Mass spectrometric study of collision interactions of fast charged particles with water and NaCl solutions

A new experimental technique has been developed for the investigation of secondary particle emission from liquid targets resulting from collision interactions with fast charged particles. Secondary ion mass spectra are presented for the first time for liquid water and 0.1 M, 1 M and 5 M NaCl aqueous solutions bombarded by 2.0 MeV He⁺ ions. The mass spectra exhibit a series of negative cluster ions of the form of X_mY_nR⁻, where X and Y stand for neutral molecules such as H₂O, NaCl and NaOH and R⁻ stands for a negative charged ion such as O⁻, OH⁻ and Cl⁻. Intensities of negative ions are found to change significantly as a function of the concentration. The results imply evidently that the cluster structure in liquid targets changes drastically depending on the abundance of NaCl in solutions.     

Alkali ion scattering from Ag(0 0 1) and Ag thin films at low and hyperthermal energies

We have investigated the scattering of K⁺ and Cs⁺ ions from a single crystal Ag(0 0 1) surface and from a Ag-Si(1 0 0) Schottky diode structure. For the K⁺ ions, incident energies of 25 eV to 1 keV were used to obtain energy-resolved spectra of scattered ions at θ_i = θ_f = 45°. These results are compared to the classical trajectory simulation safari and show features indicative of light atom-surface scattering where sequential binary collisions can describe the observed energy loss spectra. Energy-resolved spectra obtained for Cs⁺ ions at incident energies of 75 eV and 200 eV also show features consistent with binary collisions. However, for this heavy atom-surface scattering system, the dominant trajectory type involves at least two surface atoms, as large angular deflections are not classically allowed for any single scattering event. In addition, a significant deviation from the classical double-collision prediction is observed for incident energies around 100 eV, and molecular dynamics studies are proposed to investigate the role of collective lattice effects. Data are also presented for the scattering of K⁺ ions from a Schottky diode structure, which is a prototype device for the development of active targets to probe energy loss at a surface.     

Temperature-dependent surface modification of InSb(0 0 1) crystal by low-energy ion bombardment

Structural and compositional modification of InSb(0 0 1) single crystal surfaces induced by oblique incidence 2-5 keV Ar and Xe ion irradiation have been investigated by means of scanning tunneling and atomic force microscopies, and time-of-flight mass spectroscopy of secondary ion emission. In general, ion-induced patterns (networks of nanowires, or ripples) are angle of incidence- and fluence-dependent. Temperature dependences (from 300 to 600 K) of the RMS roughness and of the ripple wavelength have been determined for the samples bombarded with various fluences. Secondary ion emission from an InSb(0 0 1) surface exposed to 4.5 keV Ar⁺ ions has been investigated with a linear TOF spectrometer working in a static mode. Mass spectra of the sputtered In⁺, Sb⁺ and In₂⁺ secondary ions have been measured both for the non-bombarded (0 0 1) surface and for the surface previously exposed to a fluence of 10¹⁶ ions/cm². In⁺ and In₂⁺ intensities for the irradiated sample are much higher in comparison to the non-bombarded one, whereas Sb⁺ ions show a reversed tendency. This behavior suggests a significant In-enrichment at the InSb(0 0 1) surface caused by the ion bombardment.     

A 40 keV cyclotron for radioisotope dating

We have built and begun testing a small low energy negative ion cyclotron for direct detection of ¹⁴C. At present, the cyclotron is operated in a high resolution mode at the 31st harmonic, with 1–2 kV on the dees. The high harmonic and a minimum number of turns of approximately 100, should give a fwhm mass resolution of about $\text{1}\text{30000}$ — sufficient to suppress the background from molecular ions such as ¹³CH^?. Background such as scattered ions of ¹²C^? and ¹³C^? should be totally suppressed by the cyclotron acceleration process. (At the 88″ cyclotron at LBL we found that ions only 1% off-resonance are suppressed by more than a factor of 10¹⁷.) A miniature Cs sputter source located at the center of the cyclotron is expected to provide more than 1 μA of negative carbon ions. Negative ions are used in order to eliminate the interference from ¹⁴N. Unlike high energy cyclotrons, focussing is obtained solely from the axial components of the accelerating electric field. The magnetic field is kept flat to within 1 part in 10⁴ in order to maintain exact isochronism throughout the several thousand accelerating rf cycles. The low final energy of 40 keV eliminates any danger from radiation or need for shielding, and the final orbit radius of only 10.5 cm, reduce the size and cost of the machine to that of conventional mass spectrometers.     

Investigations of the Scattering of UF5+ Ions by N2 Molecules

The orientation-averaged classical deflection function for scattering of UF₅ ⁺ ions by N₂ molecules is calculated in the collision energy range 2–10 keV, using additive interatomic potentials to calculate the anisotropic interaction of the molecular particles. The elastic scattering cross sections are calculated for ions and the interaction potential of the colliding particles is reconstructed using the Firsov method based on classical mechanics.The results obtained are recommended for determining the isotopic sensitivity of mass spectrometers to be used for analysis of uranium compounds.     

Stopping power of 11B in Si and TiO2 measured with a bulk sample method and Bayesian inference data analysis

The popular stopping power interpolative schemes require experimental data to be developed. Where the data bases are sparse, with few experiments available, interpolations can be more inaccurate. This is the case for the stopping of heavy ions, where even for important targets such as Si there is a need for more measurements. For compounds, the situation is even worse with very few measurements available. In particular, the stopping in oxides and nitrides often deviates significantly from what would be expected using the Bragg’s rule. We apply a method that uses bulk or thick film samples to determine the stopping power of ¹¹B in Si and TiO₂. The method, which relies on Bayesian inference analysis of RBS spectra obtained at different energies, has been previously validated by verifying the results obtained in the well-known system ⁴He in Si.     

Negative Ion Confinement in the Multicusp Ion Source

To optimize the negative ion source and generate intense beams of negative ions, understanding of transport properties of both electrons and negative ions is indispensable. Transport process of negative hydrogen ions (H^?) in a multicusp H^? source, has been simulated by three-dimensional Femlab simulation software. Multipolar plasma confinement is known to result in enhanced plasma density, homogeneous plasma of a large volume, and quiescent plasmas. The effect of plasma confinement by applying multi-polar magnetic field was investigated. Results are obtained for ten different configurations of permanent magnet and discussed. Full line cusps are found to give optimum plasma density. Negative ions created on the sidewall hardly can reach the center of the source due to trapping by the multicusp magnetic field. As a result, H^? ions created on the sidewall do not have a significant effect on the H^? current.     

SIMS depth profiling of multilayer metal-oxide thin films — improved accuracy using a xenon primary ion

Multilayer thin films containing silver and copper, sandwiched in a metal oxide, have been depth profiled by secondary ion mass spectrometry (SIMS) using primary ions of differing mass, energy and chemical reactivity. These results were compared for accuracy with those obtained by Rutherford backscattering spectrometry (RBS). The use of O₂⁺ or O^? as primary ions resulted in severe distortion of the silver ion intensity distribution in the SIMS profile of a ZnO/Ag/ZnO thin film on glass; O₂⁺ bombardment at energies from 3–10 keV resulted in the detection of silver at the glass interface, while the use of O^? caused the silver to be detected closer to the outer surface than expected from RBS results. Primary beams of Ar⁺ and Xe⁺ gave progressively more accurate results for the Zn/Ag/Zn distribution; Xe⁺ at 5.0 keV energy produced profiles that agreed within 10% of RBS-derived values. The same beam conditions, used to profile a double silver layer in ZnO, resulted in some discrepancy in the position of the inner layer, compared to RBS results, and this was attributed to an enhanced sputter rate in the oxide under the outside metal film. Depth profiling of TiO₂/Cu/TiO₂ films with oxygen also resulted in significant distortion of the perceived position of the metal layer, and this was again significantly improved using Xe⁺ primary ions of 6–9 keV energy. The distorting effects of oxygen bombardment can be understood in terms of a migration of metal ions in an electrostatic field created by a charged surface. Ionization of the metallic layer is enhanced by the use of oxygen. By contrast, the use of rare gases reduces the production of ions from the metallic layer which can migrate prior to the onset of sputtering.     

Simulation of Electron Trajectories in the Multicusp Ion Source Using Geantn4 Monte Carlo Code

To optimize the negative ion source and generate intense beams of negative ions, understanding of transport properties of both electrons and negative ions is indispensable. Transport process of negative hydrogen ions (H⁻) in a multicusp H⁻ source, has been simulated by three-dimensional Femlab simulation software. Multipolar plasma confinement is known to result in enhanced plasma density, homogeneous plasma of a large volume, and quiescent plasmas. The effect of plasma confinement by applying multi-polar magnetic field was investigated. Results are obtained for ten different configurations of permanent magnet and discussed. Full line cusps are found to give optimum plasma density. Negative ions created on the sidewall hardly can reach the center of the source due to trapping by the multicusp magnetic field. As a result, H⁻ ions created on the sidewall do not have a significant effect on the H⁻ current.     

Storing Matter: A new quantitative and sensitive analytical technique based on sputtering and collection of sample material

The Storing Matter technique, which is a new analytical technique for both organic and inorganic materials, consists in decoupling the sputtering of the specimen from the subsequent analysis step. The surface of the specimen to be analysed is sputtered by means of an ion beam. The particles emitted under the impact of these primary ions are deposited at a sub-monolayer level on a dedicated collector under UHV conditions. It is only in a second step that the deposited matter is analysed in analytical instruments (mainly dynamic and static SIMS). Depositing the matter sputtered from different samples or from different layers of a sample on a same collector makes Storing Matter a powerful tool to circumvent the well-known matrix effect in SIMS. Moreover, enhanced secondary ion emission can be obtained in the different SIMS analysis modes as the collector surface and thus the matrix is chosen with respect to the elements to be analysed and the analysis mode (M⁺, M⁻, cationisation for organic information, etc.). In order to allow for the different steps of the Storing Matter technique to be performed under optimized conditions, a dedicated prototype instrument has been developed at SAM. This paper presents the Storing Matter technique and the dedicated prototype instrument with a special focus on the sputter-deposition process used in this new technique.     

Emission of large molecules from methane by ion bombardment

Condensed layers ot methane at 20 K have been bombarded by 6–8 keV Ar⁺, He⁺ and H₂⁺ ions. Mass spectra and Kinetic energy distributions of neutral species sputtered from these layers have been measured. We have found sputtered species with masses up to 72 amu and thus with at least 5 carbon atoms. In addition to this an involatile residue was formed. Analysis by pyrolysis mass spectrometry showed this residue to contain species with masses up to at least 170 amu which therefore contain at least 12 carbon atoms. The kinetic energy distributions of sputtered methane molecules lie between those of a Maxwell-Boltzmann distribution and a collision cascade. Higher values are reached for Ar⁺ than for the light ions. From these observations we conclude: for both light and heavy ions radicals are formed, which combine to new molecules. These exothermic reactions produce heat which causes desorption. The high energy tail for bombardment with argon ions shows that part of the sputtering is caused by momentum transfer.     

碳团簇及C60负离子的产生

利有铯负离子溅射源和石墨阴极产生并引出了碳团簇负离子流，观察到了一些电子亲和势很小的团簇，用Ｃ６０／Ｃ７０混合物做阴极，也引出了Ｃ６０及其碎片的负离子束，分析了碳团簇负离子束流的质谱组成特点和束流变化的一些规律。     

A comparison of secondary ion mass spectra from keV and MeV ion bombarded vanadium,niobium and copper

Positive secondary ion mass spectra have been measured from stainless steel, copper, niobium, and vanadium targets bombarded by 70 MeV ⁷⁹Br⁷⁺ and 100 keV ⁴⁰Ar⁺ ions using a modified quadrupole residual gas analyzer. Additional spectra have also been measured from the vanadium target for a number of ⁷⁹Br and ⁴⁰Ar projectile energies from 25 keV up to 5 MeV. As has been previously reported [13], under MeV ion bombardment there is an enhancement in the yield of positive ions of electronegative trace constituents relative to the yield of singly charged metal substrate ions. These data suggest that projectiles capable of large inelastic energy deposition may induce secondary ion emission by a mechanism whose contribution to the total ion yield is insignificant or absent when the projectile energy is limited to a few keV. The similarity of these data to recent results in electron- and photon-stimulated desorption is noted.     

Neutralization effect for Ne+ scattering from a Cu(110) surface

Intensity and structure of the energy spectra of Na⁺ and Ne⁺ ions scattered from a Cu(110) surface are governed by multiple scattering and neutralization effects. These were studied for ion energies between 600 eV and 1 keV and in the temperature range from 100 to 600 K in the experiment and by computer simulation. Na⁺ scattering directly reflects the crystallographic structure of the (110) surface. The temperature effects can be used to analyze thermal motions of surface atoms in terms of a surface Debye temperature for specific vibrational directions. The contributions of single and multiple scattering events to the energy spectra are analyzed and for Ne⁺ a strong trajectory-dependent neutralization is found. The comparison of the neutralization of Ne⁺ and Na⁺ leads to a Ne⁺ ion survival probability of a few percent for single scattering, less than 1% for double scattering, and a value of less than 10⁻³ for scattering from atoms below the top atomic layer. A simple neutralization model is developed to explain the observed survival probabilities.     

Localised modifications of anatase TiO2 thin films by a Focused Ion Beam

A Focused Ion Beam (FIB) has been used to implant micrometer-sized areas of polycrystalline anatase TiO₂ thin films with Ga⁺ ions using fluencies from 10¹⁵ to 10¹⁷ ions/cm². The evolution of the surface morphology was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). In addition, the chemical modifications of the surface were followed by X-ray photoelectron spectroscopy (XPS). The implanted areas show a noticeable change in surface morphology as compared to the as-deposited surface. The surface loses its grainy morphology to gradually become a smooth surface with a RMS roughness of less than 1 nm for the highest ion fluence used. The surface recession or depth of the irradiated area increases with ion fluence, but the rate with which the depth increases changes at around 5 × 10¹⁶ ions/cm². Comparison with implantation of a pre-irradiated surface indicates that the initial surface morphology may have a large effect on the surface recession rate. Detailed analysis of the XPS spectra shows that the oxidation state of Ti and O apparently does not change, whereas the implanted gallium exists in an oxidation state related to Ga₂O₃.     

The broadening of energy spectra of atoms scattered by a solid surface under molecular ion bombardment

The broadening of energy spectra of atoms scattered by a metal surface under molecular ion bombardment are calculated taking into account the molecular ions to suffer dissociative neutralization on the initial part of their scattering trajectory. The results of the calculations explain the experimental data for scattering of hydrogen atoms from aluminum, palladium and palladium covered by potassium surfaces under bombardment with H₂⁺.     

Relativistic many-body Møller-Plesset perturbation theory calculations of the energy levels and transition rates in Na-like to P-like Xe ions

Relativistic multireference many-body perturbation theory calculations have been performed for Xe⁴³⁺ to Xe³⁹⁺ ions, resulting in energy levels, electric dipole transition rates, and level lifetimes. The second-order many-body perturbation theory calculation of energy levels included mass shifts, the frequency-dependent Breit correction, and Lamb shifts. The calculated transition energies and E1 transition rates are used to present synthetic spectra in the extreme ultraviolet range for some of the Xe ions.     

Structural and compositional characterization of LiNbO3 crystals implanted with high energy iron ions

Iron ions were implanted with a total fluence of 6 × 10¹⁷ ions/m² into lithium niobate crystals by way of a sequential implantation at different energies of 95, 100 and 105 MeV respectively through an energy retarder Fe foil to get a uniform Fe doping of about few microns from the surface. The implanted crystals were then annealed in air in the range 200-400 °C for different durations to promote the crystalline quality that was damaged by implantation. In order to understand the basic phenomena underlying the implantation process, compositional in-depth profiles obtained by the secondary ion mass spectrometry were correlated to the structural properties of the implanted region measured by the high resolution X-ray diffraction depending on the process parameters. The optimised preparation conditions are outlined in order to recover the crystalline quality, essential for integrated photorefractive applications.     

Irradiation effect of carbon negative-ion implantation on polytetrafluoroethylene for controlling cell-adhesion property

We have investigated the irradiation effect of negative-ion implantation on the changes of physical surface property of polytetrafluoroethylene (PTFE) for controlling the adhesion property of stem cells. Carbon negative ions were implanted into PTFE sheets at fluences of 1 × 10¹⁴-1 × 10¹⁶ ions/cm² and energies of 5-20 keV. Wettability and atomic bonding state including the ion-induced functional groups on the modified surfaces were investigated by water contact angle measurement and XPS analysis, respectively. An initial value of water contact angles on PTFE decreased from 104° to 88° with an increase in ion influence to 1 × 10¹⁶ ions/cm², corresponding to the peak shifting of XPS C1s spectra from 292.5 eV to 285 eV with long tail on the left peak-side. The change of peak position was due to decrease of C-F₂ bonds and increase of C-C bonds with the formation of hydrophilic oxygen functional groups of OH and CO bonds after the ion implantation. After culturing rat mesenchymal stem cells (MSC) for 4 days, the cell-adhesion properties on the C⁻-patterned PTFE were observed by fluorescent microscopy with staining the cell nuclei and their actin filament (F-actin). The clear adhesion patterning of MSCs on the PTFE was obtained at energies of 5-10 keV and a fluence of 1 × 10¹⁵ ions/cm². While the sparse patterns and the uncontrollable patterns were found at a low fluence of 3 × 10¹⁴ ions/cm² and a high fluence of 3 × 10¹⁵ ions/cm², respectively. As a result, we could improve the surface wettability of PTFE to control the cell-adhesion property by carbon negative-ion implantation.