass redistribution, and ion implantation. In this contribution, we describe how most linear, non-linear, and higher order coefficients can be determined from crater function moments. We use the obtained data to compare the results of selected experimental studies with the predictions of theoretical models. We find good quantitative agreement, e.g., for irradiation of Si with Ar and Kr ions, Al2O3 with Ar and Xe ions, and amorphous carbon with Ne ions.

Pulse-duration dependence of the double-to-single ionization ratio of Ne by intense 780-nm and 800-nm laser fields: Comparison of simulations with experiments

Accurate ab initio calculations of the ratio of double-to-single ionization of Ne atoms in strong laser fields are difficult due to the many-electron nature of the target. Here, with accurate total cross sections carefully evaluated by using the state-of-the-art many-electron R-matrix theory for both electron-impact ionization and electron-impact excitation of Ne+, we simulate the total double-ionization yields of Ne2+ in strong laser fields at 780 and 800 nm for pulse durations in the range from 7.5 to 200 fs based on the improved quantitative rescattering model. The corresponding single-ionization yields of Ne+ are calculated within the nonadiabatic tunneling model of Perelomov, Popov, and Terent'ev. The ratio of double-to-single ionization of Ne is then obtained from the calculated double- and single-ionization yields. By normalizing the ratio to the one calculated from solving the time-dependent Schrodinger equation for a short few-cycle pulse, we make quantitative comparisons of our results with experimental data to show that our model predicts the experimental findings very well. Finally, we analyze the pulse-duration dependence of the double-to-single ionization ratio.

Charge-state distributions of 20Ne ions emerging from thin foils

Abstract New experimental measurements of charge state distributions produced by a 20Ne10+ beam at 15 MeV/u colliding on various thin solid targets are presented. The use of the MAGNEX magnetic spectrometer enabled measurements of the 8+ charge state down to fractions of a few 10−5. The use of different post-stripper foils located downstream of the main target is explored, showing that low Z materials are particularly effective to shift the charge state distributions towards fully stripped conditions. The dependence on the foil thickness is also studied and discussed.

The radiobiological effects of He, C and Ne ions as a function of LET on various glioblastoma cell lines.

The effects of the charged ion species 4He, 12C and 20Ne on glioblastoma multiforme (GBM) T98G, U87 and LN18 cell lines were compared with the effects of 200 kVp X-rays (1.7 keV/μm). These cell lines have different genetic profiles. Individual GBM relative biological effectiveness (RBE) was estimated in two ways: the RBE10 at 10% survival fraction and the RBE2Gy after 2 Gy doses. The linear quadratic model radiosensitivity parameters α and β and the α/β ratio of each ion type were determined as a function of LET. Mono-energetic 4He, 12C and 20Ne ions were generated by the Heavy Ion Medical Accelerator at the National Institute of Radiological Sciences in Chiba, Japan. Colony-formation assays were used to evaluate the survival fractions. The LET of the various ions used ranged from 2.3 to 100 keV/μm (covering the depth–dose plateau region to clinically relevant LET at the Bragg peak). For U87 and LN18, the RBE10 increased with LET and peaked at 85 keV/μm, whereas T98G peaked at 100 keV/μm. All three GBM α parameters peaked at 100 keV/μm. There is a statistically significant difference between the three GBM RBE10 values, except at 100 keV/μm (P < 0.01), and a statistically significant difference between the α values of the GBM cell lines, except at 85 and 100 keV/μm. The biological response varied depending on the GBM cell lines and on the ions used.