Negative Ion Fractions Research Articles

Calibration of charge state conversion surfaces for neutral particle detectors

Molecular oxygen and hydrogen ions were scattered off hydrogen terminated diamondlike carbon (DLC) charge state conversion surfaces at incident grazing angles. The energy range of the scattered particles was 390–1000 eV, and the surface roughness of the DLC surface was of the order of 1 Å rms. For all surfaces almost equal angular scattering and negative ion fractions were found within the uncertainties of the measurement. This result supports the fact that charge state conversion with DLC surfaces is a reliable technology for neutral particle sensing instruments. Furthermore, these instruments can work in the laboratory as well as in the harsh environment on board a satellite.The surfaces measured here are used in the IBEX-lo sensor, a neutral particle sensing instrument on the NASA IBEX mission, which is scheduled for launch into orbit around Earth in July 2008.

Negative helium generation upon surface scattering: Application in space science

We report the formation of negatively charged helium ions upon scattering of neutral helium atoms from silicon wafers coated with diamondlike carbon (DLC). The energy of the primary He atoms ranges from 10–1500eV. The negative ion fraction in the scattered He beam ranges from 7×10−5 down to 10−5 with increasing energy. The decreasing negative ion fraction with increasing energy we find for the DLC surfaces is in contrast with earlier results obtained for low work function metallic surfaces. The observed energy dependence cannot be explained by theory because at present there is no theory available that explains the electron transfer between diamond surfaces and scattered atoms. Although the negative ion fraction is low, this ionization technique allows the direct detection of interstellar neutral He with species identification for the first time. This measurement will be performed with the IBEX-Lo sensor on the Interstellar Boundary Explorer mission of NASA.

Nonresonant formation ofH−near unreconstructedSi(100)surfaces

We calculate ab initio the fraction of outgoing negative hydrogen ions that are normally incident on an unreconstructed Si(100) surface with kinetic energies between 50 and 150 eV. The ground-state electronic structure of the surface is derived from a self-consistent screened Thomas-Fermi-von Weizsaecker pseudopotential including Wang-Teter shell structure corrections. Orbitals and energies of the electronic states in this potential are obtained by solving Kohn-Sham equations. The dynamics of the transfer of a single electron during the ion-surface collision is represented within the Newns-Anderson model, including image-charge interactions and electron translation factor. We show that the outgoing H{sup -} fraction evolves at large distances from the surface due to nonresonant transitions from the valence band levels of the substrate into the affinity level of H{sup -}. In particular, we show that electron capture from dangling-bond surface-state resonances determines the final negative-ion fraction. We find good qualitative agreement with the experimental results of Maazouz et al. [Surf. Sci. 398, 49 (1998)] for the scattering of hydrogen atoms and ions on silicon surfaces, even though our calculations do not include the effects of reconstruction and projectile motion parallel to the surface.

Experimental and theoretical study of charge transfer in hydrogen ion scattering from a graphite surface

Ion scattering spectrometry (ISS) with time of flight (TOF) analysis is employed to measure the ion fraction of positively charged hydrogen (H +) projectiles scattered from a well characterized highly oriented pyrolitc graphite (HOPG) surface at a 45° scattering angle, various ingoing/outgoing angles and two different incoming energies (4 and 5 keV). In the theoretical approach, the negative ionization probability is calculated by employing a Green's function formalism to solve the dynamic collisional process. Both theoretical and experimental results are analyzed and contrasted. The theoretical negative ion fraction evolution during the collisional process is described in detail.

Negative ions in single and dual frequency capacitively coupled fluorocarbon plasmas

We have studied charged particle densities and fluxes in a customized industrial etch reactor, running in Ar/O2/c-C4F8 gas mixtures at pressures in the region of 50 mTorr and driven by 2 and 27 MHz RF power, either separately or simultaneously. Independent control of ion flux and ion energy is the aim of using dual frequency plasmas. However, little experimental data exists regarding the charged particle dynamics in complex industrial gas mixtures. Negative ions could play an important role in this type of plasma. The presence of negative ions will modify the positive ion flux arriving at a surface, and they may even reach the surface and participate in etching. We have measured the electron density using a microwave hairpin resonator and the positive ion flux with a RF biased ion flux probe. The ratio of these two quantities, which depends on the negative ion fractions and other factors, is seen to vary strongly with gas chemistry, giving evidence for the presence of negative ions. Our results indicate high electronegativity for high c-C4F8 flow rates. We have also examined the effect of varying the 2 and 27.12 MHz RF powers on both the electron density and the positive ion flux. This allows us to estimate the effect of varying power on the negative ion density. In addition, ultra-violet cavity ring-down spectroscopy was used to measure the F− density directly (Booth et al 2006 Appl. Phys. Lett. 88 151502). This optical measurement was compared with the probe technique.

Scattering of light molecules from Al2O3 surfaces

Various light ions were scattered at grazing incidence from a highly polished Al2O3 single crystal and from a thin film of Al2O3 on graphite. The energy of incident particles was varied from 390eV to 1200eV. For scattered positive oxygen and carbon ions, negative ion fractions of up to 14% and 3%, respectively, were recorded. For scattered positive hydrogen ions, the negative ion fractions reached the percent level.

Experimental investigation of double layers in expanding plasmas

Double layers (DLs) have been observed in a plasma reactor composed of a source chamber attached to a larger expanding chamber. Positive ion beams generated across the DL were characterized in the low plasma potential region using retarding field energy analyzers. In electropositive gases, DLs were formed at very low pressures (between 0.1 and 1mTorr) with the plasma expansion forced by a strongly diverging magnetic field. The DL remains static, robust to changes in boundary conditions, and its position is related to the magnetic field lines. The voltage drop across the DL increases with decreasing pressure; i.e., with increasing electron temperature (around 20V at 0.17mTorr). DLs were also observed in electronegative gases without a magnetic field over a greater range of pressure (0.5 to 10mTorr). The actual profile of the electronegative DL is very sensitive to external parameters and intrusive elements, and they propagate at high negative ion fraction. Electrostatic probes measurements and laser-induced photodetachment show discontinuities in all plasma parameters (electron density, electron temperature, negative ion fraction) at the DL position. The voltage drop across the electronegative DL is about 8V, is independent of the gas pressure and therefore of the electron temperature.

Modeling ion chemistry and charged species diffusion in lean methane–oxygen flames

In this work, the concentration profiles of charged species in a flat, fuel-lean ( Φ = 0.216), and laminar methane–oxygen flame are calculated using a detailed transport model and an elementary reaction mechanism. For the first time, these simulations include negative ions besides neutral species and positive ions. The chemical reaction mechanism of the charged species is compiled from different sources found in relevant literature. Overall, the ion model contains 65 reversible reactions involving 11 charged species. Special emphasis is put on the diffusion processes of the ions. An ambipolar model is developed and discussed which describes the mutual interactions of the charged species during diffusion. It allows for an arbitrary fraction of negative ions because it does not depend on the assumption that the electrons dominate the diffusion process. Reaction flow analyses show the chemical pathways taken. Special focus is put on the effects of ambipolar diffusion on the species profiles and the importance of negative ions. Our findings are compared to experimental results published in the literature.

Ion fractions in the scattering of hydrogen on different reconstructed silicon surfaces

A theoretical calculation that accounts for a fairly complete description of the resonant charge-exchange process occurring in H 0 scattering by Si surfaces is presented. Two reconstructed surfaces for the target: Si(1 0 0)2 × 1 and Si(1 1 1)7 × 7, are analyzed in this work. The interacting system is described by an extended spin-less Anderson Hamiltonian where valence as well as core states of the surface atoms are included. The interaction terms are calculated by taking into account the extended features of the surface and the localized atom–atom interactions within a mean-field approximation. The study is focused mainly in the description of the collision process in terms of short range interactions. The density of states for the surface and subsurface atoms are obtained in each case, from a molecular dynamic-density functional theory in the local density approximation. A binary elastic collision is assumed to fix the projectile trajectory, while the inelastic processes are determined by the interaction of the projectile atom with all the surface atoms ‘seen’ along its trajectory. The ion fractions are calculated by using the Keldysh–Green’s function formalism to solve the time dependent process. We analyze the negative ion fractions of hydrogen measured by Maazouz et al. By including the interaction of the ion projectile with the target atoms seen during its trajectory and averaging over a variety of scattering centers as it may occur in the experimental situation, we obtained a smooth dependence with the exit angle that does not reflect the specific details of the local density of states and the surface topography while reproducing very well the general trends of the experiment. The ion fraction is found to be almost independent on the incoming energy for large values of the exit angles, while in the opposite cases where the projectile spends longer times in contact with the surface, the effect of the parallel component of the velocity has an increasing importance. Thus, the fine details of the surface are ‘better captured’ as the parallel velocity component becomes smaller.

The electronegative character of a reflex discharge source operating in hydrogen

In this paper attention is focused on the possibility of obtaining a high production of negative hydrogen ions H− within the volume of a nonequilibrium reflex discharge plasma for gas pressure levels from 0.1 to 10 Pa. To obtain the best values for both the negative hydrogen ion density n− and the negative ion fraction n−/ne the operating conditions are optimized as a function of pressure, discharge power, and magnetic field. The experiment shows that the reflex discharge steady-state plasma is uniform for a dissipated power P = 400 W and has the best efficiency within the pressure range from 0.5 to 2.0 Pa, where the negative ion density is almost 1017 m−3, and the negative ion fraction n−/ne is almost 2.5%.

Conversion surfaces for neutral particle imaging detectors

Low-energy neutral particles must be ionized before they are analyzed by mass spectrometric means. Within the limitations regarding space, weight and power consumption onboard a spacecraft surface ionization has been identified as the only viable ionization technique. In the study presented here molecular oxygen and hydrogen ions were scattered at grazing incidence from several diamond-like carbon (DLC) surfaces in the energy range from 190 to 2400 eV. Most surfaces were hydrogen terminated. For incident positive oxygen and hydrogen ions, scattered negative ion fractions of up to 28% and more than 6%, were measured, respectively. These values are among the highest ever reported, especially for oxygen. Furthermore, though these surfaces are amorphous, due to improved polishing techniques angular scattering distributions of the scattered beam have been observed, which were comparable to distributions of scattering experiments with single-crystal surfaces. Therefore, these DLC surfaces are strong candidates to work as conversion surfaces in a neutral particle imaging detector for the BepiColombo mission.

Temporal phenomena in inductively coupled chlorine and argon–chlorine discharges

Reproducible modulations in low-pressure, inductively coupled discharges operating in chlorine and argon–chlorine mixtures have been observed and studied. Changes in the light output, floating potential, negative ion fraction, and charged particle densities were observed. Here we report two types of unstable operational modes in an inductively coupled discharge. On the one hand, when the discharge was matched, to minimize reflected power, instabilities were observed in argon–chlorine plasmas over limited operating conditions of input power and gas pressure. The instability window decreased with increasing chlorine content and was observed for chlorine concentrations between 30% and 60% only. However, when operating at pressures below 5mTorr and the discharge circuit detuned to increase the reflected power, modulations were observed in a pure chlorine discharge. These modulations varied in nature from a series of sharp bursts to a very periodic behavior and can be controlled, by variation of the matching conditions, to produce an apparent pulsed plasma environment.

High negative ion yield from light molecule scattering

Molecular oxygen and hydrogen ions were scattered at grazing incidence from several diamond-like carbon (DLC) surfaces in the energy range from 190eV to 2400eV. Most surfaces were hydrogen terminated. For incident positive oxygen ions, scattered negative ion fractions of up to 33% were recorded, and for incident positive hydrogen ions, negative ion fractions of more than 5% were measured. These values are among the highest ever reported, especially for oxygen. They have been matched only by results of scattering experiments using a hydrogen terminated surface of natural diamond.

Power Absorption of High Frequency Electromagnetic Waves in a Partially Ionized Plasma Layer in Atmosphere Conditions

We have studied the absorption, reflection, and transmission of electromagnetic waves in an unmagnetized uniform plasma layer covering a metal surface in atmosphere conditions. Instead of the absorption of the electromagnetic wave propagating only once in previous work on the plasma layer, a general formula of total power absorption by the plasma layer with an infinite time of reflections between the atmosphere-plasma interface and the metal surface has been derived for the first time. Effects of plasma parameters, especially the dependence of the fraction of positive ions, negative ions and electrons in plasmas on the power absorption processes are discussed. The results show that the existence of negative ions significantly reduces the power absorption of the electromagnetic wave. Absorptions of electromagnetic waves are calculated.

Multivariate Statistical Analysis of Concatenated Time-of-Flight Secondary Ion Mass Spectrometry Spectral Images. Complete Description of the Sample with One Analysis

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) instruments are capable of saving an entire mass spectrum at each pixel of an image, allowing for retrospective analysis of masses that were not selected for analysis during data collection. These TOF-SIMS spectral images contain a wealth of information, but few tools are available to assist the analyst in visualizing the entire raw data set and as a result, most of the data are not analyzed. Automated, nonbiased, multivariate statistical analysis (MVSA) techniques are useful for converting the massive amount of data into a smaller number of chemical components (spectra and images) that are needed to fully describe the TOF-SIMS measurement. Many samples require two back-to-back TOF-SIMS measurements in order to fully characterize the sample, one measurement of the fraction of positively charged secondary ions (positive ion fraction) and one measurement of the fraction of negatively charged secondary ions (negative ion fraction). Each measurement then needs to be individually evaluated. In this paper, we report the first MVSA analysis of a concatenated TOF-SIMS data set comprising positive ion and negative ion spectral images collected on the same region of a sample. MVSA of concatenated data sets provides results that are intuitive and fully describe the sample. The analytical insight provided by MVSA of the concatenated data set was not obtained when either polarity data set was analyzed separately.

Calculation of ion-surface collisions for a wide range of scattering geometries

A theoretical calculation that accounts for a fairly complete description of the resonant charge-exchange process occuring in the H + scattering by metal surfaces is presented. Realistic trajectories defined by the binarycollision model are considered. The interaction with nuclei and electrons of the all surface atoms that the projectile can see along its trajectory is calculated within a mean-field approximation, and in this form the contributions of the short-range interaction terms to the energy level shift are well contemplated. The long-range contributions and the motion of the projectile respect to the surface reference frame are also taken into account to define the level shift. All these ingredients are incorporated into a quantum mechanical description of the time evolution. The negative ion fractions calculated in this form show an excellent agreement with the experimental data for three different incoming energies and for a wide range of exit angles.

Mechanism of negative ion formation in low velocity collisions at surfaces

Theoretical calculations using the coupled angular modes (CAM) method have been used to interpret experimental findings related to the scattered oxygen negative ion fraction (O −) in collisions of low energy O + with an Al(1 1 0) surface. The increasing O − ion fraction observed experimentally at low O + velocities can be explained in terms of the distance of closest approach Z c to the Al surface and the specific charge state of the projectile at this distance. Both Z c and the charge state influence the charge transfer process between the projectile and the surface at low collision energies.

Scattering of slow ions from insulator surfaces at the example of molecular oxygen from LiF(1 0 0)

We scattered molecular oxygen ions at grazing incidence from a LiF(1 0 0) surface in the energy range from 880 to 3040 eV. About 50% molecular survival was obtained at the lowest primary energy of 881 eV and this dropped to about 4% at 3039 eV. Below 2 keV beam energy we found negative ion fractions of less than 2%, which increases up to 6% at about 3 keV primary energy. The molecular survival is much higher whereas the negative ion fraction is much lower compared to other insulating materials, i.e. diamond and MgO. The mean energy loss is found to increase linearly with energy and is in the order of a few percent.

Charge carrier dynamics in a pulsed inductive RF discharge in oxygen

We report on the spatio-temporal behaviour of negative oxygen ions in a pulsed inductively coupled RF excited oxygen discharge in the gaseous electronics conference cell. The temporal behaviour of the positive and negative charge carriers in the afterglow of the discharge allows identification of the dominant loss processes of negative ions. The results are in good agreement with model calculations. The negative ions are trapped for a long time in the bulk plasma without diffusion to the walls. Negative ions can diffuse to the walls only after complete decay of the electron component and formation of an ion–ion plasma. We can identify a high fraction of negative oxygen molecular ions that are generated during this phase in collisions of negative atomic oxygen ions with metastable oxygen molecules. Such a high fraction of molecular negative ions has not been observed so far in continuous discharges.

Instabilities in an inductively coupled oxygen plasma

A reproducible instability, which appears similar to those reported previously, has been observed and studied in a low-pressure 13.56 MHz inductively coupled gaseous electronics conference rf cell operating in oxygen. The instability has been observed in the form of periodic modulations in the light output, floating potential, electron and positive and negative ion densities. The magnitude and frequency of the modulations is sensitive to the plasma operating conditions and the modulation amplitude has been observed to be as high as 40%. The instability is observed in a pressure and power regime where both the capacitive and inductive modes can exist. The frequency of the oscillations increases with increase in gas pressure from 3 to 21 kHz. This pressure window coincides with the pressure regime where there exists a significant fraction of negative ions in both modes. Time-resolved measurements of the electron energy distribution functions and charged particle densities indicate that at all phases of the instability, the plasma parameters remain close to those of the inductive mode. A global model has been modified for an oxygen discharge and this provides a qualitative description of the instability. The global model predicts a smaller power and pressure window for the instability but it can provide a framework for the discussion of instabilities in weakly electronegative discharges.