Ion Kinetic Energy Distributions Research Articles

Heteroepitaxial growth of Ga2O3 thin films on Al2O3(0001) by ion beam sputter deposition

Deposition of epitaxial oxide semiconductor films using physical vapor deposition methods requires a detailed understanding of the role of energetic particles to control and optimize the film properties. In the present study, Ga2O3 thin films are heteroepitaxially grown on Al2O3(0001) substrates using oxygen ion beam sputter deposition. The influence of the following relevant process parameters on the properties of the thin films is investigated: substrate temperature, oxygen background pressure, energy of primary ions, ion beam current, and sputtering geometry. The kinetic energy distributions of ions in the film-forming flux are measured using an energy-selective mass spectrometer, and the resulting films are characterized regarding crystalline structure, microstructure, surface roughness, mass density, and growth rate. The energetic impact of film-forming particles on the thin film structure is analyzed, and a noticeable decrease in crystalline quality is observed above the average energy of film-forming Ga+ ions around 40 eV for the films grown at a substrate temperature of 725 °C.

Optimizing Ion Optical Design for Laser Ablation Source in Mass Spectrometry

The study focuses on optimizing a Laser Ablation Source (LAS) for use in mass spectrometry, particularly aiming to enhance transmission of ions and and determination of isotopic compositions in diverse matrices at TRIUMF’s Ion Trap for Atomic and Nuclear Science (TITAN). Critical variables affecting mass spectrometer resolution, such as ion kinetic energy distribution and ion transport are optimized through LAS. The paper explores the importance of consistent initial conditions, ion transport efficiency, and ion capture for accurate and reproducible measurements. Using SIMION software, we optimized ion optical design to tackle challenges like collimating ions travelling off-axis and mass filtering. A significant part of the study is dedicated to ion optic design to enable low abundance measurement of ions of interest (e.g., 10 ions of interest out of 1014 total ions). Techniques like high voltage switching and beam shaping were explored, and the final design aims for high transport efficiency and a 4 mm effective target scan area with maximum transport efficiency. The integration of LAS with a Multiple-Reflection Time-Of-Flight Mass Spectrometer (MR-TOF-MS) is expected to yield an analytical tool with high spatial resolution and transport efficiency for isotope abundance measurement.

Method of kinetic energy reconstruction from time-of-flight mass spectra.

We present a method for the reconstruction of ion kinetic energy distributions from ion time-of-flight mass spectra through ion trajectory simulations. In particular, this method is applicable to complicated spectrometer geometries with largely anisotropic ion collection efficiencies. A calibration procedure using a single ion mass peak allows the accurate determination of parameters related to the spectrometer calibration, experimental alignment, and instrument response function, which improves the agreement between simulations and experiment. The calibrated simulation is used to generate a set of basis functions for the time-of-flight spectra, which are then used to transform from time-of-flight to kinetic-energy spectra. We demonstrate this reconstruction method on a recent pump-probe experiment by Asmussen etal. [Asmussen etal., Phys. Chem. Chem. Phys., 23, 15138, (2021)] on helium nanodroplets and retrieve time-resolved kinetic-energy-release spectra for the ions from ion time-of-flight spectra.

Strongly anisotropic ion emission in the expansion of Nd:YAG-laser-produced plasma

We present results from a combined experimental and numerical simulation study of the anisotropy of the expansion of a laser-produced plasma into vacuum. Plasma is generated by nanosecond Nd:YAG laser pulse impact (laser wavelength λ=1.064 μm) onto tin microdroplets. Simultaneous measurements of ion kinetic energy distributions at seven angles with respect to the direction of the laser beam reveal strong anisotropic emission characteristics, in close agreement with the predictions of two-dimensional radiation-hydrodynamic simulations. Angle-resolved ion spectral measurements are further shown to provide an accurate prediction of the plasma propulsion of the laser-impacted droplet.

Features of ion generation by a picosecond laser in the range of 1011–1013 W cm−2 power densities

Picosecond lasers (ps-lasers) have significant advantages for the generation of low charge state ions compared to nanosecond lasers because the influence of heat conductivity on a solid target is almost negligible in the case of ps-laser ablation for laser pulse durations less than 10 ps. However, there is no comprehensive data on ion yields for different elements and target irradiation conditions for laser power densities at the target surface around and below 1013 W cm−2, which is of interest to our study of such plasmas as a source of low charge state ions for various applications, particularly for external injection of those ions into an Electron Beam Ion Source (EBIS). We investigated ion generation from Al, Ti, Cu, Nb and Ta target elements by a ps-laser with power densities in the range of 1011–1013 W cm−2 at the target surface. A ps-laser with 1.27 mJ maximum energy within an 8 ps pulse and repetition rate up to 400 Hz has been used to generate a laser-ablated plasma. Dependencies of ion current versus time, total charge of registered ions as well as ion kinetic energy distributions are characterized using a Faraday cup. Significant difference in ion current dynamics between first, second and following shots onto the same target spot was found for all five target elements. The total charge of ions registered by the Faraday cup increases linearly with increasing laser pulse energy and is almost independent of the target element and number of shots onto the same target spot for all five target elements studied. The results obtained give us a basis for specification and design of the source of low charge state ions for external injection into EBIS.

Direct inner-shell photoionization of Xe atoms embedded in helium nanodroplets

We present the first measurements of photoelectron spectra of atomic clusters embedded in superfluid helium (He) nanodroplets. Owing to the large absorption cross section of xenon (Xe) around 100 eV photon energy (4d inner-shell ionization), direct dopant photoionization exceeds charge transfer ionization via the ionized He droplets. Despite the predominant creation of Xe2+ and Xe3+ by subsequent Auger decay of free Xe atoms, for Xe embedded in He droplets only singly charged , k = 1, 2, 3 fragments are observed. Broad Xe+ ion kinetic-energy distributions indicate Coulomb explosion of the ions due to electron transfer to the primary Auger ions from surrounding neutral atoms. The electron spectra correlated with Xe ions emitted from the He nanodroplets contain a low-energy feature and nearly unshifted Xe photolines. These results pave the way to extreme ultraviolet and x-ray photoelectron spectroscopy of clusters and molecular complexes embedded in He nanodroplets.

Unexpected Indirect Dynamics in Base-Induced Elimination.

Base-induced elimination (E2) and bimolecular nucleophilic substitution (SN2) are two of the most versatile reactions that are important in preparative organic chemistry. These stereospecific reactions are often found in direct competition with each other. Elimination can proceed via two distinct transition states, referred to as anti and syn, of which anti is commonly energetically favored. To investigate the intrinsic dynamics of base-induced elimination, reactions under single-collision conditions are required. Here, we present reactive scattering results on the prototype reaction of the fluoride anion with tert-butyl halides. The observed mechanistic fingerprints are associated with the E2 reaction, because steric hindrance at the α-carbon suppresses the SN2 reaction [Carrascosa, E.; Meyer, J.; Zhang, J.; Stei, M.; Michaelsen, T.; Hase, W. L.; Yang, L.; Wester, R. Nat. Commun. 2017, 8, 25]. The reaction coordinate shows energetically submerged transition states, with anti favored over syn, and we found a very shallow prereaction well for anti. We predominantly found indirect dynamics for a range of collision energies, which can be separated into three remarkably different mechanisms. At low collision energies, the first is a large impact parameter indirect mechanism which leads to a forward-backward symmetric scattering signature. The second mechanism is attributed to low-impact parameter reactions with a near-statistical partitioning of the total available energy. The majority of events are associated with widespread isotropic scattering. Unexpectedly, the product ion kinetic energy distributions are independent of collision energy. We associate this with dynamic trapping in a prereaction well supported by a large centrifugal potential. These measured fingerprints support that atomistic reaction dynamics cannot be predicted based on stationary arguments alone.

Controlling ion kinetic energy distributions in laser produced plasma sources by means of a picosecond pulse pair

The effect of a pair of picosecond pulses on the ionization and deformation of a liquid tin microdroplet is studied for a range of incident pulse parameters. Faraday cups are used to measure ion kinetic energy distributions, together with high-resolution shadowgraphy to monitor target deformation and expansion. It is found that the introduction of a relatively weak first pulse results in an order-of-magnitude reduction of the number of ions with kinetic energies above 1 keV, and a strong shift of the kinetic energy distribution towards lower energies, while the expansion dynamics of the droplet can be kept similar to the single-pulse case. By controlling the relative intensity and the time delay between pairs of pulses with 52 ps duration, regimes are identified in which spherical final target shapes are combined with a reduced high-energy ion yield. The high-energy part of the observed ion distributions has been fitted with a self-similar expansion model, showing a 30-fold decrease in characteristic ion energy for pulse pairs. This combination of results is of particular importance for plasma sources of EUV radiation for nanolithography applications, in which picosecond-laser-produced target shapes can lead to significant improvements in source conversion efficiency, while a low high-energy ion yield is desirable from a source lifetime perspective.

Electron-impact-ionization dynamics of SF6

A detailed understanding of the dissociative electron ionization dynamics of $\mathrm{S}{\mathrm{F}}_{6}$ is important in the modeling and tuning of dry-etching plasmas used in the semiconductor manufacture industry. This paper reports a crossed-beam electron ionization velocity-map imaging study on the dissociative ionization of cold $\mathrm{S}{\mathrm{F}}_{6}$ molecules, providing complete, unbiased kinetic energy distributions for all significant product ions. Analysis of these distributions suggests that fragmentation following single ionization proceeds via formation of $\mathrm{S}{{\mathrm{F}}_{5}}^{+}$ or $\mathrm{S}{{\mathrm{F}}_{3}}^{+}$ ions that then dissociate in a statistical manner through loss of F atoms or ${\mathrm{F}}_{2}$, until most internal energy has been liberated. Similarly, formation of stable dications is consistent with initial formation of $\mathrm{S}{{\mathrm{F}}_{4}}^{2+}$ ions, which then dissociate on a longer time scale. These data allow a comparison between electron ionization and photoionization dynamics, revealing similar dynamical behavior. In parallel with the ion kinetic energy distributions, the velocity-map imaging approach provides a set of partial ionization cross sections for all detected ionic fragments over an electron energy range of 50--100 eV, providing partial cross sections for ${\mathrm{S}}^{2+}$, and enables the cross sections for $\mathrm{S}{{\mathrm{F}}_{4}}^{2+}$ from $\mathrm{S}{\mathrm{F}}^{+}$ to be resolved.

Characterization of charge and kinetic energy distribution of ions emitted during nanosecond pulsed laser ablation of several metals

The ion flux from various metals (Al, Ti, Cu, Sn and W) ablated with 20 ns Nd:YAG laser radiation at a wavelength of 1064 nm was investigated by an ion collector operating in time-of-flight (TOF) configuration. The laser irradiance at the target was varied in the range of 1.7 × 108–5.73 × 108 W cm−2. Ion yield from various metals showed a linearly increasing trend with increasing laser irradiance, whereas ion yield was found to decrease with an increasing atomic mass of the target. Our results clearly indicate that ion yield is not a function of the volatility of the metal. TOF ion spectra showed at least two groups of low intensity peaks due to fast ions. The first group of ion peaks, which was present in the spectra of all five metals, was due to surface contamination. The additional fast ion structures in the spectra of Sn and W can be related to the ion acceleration due to the prompt electron emission from these high-Z metals. The ion velocity follows the anticipated inverse square root dependence on the ion mass. For the range of laser irradiance investigated here, the most probable energy of the Cu ions increases from about 100–600 eV. The fast increase in ion energy above ~3 × 108 W cm−2 is related to the increase of the Columb part of the ion energy due to the production of multiply charged ions.

Multicharged carbon ion generation from laser plasma.

Carbon ions generated by ablation of a carbon target using an Nd:YAG laser pulse (wavelength λ = 1064 nm, pulse width τ = 7 ns, and laser fluence of 10-110 J cm-2) are characterized. Time-of-flight analyzer, a three-mesh retarding field analyzer, and an electrostatic ion energy analyzer are used to study the charge and energy of carbon ions generated by laser ablation. The dependencies of the ion signal on the laser fluence, laser focal point position relative to target surface, and the acceleration voltage are described. Up to C4+ ions are observed. When no acceleration voltage is applied between the carbon target and a grounded mesh in front of the target, ion energies up to ∼400 eV/charge are observed. The time-of-flight signal is analyzed for different retarding field voltages in order to obtain the ion kinetic energy distribution. The ablation and Coulomb energies developed in the laser plasma are obtained from deconvolution of the ion time-of-flight signal. Deconvolution of the time-of-flight ion signal to resolve the contribution of each ion charge is accomplished using data from a retarding field analysis combined with the time-of-flight signal. The ion energy and charge state increase with the laser fluence. The position of the laser focal spot affects the ion generation, with focusing ∼1.9 mm in front of the target surface yielding maximum ions. When an external electric field is applied in an ion drift region between the target and a grounded mesh parallel to the target, fast ions are extracted and separated, in time, due to increased acceleration with charge state.

Thermal and log-normal distributions of plasma in laser driven Coulomb explosions of deuterium clusters

In this work, we explore the possibility that the motion of the deuterium ions emitted from Coulomb cluster explosions is highly disordered enough to resemble thermalization. We analyze the process of nuclear fusion reactions driven by laser–cluster interactions in experiments conducted at the Texas Petawatt laser facility using a mixture of D2+3He and CD4+3He cluster targets. When clusters explode by Coulomb repulsion, the emission of the energetic ions is “nearly” isotropic. In the framework of cluster Coulomb explosions, we analyze the energy distributions of the ions using a Maxwell–Boltzmann (MB) distribution, a shifted MB distribution (sMB), and the energy distribution derived from a log-normal (LN) size distribution of clusters. We show that the first two distributions reproduce well the experimentally measured ion energy distributions and the number of fusions from d–d and d-3He reactions. The LN distribution is a good representation of the ion kinetic energy distribution well up to high momenta where the noise becomes dominant, but overestimates both the neutron and the proton yields. If the parameters of the LN distributions are chosen to reproduce the fusion yields correctly, the experimentally measured high energy ion spectrum is not well represented. We conclude that the ion kinetic energy distribution is highly disordered and practically not distinguishable from a thermalized one.

Features in the ion emission of Cu, Al, and C plasmas produced by ultrafast laser ablation

The bi-modal nature of charge integrated ion kinetic energy distributions, which result from ultrafast laser produced plasmas, is discussed in this paper. A negatively biased Faraday cup was used as a charge collector to measure ion distributions from three different solid targets that had been irradiated with an ultrafast laser in the fluence range 0.1–1 J/cm2. A bi-modal time of flight distribution is found for all three targets (C, Al, and Cu). In the case of the metallic targets (Al and Cu), high- and low-kinetic energy peaks exhibit quite different dependencies on laser fluence, whereas for the semi-metallic target (C), both peaks scale similarly with ultrafast laser fluence. The results are discussed within the framework of a one dimensional capacitor model resulting in ion acceleration.

High-resolution (e, 2e + ion) study of electron-impact ionization and fragmentation of methane.

The ionization and fragmentation of methane induced by low-energy (E0 = 66 eV) electron-impact is investigated using a reaction microscope. The momentum vectors of all three charged final state particles, two outgoing electrons, and one fragment ion, are detected in coincidence. Compared to the earlier study [Xu et al., J. Chem. Phys. 138, 134307 (2013)], considerable improvements to the instrumental mass and energy resolutions have been achieved. The fragment products CH4 (+), CH3 (+), CH2 (+), CH(+), and C(+) are clearly resolved. The binding energy resolution of ΔE = 2.0 eV is a factor of three better than in the earlier measurements. The fragmentation channels are investigated by measuring the ion kinetic energy distributions and the binding energy spectra. While being mostly in consistence with existing photoionization studies the results show differences including missing fragmentation channels and previously unseen channels.

Experimentally established correlation between ion charge state distributions and kinetic ion energy distributions in a direct current vacuum arc discharge

DC arc plasmas from Al, Ti, Cu, Mo, and W cathodes have been characterized with respect to plasma chemistry and charge-state-resolved ion energy. The evaluated average ionization energies in the plasmas were found to be linearly correlated with the kinetic ion energies. This was further supported by evaluation of previously published data for 42 elements. A comparison of the total ion kinetic energy distribution and the corresponding ion charge state distribution, as defined by the ionization energies of the constituent ions, showed close to equivalent shapes and widths, for all cathodes analyzed. This suggests that the energy provided for ionization and acceleration varies simultaneously during plasma generation in the arc spot. The presented results provide a link between the ionization and acceleration processes, and may provide further insight into the fundamentals of cathode spot evolution and plasma generation.

Development of a miniature time-of-flight mass/charge spectrometer for ion beam source analyzing

To diagnose the vacuum arc discharge process, a miniature time-of-flight mass spectrometer (TOF MS) with compact structure was developed for cathodic vacuum arc discharge ion source. An orthogonal acceleration region and an angle reflector were employed for the miniature TOF MS instrument. Especially, a small extraction hole in the acceleration region was set for getting relative high mass resolution, an ion detector with a large rectangular effective area were designed for getting wide ion kinetic energy distribution measurement capability. The compact size of the TOF MS is 413mm×250mm×414mm, the mass resolution is 600 (FWHM) and the detection range of ion kinetic energy is two orders of magnitude. A copper cathodic vacuum arc discharge ion source was tested and the charge states of Cu ions from 1+ to 3+ were detected. The miniature TOF MS was successfully developed with relative high mass resolution and wide ion kinetic energy distribution measurement capability which could be used to diagnose the cathodic vacuum arc discharge plasma.

A hemispherical Langmuir probe array detector for angular resolved measurements on droplet-based laser-produced plasmas.

In this work, a new diagnostic tool for laser-produced plasmas (LPPs) is presented. The detector is based on a multiple array of six motorized Langmuir probes. It allows to measure the dynamics of a LPP in terms of charged particles detection with particular attention to droplet-based LPP sources for EUV lithography. The system design permits to temporally resolve the angular and radial plasma charge distribution and to obtain a hemispherical mapping of the ions and electrons around the droplet plasma. The understanding of these dynamics is fundamental to improve the debris mitigation techniques for droplet-based LPP sources. The device has been developed, built, and employed at the Laboratory for Energy Conversion, ETH Zürich. The experimental results have been obtained on the droplet-based LPP source ALPS II. For the first time, 2D mappings of the ion kinetic energy distribution around the droplet plasma have been obtained with an array of multiple Langmuir probes. These measurements show an anisotropic expansion of the ions in terms of kinetic energy and amount of ion charge around the droplet target. First estimations of the plasma density and electron temperature were also obtained from the analysis of the probe current signals.

Signal enhancement in laser ablation inductively coupled plasma-mass spectrometry using water and/or ethanol vapor in combination with a shielded torch

To improve the precision and accuracy of in situ trace element and isotope analysis and to apply LA-ICP-MS to small or isotopically heterogeneous samples such as minerals, greater analytical sensitivity is required. In this study, we investigate the effects of the addition of water and/or ethanol vapor to the central gas flow (Ar + He) of an Ar plasma in combination with a shielded torch on the signal intensity of laser-generation using an 193 nm ArF laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) setup. A custom-built sample introduction system was developed to add water and/or ethanol vapor to dry aerosol at different and constant rates. The optimum makeup gas flow rate is negatively correlated with the water and/or ethanol load into the ICP. The effect of the shielded torch on the LA-ICP-MS sensitivity is minimal for dry plasma conditions but becomes significant in the presence of water and/or 1–4% (v/v) ethanol vapor. For most of the 60 investigated elements reported in this study, the addition of small amounts of water and/or 1–4% (v/v) ethanol vapor into the carrier gas increases the sensitivity by a factor of 2.5–3.0 or 1.2–1.5 with or without the shielded torch, respectively. The suppression of the ion kinetic energy distribution by the shield in the presence of water and/or 1–4% ethanol vapor may account for the increase in sensitivity. With the shielded torch, the oxide ratio increases from 0.47% in dry plasma to 6.41% in water plasma and decreases to 0.81% in 4% ethanol plasma. Hydride formation increases by a factor of 27 in the presence of 4% ethanol vapor relative to dry plasma. The yield of doubly charged ions decreases from 1.2% in dry plasma to 0.8% and 0.7% in water and 4% ethanol plasmas with the shielded torch, respectively. Relative to the spatial profiles of the ion distributions in dry plasma, the addition of water and/or 2% (v/v) ethanol vapor in combination with a shielded torch shows several effects: (1) axial profiles are significantly narrower; (2) the maximum ion density zone shifts away from the load coil; and (3) there are more uniform ion distributions for elements with different physical and chemical properties. The combination of changes in ICP geometry, enhanced vaporization, atomization and ionization of analyte, and efficient ion transmission with a shielded torch contributes to the sensitivity enhancement observed in this study.

Ion energetics in electron-rich nanoplasmas

Based on trajectory calculations of xenon clusters up to ≈6000 atoms irradiated by laser pulses (peak intensities IM = 1014–1016 W cm−2, Gaussian pulse lengths τ = 10–230 fs and frequency 0.35 fs−1), we have analyzed the interrelation between outer ionization and ion kinetic energies. The following three main categories have been identified. (A) For short pulses (τ = 10 fs) of higher intensity IM = 1016 W cm−2, the outer ionization level leads to a sufficiently high positive cluster charge, which confines the remaining nanoplasma electrons to the cluster center. In this case, ion energies can be reasonably well accounted for by a multi-charge state lychee model, according to which outer ionization is vertical and the nanoplasma can be described by a non-expanding neutral cluster interior, causing a zero-energy component in the ion kinetic energy distribution and an expanding electron-free cluster periphery. (B) For a very low outer ionization level, which is realized for short pulses of low intensity (IM = 1014 W cm−2) and/or large clusters, a slow gradual evaporation of nanoplasma electrons under laser-free conditions on the picosecond time scale is observed, making the entire outer ionization process highly non-vertical despite the short laser pulse. Accordingly, ions are accelerated only by a gradual buildup of the total cluster charge. (C) For long pulses (τ = 230 fs), the cluster expansion during the laser pulse is large and outer ionization is non-vertical. The nanoplasma electrons attain high kinetic energies by resonance heating and are distributed over the entire ion framework without a neutral cluster interior. Consequently, a zero-energy component in the ion energy distribution is missing.

Characterization of kinetic energy distributions of ions in high laser irradiance via orthogonal time-of-flight mass spectrometry

The characteristics of laser ionization in low pressure background gas have been investigated through the measurement of temporal and kinetic energy distributions of Al+, Mn+, Nb+, In+, Ta+, and Bi+ produced from a disk comprised of these metallic elements. A Q-switched Nd:YAG laser was utilized with a laser beam at 532nm of wavelength and 1.0×1010W/cm2 of laser irradiance. The kinetic energy was found to be the same for all ablated species at the given pressure, regardless of the atomic mass. The plume propagation translates from a free expansion at 0.5Pa to a collisional and shockwave-like hydrodynamic expansion at 50Pa. A plume splitting exists at 500–1500Pa where only the fast component can be observed with a grounded nozzle voltage. As the nozzle voltage grows up, the thermalized component with increased kinetic energy is found depending on the nozzle voltage.