Charge exchange in low-energy ion-surface scattering

Charge exchange between low-energy projectiles and solid materials is explored for systems in which there is overlap between the ionization level and the surface bands. Time-of-flight spectroscopy is used to measure the charge state and kinetic energy distributions of scattered and recoiled atomic particles. Alkali ions scattered from alkali and halogen adatoms have larger neutralization probabilities than for scattering from the substrate, indicating that they both have positively charged regions at the top, which is surprising for the negatively charged halogen adsorbates. Because of strong overlap, Na⁺ ions scattered from Au nanocrystals neutralize by interaction with the quantum-confined states. Charge exchange in symmetric systems was investigated by measurements of the charge state of Al emitted from clean Al(1 0 0) by direct recoil induced by 5 keV Xe⁺ ions. The neutralization of recoiled Al is basically consistent with a resonant process resulting from the overlap of the Al atomic and solid states.     

Sassy,a gas-filled magnetic separator for the study of fusion reaction products

A gas-filled on-line recoil separator at the Berkeley SuperHILAC is described. The separator consists of a magnetic dipole and a quadrupole doublet. The system is filled with He at a pressure of about 1 Torr (∼ 130 Pa). It separates particles according to their average magnetic rigidity. This allows a separation of evaporation residues, scattered target atoms and beam projectiles in a nuclear reaction.The separation time is in the order of 10⁻⁶ s; the transmission is about 50% for evaporation residues, less than 10⁻³ for scattered target-like recoil ions and transfer products, and about 10⁻¹⁵ for full energy beam projectiles. Experimental data over a wide range of reactions are given and it is shown that the average charge of the recoils is determined by the atomic shell structure of the moving ionized atom in the gas.     

Guided transmission of Ne ions through nanocapillaries in PET: dependence on the tilt angle

We measured the transmission of 3 keV Ne⁷⁺ ions through nanocapillaries of 100 nm diameter and 10 μm length in a PET polymer foil. The capillaries were produced before the transmission experiments by etching tracks of fast xenon ions. The foils were tilted up to 15° for which the incident ions are forced to approach the capillary surface. After a characteristic time the majority of Ne⁷⁺ ions were found to be transmitted in their initial charge state. The angular distributions of the transmitted Ne⁷⁺ projectiles reveals a propagation of the ions parallel to the capillary axis. This indicates that the ions are guided through the capillaries. The capillary guiding is the result of deposit of charges in a self-organizing process. A non-linear model is presented to interpret the experimental observation. The analysis exhibits a significant dependence of the ion transmission on the tilt angle.     

A new time-of-flight instrument capable of in situ and real-time studies of plasma-treated surfaces

We introduce a new time-of-flight (TOF) instrument that has been constructed to study the dynamics of plasma-surface interactions. The instrument uses a well-defined ion beam at a grazing incidence as a surface probe. Real-space and real-time profiles of scattered particles are created from the output of a position-sensitive detector. The set-up permits the recording of energy and angular distributions of scattered ions and neutrals. Changes in energy and angular distribution as a function of time can be used to monitor real-time and in situ the interaction between plasma and surfaces. The performance of the set-up is tested and illustrative spectra for Ar⁺ scattering from Si (1 0 0) surfaces that were subjected to different pre-treatments are shown.     

Low emittance ion beams by laser interaction

Laser ion sources offer the possibility to get ion beam utilizable to improve particle accelerators. Pulsed lasers at intensities of the order of 10⁸ W/cm² and of ns pulse duration interact with solid matter in vacuum to produce plasma of high temperature and density. The charge state distribution of the plasma generates high electric fields which accelerate ions along the normal to the target surface. The energy of emitted ions has a Maxwell Boltzmann distribution, which depends on the ion charge state. To increase the ion energy, a post-acceleration system can be employed by means of high voltage power supplies of about 100 kV. The post-acceleration system results in a good method to obtain high ion currents by an inexpensive system and the final ion beams find interesting applications in the field of the ion implantations, scientific applications and industrial use. In this work we compare the electromagnetic and geometric properties, like the emittance, of the beams delivered by a Cu and Y target. The characterization of the plasma was performed by a Faraday cup for the electromagnetic characteristics and a pepper pot system for the geometric ones. At 60 kV accelerating voltage and 5.5 mA output current the normalized beam emittance resulted in 0.22 π mm mrad for the Cu target, while under the same accelerating voltage but with 7.4 mA output current, a lower normalized beam emittance value was reached for the Y target. It resulted in 0.14 π mm mrad. The brightness of the beams was of 114 and 378 mA (π mm mrad)⁻² for the Cu and Y targets, respectively.     

Optical properties of Cu and Ag nanoparticles synthesized in glass by ion implantation

Metal nanoparticles synthesized by sequentially ion-implanted Ag and Cu into silica glasses have been studied. The implantation doses (×10¹⁶ ions/cm²) were 5Ag, 5Cu and 5Ag/5Cu, respectively. The optical and microstructural properties of the nanoparticles were characterized by optical absorption spectra and transmission electron microscopy (TEM), respectively. Fast nonlinear optical refraction and nonlinear optical absorption coefficients were measured at 1064 nm of wavelength using Z-scan technique. Results in this paper indicate that the nonlinear refractive index for the Ag/Cu implanted system has a higher value compared to single Ag or Cu implantation nanoparticles.     

In situ spectroelectrochemistry of poly(N,N'-ethylenebis(salicylideneiminato)Cu(II))

In situ ESR, UV-visible, and FT-IR-ATR spectroelectrochemistry were used to study the charge transfer for electrochemically synthesized poly(N,N'-ethylenebis(salicylideneiminato)Cu(II)), which represents a macromolecular metal chelate with ESR-active central metal ions. Structural evidence for different charged states in electroactive poly(Cu(II)-salen) was obtained from ESR, UV-visible, and FT-IR spectra under reversible redox cycling. The changes in a characteristic broad ESR line without hyperfine splitting originating from Cu(II) as well as in the corresponding UV-visible and infrared spectra are discussed in order to describe an electron transfer to the redox-active sites within the polymer chains in detail.     

Spatial distribution of thorium fission rate in a fast spallation and fission neutron field: An experimental and Monte Carlo study

The Energy plus Transmutation (EpT) set-up of the Joint Institute for Nuclear Research (JINR), Dubna, Russia is composed of a lead spallation target surrounded by a blanket of natural uranium. The resultant neutron spectrum is a combination of spallation and fission spectra, modified by a reflective external layer of polyethylene and an internal absorbing layer of cadmium. The EpT set-up was irradiated with a beam of 4 GeV deuterons from the Nuclotron Accelerator at JINR. The spatial distribution of thorium fission rate within the assembly was determined experimentally, using a fission track detector technique, and compared with Monte Carlo predictions of the MCNPX code. Contributions of neutrons, protons, deuterons, photons and pions to total fission were taken into account. Close agreement between the experimental and calculated results was found.     

Thermal evolution and optical properties of Cu nanoparticles in SiO2 by ion implantation

SiO₂ samples were implanted by 45 keV Cu ions at a dose of 1 × 10¹⁷ /cm², and subjected to furnace annealing at temperatures ranging from 200 to 600 °C in nitrogen atmosphere. The results indicate that the Cu nanoparticles have been synthesized by Cu ion implantation, and subsequent annealing induces the diffusion and nucleation of nanoparticles partially. The results from XPS measurements show that the Cu⁰ is the dominate charge state in the implanted and subsequent annealed samples. With increasing annealing temperature, the size and distribution of Cu nanoparticles have been modified gradually. The surface plasmon resonance (SPR) of Cu nanoparticles at 570 nm has been observed by optical transmission spectroscopy. The strongest SPR signal at 400-600 °C indicates that lots of Cu nanoparticles have grown and show good optical properties. Moreover, the luminescence has been investigated in Cu implanted and subsequent annealed samples. Possible luminescence mechanisms, such as radiation induced defects, Cu (ions or atoms) related luminescence centers, etc., have been discussed.     

Photoinduced polarization investigations in 75 MeV oxygen ion irradiated kapton-H polyimide

The photoinduced polarization in 75 MeV oxygen ion irradiated (fluence: 1.8 × 10¹¹, 1.8 × 10¹² and 1.8 × 10¹³ ions/cm²) kapton-H polyimide has been investigated by analyzing charge decay characteristics for different polarizing parameters viz. electric fields (40 to 600 kV/cm), temperature (40 to 250 °C) and illumination intensity (1200 to 2800 1x). The fields induced as well as thermal ionization of excitons under illumination are the main causes which provide photopolarization. The charge decay spectra reveal the presence of both shallow and deep trapping sites in pristine and irradiated kapton-H polyimide. The variation in the photopolarization with fluence shows the occurrence of secondary radiation induced crystallinity (SRIC). The SRIC is also responsible for the increase in initial current (I_0P) with intensity of illumination in low fluence irradiated samples. A decrease in I_0P with intensity of illumination in high fluence irradiated samples has been associated to the conversion of trapping sites into recombination centers.     

Surface analysis with heavy ion induced Auger electrons; basic properties of a new method for materials sciences

Heavy ion induced Auger electron spectroscopy is presented as a new tool for surface analysis. In contrast to conventional Auger electron spectroscopy using electron excitation, the new technique offers the advantage of selective excitation of specific impurity elements present in a solid matrix by choosing a proper value of projectile mass and projective energy. The technique is characterized by very fast rising cross-sections at threshold. The cross-sections can be very large in comparison to electron excitation. Due to the high energy loss of heavy ions at the energies applied, the projectiles are probing a very thin surface layer of the sample. Another benefit of heavy ion excitation is the possibility of simultaneous detection of back scattered heavy ions as well as the possibility for detection of ultra soft X-rays. In this way three complementary techniques are available simultaneously for mutual comparison.     

Depth profile analysis of electrodeposited nanoscale multilayers by SNMS

As early as 10 years after the discovery of the giant magnetoresistance (GMR) the magnetic/non-magnetic multilayers found their first application in the read-out units of magnetic recording media, and the hard disk drives with GMR-based sensors since gained a dominating market share. In spite of the large number of works published on nanoscale multilayers, data on the depth profile of electrodeposited multilayer samples are very scarce. This work deals with the depth profile analysis of electrodeposited CoNiCu/Cu and Co/Cu multilayers films. Commercial Cu sheet and a Cr/Cu layer evaporated onto Si (1 1 1) surface were used as substrates with high and low roughness, respectively. The Secondary Neutral Mass Spectrometry (SNMS) depth profile analysis clearly revealed the layered structure of the samples. The resolution of the individual layers varied with the initial roughness of the substrate. The SNMS spectra showed that the oxygen incorporation into the layers is insignificant. When both Ni and Co are present in the magnetic layer, the composition of the samples is influenced by both the anomalous codeposition properties of the iron-group elements and the mass transport of the corresponding ions in the electrolyte. This observation draws the attention to the possible inhomogeneity of the magnetic layers in electrodeposited samples.     

Target set-up for measurements with the use of a Si-ball detector

The thin n-p⁺ layer of Si n⁺-n-p⁺ wafers composing the light charged particles detectors can be easily damaged by scattered beam projectiles or heavy products of the nuclear reaction in target material. Such damages of the sensitive part of the wafer may in consequence shorten the detector life-time. This could be avoided by protecting the sensitive part of the detector with shielding that stops the heavy projectiles.Due to the compact configuration of the detectors in a small Si-ball counters placing the shielding in front of each wafer could be difficult and therefore a target-shielding assembly is proposed.     

Enhanced nanoparticle formation by indentation and annealing on 2 MeV Cu ion-implanted SiO2

Enhancement of surface plasmon resonance (SPR) in optical absorption has been found on Cu ion-implanted SiO₂ substrate modified by micro-indentation and post-annealing. Micro-indentation effects on surface plasmon resonance (SPR) in optical absorption have been studied to control nanoparticle formation in Cu ion-implanted SiO₂ substrate. The SiO₂ was firstly implanted with 2 MeV Cu²⁺ ions at an ion flux of 4 μA/cm², up to a fluence of 6 × 10¹⁶ ions/cm². After the ion implantation, dot-array patterns of micro-indents were made by a micro-Vickers hardness tester, and followed by annealing at 600 °C in vacuum for 1 h. The optical absorption spectra of the indented region and the non-indented flat region were measured and compared with each other. After post-annealing at 600 °C, the indented area showed higher absorbance of SPR at 2.2 eV than that of the flat region annealed under the same annealing conditions. The TEM study shows larger and denser Cu precipitates inside the indentation than those in the flat area. The results indicate that the defects produced by indentation enhance the atomic migration in the plastic zone during thermal relaxation process, resulting in promoting the enhanced precipitation of Cu nanoparticles.     

Ultra-high velocity impacts: cratering studies of microscopic impacts from 3 km/s to 30 km/s

Cratering experiments performed under carefully controlled conditions at impact velocities ranging from 3 km/s to 30 km/s into a wide variety of target materials are presented. These impact experiments use the 6 MV vertical Van de Graaff accelerator of the Ion Beam Facility at the Los Alamos National Laboratory to electrostatically accelerate highly charged iron micro-spheres. The sub-micron spheres, from a random size distribution, are shocklessly accelerated along an 8 m flight path. Ultra-sensitive charge detectors monitor the passage of the projectiles at a rate of up to 100 projectiles/second. An online computer records and displays in real time the charge, velocity and mass of the projectiles and provides cross correlation between the events observed by the several in-flight charge detectors and impact detectors. Real-time logic controls an electrostatic kicker which deflects projectiles of selected charge and velocity onto the target. Thus each experiment consists of an ensemble of 10 to 40 impacts onto a single target within a narrow window of the projectile parameter space, providing excellent statistical resolution of each data point.

The target materials used include single crystal copper and single crystal aluminum, gold, and quartz as well as pyrolytic graphic and anoxy used in composite materials of interest to space applications. We also conducted impact experiments onto thin Mylar and nickel foils. This paper presents these experiments and summarizes the cratering characterization performed to date. Emphasis is placed on cratering results in several materials over a range of impact velocities.     

Role of electrons in laser desorption/ionization mass spectrometry

A nonmetallic sample support for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry enhances the positive ion yield by 2 orders of magnitude and generally affects the charge balance in the desorption plume. We interpret the effects of the target material and of the sample preparation on MALDI mass spectra as a result of photoelectrons emitted upon laser irradiation of a metal target covered by a thin sample layer. These electrons are shown to play an important role in MALDI and laser desorption/ionization because they decrease the yield of positive ions, reduce ions with higher oxidation states, and affect the ion velocity distribution as well as the mass resolution. Understanding the role of these photoelectrons helps to clarify previously obscure aspects of the ion formation mechanism in MALDI.     

Features of spectra of low-energy neon ions scattered from gallium phosphide

Using mass-resolved ion scattering spectrometry, spectra of Ne⁺ ions scattered at an angle of 120° from the surface of GaP in the energy range of 0.4–1.96 keV have been studied in detail. In the spectra, in addition to the peaks of elastic binary Ne⁺/P and Ne⁺/Ga collisions, the peak of sputtered neon ions has been found, as well as the wide peak (a “hump”), the energy of which slightly depends on the energy of primary ions and the intensity considerably increases with an increase in this energy. In our opinion, the main contribution to this peak is made by neon ions that undergo multiple collisions with gallium and phosphorus atoms on the surface and deeper layers of the sample and keep their charge due to reionization processes.     

Corona discharge in charge reduction electrospray mass spectrometry

Corona discharge is applied to charge reduction electrospray mass spectrometry for the analysis of complex mixtures of biological molecules. Recent work has described a method of charge reduction (reducing the charge states of analyte ions generated by the electrospray process) employing the radioactive isotope 210Po to produce neutralizing species. A variation to this approach is presented, in which charge neutralization is mediated by ions produced in a corona discharge. Varying the corona discharge voltage controls the current and the degree of charge reduction, providing predominantly singly charged ions that are detected by a commercial electrospray time-of-flight mass spectrometer. This technique provides charge reduction for the simplification of ESI spectra, without need for any radioactive material.     

In-situ analyses on the reactive sputtering process to deposit Al-doped ZnO films using an Al-Zn alloy target

The kinetic energies of generated ions were investigated during the reactive sputtering process to deposit Al-doped ZnO (AZO) films using an Al-Zn alloy target. The sputtering system was equipped with specially designed double feedback system to stabilise the reactive sputtering processes and analysis was performed with a quadrupole mass spectrometer combined with an energy analyser. Negative ions O⁻, O₂⁻, AlO⁻ and AlO₂⁻ with high kinetic energies corresponding to cathode voltage are generated at the partially oxidised target surface, after which some of the ions undergo subsequent charge exchange and/or dissociation. Positive ions O⁺, Ar⁺, Zn⁺ and Al⁺ with lower kinetic energies (around 10 eV) are generated by charge exchange of sputtered neutral O, Ar, Zn and Al atoms, respectively. As the target surface oxidises, cathode voltage decrease, the flux of high-energy negative ions increases and the electrical properties of the AZO degrade by ion bombardment as well as the AZO films that are deposited by conventional magnetron sputtering using an AZO target.