Ion Trajectory Calculations Research Articles

Charge states of energetic ions in Jupiter's radiation belt inferred from absorption microsignatures of Io

The Heavy Ion Counter on the Galileo Jupiter orbiter observed depletions of energetic (≳5 MeV/nucleon) oxygen and sulfur ions near Io that are interpreted as absorption microsignatures. The two elements were equally abundant in this region of Jupiter's radiation belt. Numerical simulations of microsignatures from Galileo orbits 27, 31, and 32, based on calculations of ion trajectories in models of the magnetospheric and Alfvén wing magnetic fields, show that the data are most consistent with the ions being fully ionized or nearly so. The conclusion is independent of field model inaccuracies, which cause some discrepancy between simulations and data, because the Alfvén wing field significantly deflects only those trajectories with relatively small gyroradii.

Realistic modeling of ion cloud motion in a Fourier transform ion cyclotron resonance cell by use of a particle‐in‐cell approach

Using a 'Particle-In-Cell' approach taken from plasma physics we have developed a new three-dimensional (3D) parallel computer code that today yields the highest possible accuracy of ion trajectory calculations in electromagnetic fields. This approach incorporates coulombic ion-ion and ion-image charge interactions into the calculation. The accuracy is achieved through the implementation of an improved algorithm (the so-called Boris algorithm) that mathematically eliminates cyclotron motion in a magnetic field from digital equations for ion motion dynamics. It facilitates the calculation of the cyclotron motion without numerical errors. At every time-step in the simulation the electric potential inside the cell is calculated by direct solution of Poisson's equation. Calculations are performed on a computational grid with up to 128 x 128 x 128 nodes using a fast Fourier transform algorithm. The ion populations in these simulations ranged from 1000 up to 1,000,000 ions. A maximum of 3,000,000 time-steps were employed in the ion trajectory calculations. This corresponds to an experimental detection time-scale of seconds. In addition to the ion trajectories integral time-domain signals and mass spectra were calculated. The phenomena observed include phase locking of particular m/z ions (high-resolution regime) inside larger ion clouds. A focus was placed on behavior of a cloud of ions of a single m/z value to understand the nature of Fourier transform ion cyclotron resonance (FTICR) resolution and mass accuracy in selected ion mode detection. The behavior of two and three ion clouds of different but close m/z was investigated as well. Peak coalescence effects were observed in both cases. Very complicated ion cloud dynamics in the case of three ion clouds was demonstrated. It was found that magnetic field does not influence phase locking for a cloud of ions of a single m/z. The ion cloud evolution time-scale is inversely proportional to magnetic field. The number of ions needed for peak coalescence depends quadratically on the magnetic field.

A novel surface-induced dissociation instrument for ion mobility-time-of-flight mass spectrometry

A surface-normal surface-induced dissociation (SID) configuration specifically designed for coupling ion mobility spectrometry (IMS) and orthogonal time-of-flight (TOF) mass spectrometer is described. The instrument configuration and the effects of various operating parameters are critically evaluated using ion trajectory calculations (SIMION) and SID spectra of a series of model peptides. The utility of the instrument configuration for simultaneous acquisition of MS and MS–MS spectra in both data-dependent and non-data-dependent modes are also discussed.

Superconducting magnet for a cyclotron Rutherford backscattering spectrometry system

We have developed a superconducting magnet for a high-speed and high-resolution Rutherford backscattering spectrometry system that utilizes the ion motion along the cyclotron trajectory in a magnetic field. The superconducting magnet without cryogen consists of a main coil, shim coils, a 4K refrigerator and pole pieces. It generates a 2T magnetic field in the spectrometer of ϕ 340×750mm. The shapes of the pole pieces and the coil arrangements are optimized by a magnetic field analysis to generate a uniform field. The magnetic field distribution is made to be so uniform that the scattered ions can pass through a ϕ 1mm aperture, by ion trajectory calculation. The trajectory of the incident beam is also computed, and the position of a specimen holder is determined so that the incident beam is well focused on the specimen.

TOF-SIMS: Accurate Mass Scale Calibration

A study is presented of the factors affecting the calibration of the mass scale in time-of-flight secondary ion mass spectrometry (TOF-SIMS). At the present time, TOF-SIMS analysts using local calibration procedures achieve a rather poor relative mass accuracy of only 150 ppm for large molecules (647 u) whereas for smaller fragments of <200 u this figure only improves to 60 ppm. The instrumental stability is 1 ppm and better than 10 ppm is necessary for unique identification of species. The above experimental uncertainty can lead to unnecessary confusion where peaks are wrongly identified or peaks are ambiguously assigned. Here we study, in detail, the instrumental parameters of a popular single stage reflection TOF-SIMS instrument with ion trajectory calculations using SIMION. The effect of the ion kinetic energy, emission angle, and other instrumental operating parameters on the measured peak position are determined. This shows clearly why molecular and atomic ions have different relative peak positions and the need for an aperture to restrict ions at large emission angles. These data provide the basis for a coherent procedure for optimizing the settings for accurate mass calibration and rules by which calibrations for inorganics and organics may be incorporated. This leads to a new generic set of ions for mass calibration that improves the mass accuracy in our interlaboratory study by a factor of 5. A calibration protocol is developed, which gives a relative mass accuracy of better than 10 ppm for masses up to 140 u. The effects of extrapolation beyond the calibration range are discussed and a recommended procedure is given to ensure that accurate mass is achieved within a selectable uncertainty for large molecules. Additionally, we can alternatively operate our instrument in a regime with good energy discrimination (i.e., poor energy compensation) to study the fragmented energies of molecules. This leads to data that support previous concepts developed in G-SIMS.

Determination of the Size Distribution of Polydisperse Nanoparticles with Single-Particle Mass Spectrometry: The Role of Ion Kinetic Energy

We develop a method to determine size and size distribution (30–150 nm) of polydisperse nanoparticles using a laser ablation/ ionization time-of-flight single-particle mass spectrometer that extends the work first described by Reents and Ge. We found a composition independent “power law” dependence between the total peak area and original particle volume that enables one to determine particle volume directly from a particles mass spectrum. This power-law relationship suggests that some ions ablated and ionized from a particle are selectively lost during transport from the laser ablation/ionization region to the detector. A numerical calculation of ion trajectories shows that ion loss is highly dependent on the initial kinetic energy of ions. We show that the size-dependent energetic ions formed by the laser-particle interaction lead to power-law relationship between the cube root of peak area and particle diameter. The results demonstrate that particle size distributions measured with the mass spectromete...

High-resolution state-selected ion-molecule reaction studies using pulsed field ionization photoelectron-secondary ion coincidence method

We have developed an octopole-quadrupole photoionization apparatus at the Advanced Light Source for absolute integral cross-section measurements of rovibrational-state-selected ion-molecule reactions. This apparatus consists of a high-resolution photoionization ion source, a wired ion gate lens, a dual radio-frequency (rf) octopole ion guide reaction gas cell, and a quadrupole mass spectrometer for reactant and product ion detection. The unique feature of this apparatus is the implementation of the high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) technique, which has allowed the rotational-state selection of diatomic ions for ion-molecule reaction studies. The novel application of the wired ion gate lens for the rejection of false coincidence background ions is described. This application, along with the differential-ion-gate scheme, has made possible the measurements of rovibrational-state-selected absolute integral reaction cross sections for ion-molecule collisions using the PFI-PE-secondary ion coincidence (PFI-PESICO) method. The successful measurement of absolute state-selected cross sections for H2+(X,v+,N+)+Ar(Ne) with v+ up to 17 [the third to the last vibrational state of H2+(X)] demonstrates the high sensitivity of this differential-ion-gate PFI-PESICO method. In order to gain a detailed understanding and to obtain optimal performance of the wired ion gate lens for PFI-PESICO measurements, we have carried out ion trajectory calculations of reactant ions between the photoionization region and the rf-octopole ion guide. On the basis of these calculations, possible future improvements for the application of this differential-ion-gate PFI-PESICO scheme are discussed.

A second-generation ion beam buncher and cooler

A radiofrequency quadrupole (RFQ) ion accumulator and buncher has been designed for the low-energy beam and ion-trap (LEBIT) facility which is being set up at the NSCL/MSU. The LEBIT buncher will be a cryogenic system. Compared to room-temperature systems an improved beam quality and overall efficiency are expected. It will feature a novel electrode structure with a drastically reduced number of electrodes for simplified operation. Its design is presented and Monte-Carlo type ion-trajectory calculations are discussed which predict excellent beam quality and high performance.

Modeling the ion density distribution in collisional cooling RF multipole ion guides

Collisional cooling radio frequency (RF) multipoles are widely used in mass spectrometry, as ion guides and two-dimensional ion traps. Understanding the behavior of ions in these devices is important in choosing a multipole configuration and optimizing its performance. We have developed a computer model based on ion trajectory calculations in the RF multipole electric field, taking into account ion–ion and ion–neutral interactions. The two-dimensional model for idealized infinite RF multipoles gives an accurate description of the ion density distribution. We consider first a basic case of a single m/ z ion cloud in the two-dimensional RF quadrupole after equilibrium is reached. Approximate theoretical relationships for the ion cloud configuration in the two-dimensional ion trap are tested based on the simulation results. Next we consider the case of an ion cloud consisting of several different m/ z ion species. The ion relaxation dynamics and the process of establishing the stratified ion density distribution are observed. Simulations reveal that the ion kinetic energy relaxation dynamics are dependent on the ion population and bath gas pressure. The equilibrium distribution agrees well with the ion stratification theory, as demonstrated by simulations for RF quadrupole and octopole two-dimensional ion traps.

Influence of the 6th and 10th spatial harmonics on the peak shape of a quadrupole mass filter with round rods.

The influence of the ratio of the rod radius, r, to field radius, r(0), on the peak shape for a linear quadrupole mass filter constructed with round rods has been investigated. The expansion of the potential in multipoles, phi(N),Phi(x, y) = sum(infinity)(N=0)A(N)phi(N)/r(N)(0) has been considered, and the peak shape and resolution have been determined by numerical calculation of ion trajectories in quadrupoles with different ratios, r/r(0). Geometries that make the dodecapole term (A(6)) zero (r/r(0) = 1.14511) do not give the best performance because the contribution of the 20-pole term, A(10), must be considered. The optimum ratio is r/r(0) approximately 1.13. With this ratio the dodecapole term (A(6)) is ca. 1 x 10(-3), but its effects are partially compensated by the A(10) term which has similar magnitude, but opposite sign.

Quadrupole Mass Filter Transmission in Island a of the First Stability Region with Quadrupolar Excitation

With quadrupolar excitation of ions by a small auxiliary RF field, the first stability region of the quadrupole mass filter splits into islands of stability. This study is of the first upper island, “A”. We calculate the location of this island for different values of the excitation parameter, q‘. Ion trajectory calculations are used to study the influence of the input fringing fields and the separation time. The Runge-Kutta–Nustrom–Dormand-Prince (RK–N–DP) method of order 6(7) is used for integration of the equations of ion motion. It is found that a weak auxiliary RF signal improves the peak shape for quadrupole mass filters that have weak field distortions.

Mapping Jupiter's outer radiation belt

Flux maps of energetic ions in Jupiter's outer radiation belt, calculated with the aid of a model of the current sheet magnetic field, are consistent with a spatial distribution that is uniform along field lines and monotonically decreasing with radial distance. This result is supported by numerical ion trajectory calculations that predict uniform filling of drift shells at high L values. Adiabatic compression by radial transport then provides predictions of radial energy dependence and flux gradients. At lower L values, conservation of the second adiabatic invariant predicts some anisotropy with higher equatorial fluxes.

Mechanism of Collisional Heating in Electrospray Mass Spectrometry: Ion Trajectory Calculations

To simulate the multicollisional heating process taking place in the intermediate pressure region of an electrospray source, ion trajectory calculations have been performed by introducing in the SIMION program a subroutine for handling the collision dynamics. The simulated internal energy distributions are compared with already available experimental distributions obtained by the “survival ion yield” method (Collette, C.; Drahos, L.; De Pauw, E.; Vékey, K. Rapid Commun. Mass Spectrom. 1998, 12, 1673). The latter have been satisfactorily reproduced by using a statistical model for the energy redistribution between the ion and the target gas. This model takes into account both activation and deactivation processes occurring upon collision in the acceleration/fragmentation region. It accounts for the important role of the number of degrees of freedom of the target on the shape of the internal energy distribution.

Reducing grid dispersion of ions in orthogonal acceleration time-of-flight mass spectrometry: advantage of grids with rectangular repeat cells

In orthogonal acceleration time-of-flight mass spectrometers (oa-TOFMSs), where ion velocity in the axis of the ion source is preserved, the ions approach grids at an angle not equal to 90°. In this situation ions are expected to be dispersed more than in the case of the normal approach. This may be attributed primarily to the effect of grid wires that are not parallel with the source axis. The dispersion leads to a broadening of the flight-time distribution for ions of a given mass-to-charge ratio and hence it degrades mass resolving power. A novel 20 kV matrix-assisted laser desorption/ionisation (MALDI) oa-TOFMS instrument has been used in this study to investigate grid dispersion. The results show that the dispersions that ions experience as they pass through grids/meshes that divide regions of different electric field strength have a significant effect on the resolving power when the majority of the wires are not aligned with the source. Numerical simulations of ion motion point to an advantage in using ideal parallel wire grids with line densities in excess of one hundred lines per centimeter, orientated with the ion source axis. The orientation of parallel wire grids has previously been predicted to significantly affect resolving power in oa-TOF instruments. This article presents the first experimental data to demonstrate the effect. Parallel wire grids of such high line densities are impractical to construct and support so a grid design that approximates parallel wires has been lithographically mastered for electroforming 70% transmission grids with 120 lines per centimeter in one direction and 12 lines per centimeter at right angles to this direction. A resolution loss of ∼40% (from a resolution of m/Δ m = ∼8000 at full width at half maximum) was observed when a three-grid orthogonal accelerator constructed with this material was rotated through 90° from the predicted ideal orientation. The observed linewidths were in reasonable agreement with those predicted by ion trajectory calculations.

Ion-source modeling and improved performance of the CAMS high-intensity Cs-sputter ion source

The interior of the high-intensity Cs-sputter source used in routine operations at the Center for Accelerator Mass Spectrometry (CAMS) has been computer modeled using the program NEDLab, with the aim of improving negative ion output. Space charge effects on ion trajectories within the source were modeled through a successive iteration process involving the calculation of ion trajectories through Poisson-equation-determined electric fields, followed by calculation of modified electric fields incorporating the charge distribution from the previously calculated ion trajectories. The program has several additional features that are useful in ion source modeling: (1) averaging of space charge distributions over successive iterations to suppress instabilities, (2) Child’s Law modeling of space charge limited ion emission from surfaces, and (3) emission of particular ion groups with a thermal energy distribution and at randomized angles. The results of the modeling effort indicated that significant modification of the interior geometry of the source would double Cs + ion production from our spherical ionizer and produce a significant increase in negative ion output from the source. The results of the implementation of the new geometry were found to be consistent with the model results.

Plasma interactions with high aspect ratio patterned surfaces: ion transport, scattering, and the role of charging

Etching of a patterned semiconductor surface in a plasma depends strongly on the transport of ion and neutral species between the features. Factors altering ion scattering at the sidewalls of high aspect ratio features, such as ion temperature, mask erosion, and charging, influence significantly the final profile shape. Models to describe the inelastic and reactive scattering at various surfaces, coupled to ion trajectory calculations in the sheath and between features, are used to illustrate the contribution of these factors to microtrenching and sidewall bowing. A transition in the profile shape from exhibiting microtrenching to having a rounded bottom is predicted as a result of plasma-induced charging in very high aspect ratio insulating masks.

The shape of field emitters and the ion trajectories in three-dimensional atom probes

The lateral resolution of three-dimensional atom probes is mainly controlled by the aberrations of the ion trajectories near the specimen surface. For the first time, a simulation program has been developed to reconstruct the ion trajectories near a sharp hemispherical electrode defined at the atomic scale. Surface atoms submitted to the highest field were removed one by one. The consecutive gradual change of the surface topology was taken into account in the calculation of ion trajectories. As the tip was 'field evaporated', the initial spherical shape of the emitter was observed to transform gradually into a polygonal shape. When the tip reached its equilibrium shape, the field distribution at the tip surface was found to be much more uniform compared to the initial distribution. The calculated distribution of ion impacts on the detector exhibits the presence of depleted zones both at the centre of low index poles and along <001> zone axes. These predictions are in excellent agreement with experiments.

Simulation of ion trajectories in a quadrupole ion trap: a comparison of three simulation programs

An attempt has been made to compare the performance, design and operation of three simulators, ISIS, ITSIM and SIMION-3D, when applied to the calculation of ion trajectories in a quadrupole ion trap. For the simulation of the trajectory of a single ion in a collision-free system, the calculated spatial trajectory components, kinetic energies and secular frequencies from the three simulators were virtually identical. It is concluded that, despite the various approaches to electrode design, calculation of fields, integration methods and ion generation tactics, there is a remarkable degree of consistency among the products of the simulators when dealing with collision-free conditions. The results of the ion injection simulations under collisional conditions were indicative of the complexity that can be introduced into the simulations with little effort. Random effects such as collisions of ions with He buffer gas and accumulated calculation errors together with the different collision model settings and the different approaches to field calculation are thought to have contributed to the somewhat minor differences in trapping efficiency. SIMION is the simulator of choice for the simulation of ion trajectories in hybrid instruments and in custom-designed assemblies of electrodes; and ITSIM would appear to be the best choice on the basis of computational speed for running multiparticle simulations and user friendliness. Both ISIS and ITSIM are adept at providing detailed information of collision events. Copyright 1999 John Wiley & Sons, Ltd.

Confinement physics for thermal, neutral, high-charge-state plasmas in nested-well solenoidal traps.

A theoretical study is presented which indicates that it is possible to confine a neutral plasma using static electric and solenoidal magnetic fields. The plasma consists of equal temperature electrons and highly stripped ions. The solenoidal magnetic field provides radial confinement, while the electric field, which produces an axial nested-well potential profile, provides axial confinement. A self-consistent, multidimensional numerical solution for the electric potential is obtained, and a fully kinetic theoretical treatment on axial transport is used to determine an axial confinement time scale. The effect on confinement of the presence of a radial electric field is explored with the use of ion trajectory calculations. A thermal, neutral, high-charge-state plasma confined in a nested-well trap opens new possibilities for fundamental studies on plasma recombination and cross-field transport processes under highly controlled conditions.

Development of a three-electrode-lens drift tube for time-of-flight mass spectrometry.

A three-electrode-lens drift tube for time-of-flight mass spectrometry (TOF-MS) has been developed for utilizing a detector to observe photon-stimulated desorption (PSD). In spite of a small detection area, the detector has a high detection efficiency and durability to reactive gas atmosphere at high pressure. The TOF-MS performance of the drift tube was examined for PSD using single-bunch-mode synchrotron radiation on a dichlorosilane (SiH(2)Cl(2))-saturated Si(001) surface. The measured acceleration and focusing-voltage dependences of the time of flight, intensity and full width at half-maximum for the peak of H(+) and Cl(+) PSD ions are discussed in terms of the numerical calculations of ion trajectories and focusing characteristic of the drift tube.