Ion Yield Research Articles

Sputtering, Cluster Primary Ions and Static SIMS

Sputtering using cluster primary ion beams is very important for the future development of static SIMS and the SIMS depth profiling of organic layers. However, different results from different laboratories may be confusing. Analytical models have an important function for enabling the prediction of behaviour for practical analysis. Sigmund's model for sputtering, often used in surface analysis, is helpful and accurate in the linear cascade regime. However, for cluster sputtering this is no longer the case and spike effects need evaluation. Evidence will be presented of the spike model validity for clusters of up to more than 10 atoms over 3 orders of magnitude in sputtering yield. Using data from one primary ion, extremely good descriptions of measurements reported with other primary ions can then be achieved. This theory is then used to evaluate the molecular ion yield behaviour of interest in the static SIMS of organics. This leads to universal dependencies for the de-protonated molecular ion yields, relating all primary ions, both single atom and cluster, which are illustrated by experimental data over 5 decades of emission intensity. This formulation permits the prediction of the (M-H)- secondary ion yield for different, or new, primary ion sources. It is shown how further gains are predicted. For analysing materials, raising the molecular secondary ion yield is extremely helpful but it is the ratio of this yield to the disappearance cross-section (the efficiency) that is critical. The relation of the damage and disappearance cross sections is formulated. Data are evaluated and a description is given to show how these cross sections are related and to provide a further universal relation for the efficiency/yield dependence of all cluster ions.

Fabrication of nanoscale multimolecular structures of lanthanum stearate using Langmuir monolayers for laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization (MALDI) from the surface of nanosized multimolecular structures based on lanthanum stearate monolayers (FLa) has been studied. The presence of FLa on the surface of MALDI target was confirmed experimentally by laser desorption of lanthanum-containing organic ions. MALDI target functionalization with FLa is shown to significantly increase the yield of target peptide ions, with the optimum being achieved at a film thickness of about 6 monolayers. An approach is proposed in a "lab-on-a-plate" format, which allows specific extraction of peptides modified with chlorine-containing compounds and includes the following steps: functionalization of the target surface, metal affinity extraction, matrix deposition and MALDI mass spectrometric analysis. Keywords: mass spectrometry, matrix assisted laser desorption/ionization (MALDI), surface, Langmuir monolayers, metal affinity chromatography.

Micro-tapered aperture nebulization ionization for versatile mass spectrometry analysis.

The bloom of new ionization sources is promoting the prosperity and application of mass spectrometry analysis. In this study, we introduced a new (high) voltage-free ionization method termed micro-tapered aperture nebulization ionization (MANI), which only requires the use of a common micro-tapered aperture atomizer to operate. It is found that liquid nebulization on this type of atomizer can induce ionization of the dissolved analytes in the droplets without the assistance of additional voltage and gas. By assembling the commercially available atomizer module into a 3D-printed chamber, a compact MANI source was constructed and then characterized. This source has a high ion yield and satisfactory quantitative performance, and it is preferably used to analyze aqueous solutions. Furthermore, it exhibits broad applicability and can be easily extended to multiple applications, including liquid extraction surface analysis of solid samples and direct analysis of gaseous analytes via secondary spray ionization. In short, the MANI source is a simple, safe, green, and versatile tool that can assist mass spectrometers to perform routine and diverse analysis.

Stabilization of benzene radical anion in ammonia clusters.

We investigate electron attachment to large ammonia clusters doped with a single benzene (Bz) molecule (NH3)N·Bz, N̄ ≈ 320. Negatively charged clusters are probed by mass spectrometry, and the energy-dependent ion yields are derived from mass spectra measured at different electron energies. The ion efficiency curves of pure ammonia clusters exhibit two maxima. At around 6 eV, (NH3)n-1NH2- ions are produced via dissociative electron attachment (DEA) to NH3 molecules. (NH3)n- ions produced at this energy are formed by DEA followed by fragment caging. At low energies around 1.3 eV, only (NH3)n- ions are formed for cluster sizes n ≥ 35 that correspond to solvated electrons in ammonia clusters. The doped (NH3)n·Bz- cluster ions exhibit essentially the same energy dependence. The (NH3)n·Bz- ions are metastable and evaporate NH3 molecule(s), while pure (NH3)n- ions are stable. The lifetime for NH3 molecule evaporation from the Bz-doped clusters was estimated as τ ≈ 18 μs. We interpret the metastability of the doped clusters by the charge localization on a Bz- ion solvated in the ammonia, which is accompanied by an energy release leading to the evaporation of NH3 molecule(s).

Solvated Electron in Acetonitrile: Radiation Yield, Absorption Spectrum, and Equilibrium between Cavity- and Solvent-Localized States.

The equilibrium between a solvent cavity-localized electron, ecav-, and a dimeric solvent anion, (CH3CN)2•-, which are the two lowest energy states of the solvated electron in acetonitrile, has been investigated by pulse radiolysis at 233-353 K. The enthalpy and entropy for the ecav- to (CH3CN)2•- conversion amount to -11.2 ± 0.3 kcal/mol and -39.3 ± 1.2 cal/(mol K), corresponding to a 0.44 ± 0.35 equilibrium constant at 25 °C. The radiation yield of the solvated electron has been quantified using a Co(II) macrocycle that scavenges electrons with a 1.55 × 1011 M-1 s-1 rate constant. The apparent yield increases without saturation over the attainable scavenger concentration range, reaching 2.8 per 100 eV; this value represents the lower limit for the acetonitrile ionization yield in pulse radiolysis. The apparent molar absorption coefficient of (20.8 ± 1.5) × 103 M-1 cm-1 at 1450 nm and 20 °C for the solvated electron and individual vis-near-infrared (NIR) absorption spectra of ecav- and (CH3CN)2•- are derived from the data. Variances with previous reports are thoroughly discussed. Collectively, these results resolve several controversies concerning the solvated electron properties in acetonitrile and furnish requisite data for quantitative pulse radiolysis investigations in this commonly used solvent.

Nanoscale Three-Dimensional Imaging of Drug Distributions in Single Cells via Laser Desorption Post-Ionization Mass Spectrometry.

Exploring the three-dimensional (3D) drug distribution within a single cell at nanoscale resolution with mass spectrometry imaging (MSI) techniques is crucial in cellular biology, yet it remains a great challenge due to limited lateral resolution, detection sensitivities, and reconstruction problems. Herein, a microlensed fiber laser desorption post-ionization time-of-flight mass spectrometer (MLF-LDPI-TOFMS) was developed for the 3D imaging of two anticancer drugs within single cells at a 500 × 500 × 500 nm3 voxel resolution. Nanoscale desorption was obtained with a microlensed fiber (MLF), and a 157 nm post-ionization laser was introduced to enhance the ionization yield. Furthermore, a new type of alignment method for 3D reconstruction was developed on the basis of our embedded uniform circular polystyrene microspheres (PMs). Our findings demonstrate that this 3D imaging technique has the potential to provide information about the 3D distributions of specific molecules at the nanoscale level.

MALDI-2 and t-MALDI-2 Mass Spectrometry Imaging.

Matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) combined with laser-induced postionization for MALDI-2 enables the simultaneous registration of numerous classes of small molecules (e.g., secondary metabolites including sterols) as well as phospholipids, glycolipids, and glycans from tissue sections and from cell cultures with strongly boosted ion yields. Here, we describe methodological aspects that are key for optimizing the analytical sensitivity and spatial resolution of a MALDI-2 imaging experiment. We will include both top-illumination MALDI-2 as well as the recently introduced transmission (t-) mode MALDI-2 approach.

Space weathering on inner planetary surface analogues induced by swift multicharged heavy ion bombardment

Silicates are ubiquitous in space. They dominate the surfaces of the inner rocky planets (Mercury in the case of the Solar System), the Moon, and asteroids, forming the major part of the non-volatile material. The physical and chemical properties of the rocky surfaces are determined not only by their initial composition but also by the processes occurring on them. Here we discuss one of these processes; irradiation by energetic cosmic particles that induces many effects among which structural changes and sputtering, the latter contributing to the formation of exospheres. In the current work we report the results of experiments conducted on anorthite, jadeite and nepheline silicates that have been irradiated with energetic heavy ions with the aim to better understand the interaction of galactic cosmic rays, solar wind, and solar energetic particles with planetary and small body surfaces. The sputtering effects induced by energetic (MeV/u) multicharged heavy ions (e.g., 105Rh and 140Ba) were analyzed by the PDMS-TOF-SIMS technique (plasma desorption mass spectrometry - time-of-flight secondary ion mass spectrometry). Positive and negative secondary ionic species are identified: Na+, K+, Al+, Ca+, SiO2−. Ejection of (SiO2)n− and (AlSi)Om− cluster series are also observed. Less frequent, negative ion yields are one order of magnitude less than positive ones, or greater, which is the case for nepheline, with 0.671 ions impact−1 for positive and 0.126 ions impact−1 for negative ions. The results concerning ejection of ionic species show, for instance, that the Na+/K+ ratio is ~2.5, which is in very good agreement with that observed in the Hermean exosphere found to be ~2.3.

Criterion for the yield of micro-object ionization driven by few- and subcycle radiation pulses with nonzero electric area

The photoionization yield is analyzed for three-dimensional quantum systems with finite number of discrete spectrum states driven by unipolar subcycle and few-cycle electromagnetic pulse with a duration much less than ``the Kepler period,'' electron oscillation period in the ground state. The yield for such objects---symmetric quantum dots and those described by the zero-radius potential---is compared with the yield for hydrogen atom. In all these cases, the standard Keldysh ionization theory is inapplicable. It is shown that ionization probability is determined by the ratio of the electric pulse area (integral of the electric field strength over time) and its characteristic value inversely proportional to the size of the electron localization, and not by the pulse energy or its maximum intensity.

Unraveling metabolic alterations in transgenic mouse model of Alzheimer's disease using MALDI MS imaging with 4-aminocinnoline-3-carboxamide matrix

Revealing the metabolic abnormalities of central and peripheral systems in Alzheimer's disease (AD) mouse model is of paramount importance for understanding AD disease. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) is a powerful label-free technique that has been extensively utilized for the interrogation of spatial changes of various metabolites in neurodegenerative disease. However, technical limitations still exist in MALDI MS, and there is a need to improve the performance of traditional MALDI for a deeper investigation of metabolic alterations in the AD mouse model. In this work, 4-aminocinnoline-3-carboxamide (4-AC) was developed into a novel dual-polarity MALDI matrix. Compared with traditionally used MALDI matrices such as 2,5-dihydroxybenzoic acid (DHB) and 9-aminoacridine (9-AA), 4-AC exhibited superior performance in UV absorption at 355 nm, ion yields, background interference, and vacuum stability, making it an ideal MALDI matrix for comprehensive evaluation of metabolic alteration in the brain and serum of APP/PS1 transgenic mouse model of AD. In total, 93 metabolites exhibited different levels of regional changes in the brain of AD mice as compared to the age-matched controls. Moreover, in the serum of AD mice, 81 altered metabolites distinguishing the AD group from the control were observed by using multivariate statistical analysis. It is expected that the application of the MALDI MSI method developed in this work to visualize the spatio-chemical change of various metabolites may improve our understanding of the etiopathogenesis of AD.

Energy imparted and ionization yield in nanometre-sized volumes

In radiation physics, the biological effectiveness of radiation is generally estimated from the energy imparted to a sensitive volume of given size. However, measurements of this quantity are carried out almost exclusively with ionization-based techniques, by converting the ionization charge signal to energy imparted by means of a constant calibration factor, independent of particle type and energy. This procedure is reliable when the tissue-equivalent size of the sensitive volume is of the order of 1 μm and larger, since in this case the number of radiation-induced collisions is high and the distribution of energy transfer per single collision is masked by the multiple averaging. However, its reliability can be called into question when the size of the sensitive volume is decreased to the nanometre scale, as it is the case in experimental nanodosimetry. For this reason, the present study investigates the relationship between the two stochastic quantities energy imparted and ionization yield, calculated by Monte Carlo simulations in spherical sensitive volumes with diameter ranging from 100 nm down to 1 nm. The analysis was carried out for protons and carbon ions in the energy range from 1 to 100 MeV/u, crossing liquid-water sensitive spheres along their diameter. Simulations were done by means of the Geant4-DNA Monte Carlo code, with two different physics lists. A strong correlation between the energy imparted and the ionization yield was found, which comprises not only their mean values, but their entire stochastic distributions, even when the diameter of the sensitive volume is as small as 1 nm.

Probing Surface Information of Alloy by Time of Flight-Secondary Ion Mass Spectrometer

In recent years, time of flight-secondary ion mass spectrometer (ToF-SIMS) has been widely employed to acquire surface information of materials. Here, we investigated the alloy surface by combining the mass spectra and 2D mapping images of ToF-SIMS. We found by surprise that these two results seem to be inconsistent with each other. Therefore, other surface characteristic tools such as SEM-EDS were further used to provide additional supports. The results indicated that such differences may originate from the variance of secondary ion yields, which might be affected by crystal orientation.

Resonance-Enhanced Multiphoton Ionization in the X-Ray Regime.

Here, we report on the nonlinear ionization of argon atoms in the short wavelength regime using ultraintense x rays from the European XFEL. After sequential multiphoton ionization, high charge states are obtained. For photon energies that are insufficient to directly ionize a 1s electron, a different mechanism is required to obtain ionization to Ar^{17+}. We propose this occurs through a two-color process where the second harmonic of the FEL pulse resonantly excites the system via a 1s→2p transition followed by ionization by the fundamental FEL pulse, which is a type of x-ray resonance-enhanced multiphoton ionization (REMPI). This resonant phenomenon occurs not only for Ar^{16+}, but also through lower charge states, where multiple ionization competes with decay lifetimes, making x-ray REMPI distinctive from conventional REMPI. With the aid of state-of-the-art theoretical calculations, we explain the effects of x-ray REMPI on the relevant ion yields and spectral profile.

Molecular speciation analysis of oxidized metal surfaces by TOF SIMS

Molecular speciation by Time of Flight Secondary Ion Mass Spectrometry (TOF SIMS) was applied to oxidized bismuth, lead, tin, and tellurium. In their pure elemental form, they are basic constituent elements of both topological insulators and topological crystalline insulators, which are very hot topics in the last decade.The range of specific ions masses used to evaluate the valence of investigated oxides has been significantly expanded towards high-mass clusters MexOy (2 ≤ x ≤ 5). The significant enhancement in the secondary high-mass ion yield by using the Bi cluster ion source reflects the valence of primary ion clusters used in molecular speciation of inorganic species. The results show that chemical composition of clusters differs in three 2, 3, and 4 oxidation states studied. This implies that each oxidation state possesses its own set of the high-mass clusters extracted from oxidized metal surfaces during measurement.Such an approach allows for the first time to perform a semi-quantitatively molecular speciation of oxides exploiting the empirical Plog's model. It describes the secondary ion yield as a function of the metal valency in the oxide. We found that the estimated lattice valence state G0 is close to the valence state of the metal in stable oxides.

Nontargeted Screening Using Gas Chromatography-Atmospheric Pressure Ionization Mass Spectrometry: Recent Trends and Emerging Potential.

Gas chromatography–high-resolution mass spectrometry (GC–HRMS) is a powerful nontargeted screening technique that promises to accelerate the identification of environmental pollutants. Currently, most GC–HRMS instruments are equipped with electron ionization (EI), but atmospheric pressure ionization (API) ion sources have attracted renewed interest because: (i) collisional cooling at atmospheric pressure minimizes fragmentation, resulting in an increased yield of molecular ions for elemental composition determination and improved detection limits; (ii) a wide range of sophisticated tandem (ion mobility) mass spectrometers can be easily adapted for operation with GC–API; and (iii) the conditions of an atmospheric pressure ion source can promote structure diagnostic ion–molecule reactions that are otherwise difficult to perform using conventional GC–MS instrumentation. This literature review addresses the merits of GC–API for nontargeted screening while summarizing recent applications using various GC–API techniques. One perceived drawback of GC–API is the paucity of spectral libraries that can be used to guide structure elucidation. Herein, novel data acquisition, deconvolution and spectral prediction tools will be reviewed. With continued development, it is anticipated that API may eventually supplant EI as the de facto GC–MS ion source used to identify unknowns.

Chemical vapor deposition synthesis of high-quality Ni3C/GNPs composite material: Effect of growth time on the yield, morphology and adsorption behavior of metal ions

Chemical vapor deposition synthesis of high-quality Ni3C/GNPs composite material: Effect of growth time on the yield, morphology and adsorption behavior of metal ions

Survival of rotational alignment in H2 scattering from Si(100).

We report a state-prepared, state-resolved study of rotational scattering of a diatomic molecule from a solid surface. Specifically, H2 molecules with 80meV kinetic energy are rotationally aligned in the j = 3 rotational state via stimulated Raman pumping and then scattered from a Si(100) surface at normal incidence. The rotational alignment of the scattered molecules is determined by measuring, for both the incident and scattered molecules, the ionization yield of a probe laser, tuned to selectively ionize molecules in the j = 3 rotation level, as the probe laser polarization is rotated. The measurement is performed for two initial rotational alignments: a "helicoptering" alignment with the bonds constrained to lie primarily parallel to the surface and a "cartwheeling" alignment with the bonds lying primarily normal to the surface. For both initial alignments, the modulation of the probe ionization yield with laser polarization for the scattered molecules is pronounced, although significantly weakerthan the modulation measured for the incident molecules. This indicates a significant modification but not a complete elimination of the initial alignment. The modulation is found to be stronger for the scattered molecules originating in the cartwheeling alignment than for the helicoptering alignment. These results contribute toward an improved understanding of the role of rotational motion in molecule-surface dynamics.

Chemoheteroepitaxy of 3C‐SiC(111) on Si(111): Influence of Predeposited Ge on Structure and Composition

Secondary ion mass spectroscopy, Fourier transformed infrared spectroscopy, ellipsometry, reflection high energy diffraction and transmission electron microscopy are used to gain inside into the effect of Ge on the formation of ultrathin 3C‐SiC layers on Si(111) substrates. Accompanying the experimental investigations with simulations it is found that the ultrathin single crystalline 3C‐SiC layer is formed on top of a gradient Si1–x–yGexCy buffer layer due to a complex alloying and alloy decomposition processes promoted by carbon and germanium interdiffusion and SiC nucleation. This approach allows tuning residual stress at very early growth stages as well as the interface properties of the 3C‐SiC/Si heterostructure. Useful yields of secondary ions of Ge in Si matrix and Si dimer are estimated.

Generation of ultrafast, transient, highly acidic pH spikes in the radiolysis of water at very high dose rates: relevance for FLASH radiotherapy

Monte Carlo multi-track chemistry simulations were carried out to study the effects of high dose rates on the transient yields of hydronium ions (H3O+) formed during low linear energy transfer (LET) radiolysis of both pure, deaerated and aerated liquid water at 25 °C, in the interval ∼1 ps – 10 μs. Our simulation model consisted of randomly irradiating water with N interactive tracks of 300-MeV incident protons (LET ∼ 0.3 keV/µm), which simultaneously impact perpendicularly on the water within a circular surface. The effect of the dose rate was studied by varying N. Our calculations showed that the radiolytic formation of H3O+ causes the entire irradiated volume to temporarily become very acidic. The magnitude and duration of this abrupt “acid-spike” response depend on the value of N. It is most intense at times less than ∼10–100 ns, equal to ∼3.4 and 2.8 for N = 500 and 2000 (i.e., for dose rates of ∼1.9 × 109 and 8.7 × 109 Gy/s, respectively). At longer times, the pH gradually increases for all N values and eventually returns to the neutral value of seven, which corresponds to the non-radiolytic, pre-irradiation concentration of H3O+. It is worth noting that these early acidic pH responses are very little dependent on the presence or absence of oxygen. Finally, given the importance of pH for many cellular functions, this study suggests that these acidic pH spikes may contribute to the normal tissue-sparing effect of FLASH radiotherapy.

Multiorbital impact on strong-field ionization of HCl molecules

We have put forward the time-dependent Hartree-Fock approach to investigate ionization dynamics of HCl molecules subjected to strong few-cycle laser pulses, which is based on the finite-element discrete-variable representation in prolate spheroidal coordinates and the Crank-Nicolson method. It is demonstrated that the electron is favored to be freed by the peak electric field pointing towards the H atom for the HCl molecule possessing the bonding length $R=2.5$ a.u., while the ionization prefers to occur for the maximum electric field pointing from the H nucleus to Cl nucleus with increasing internuclear separations. Analysis indicates that the ionization yield of $5\ensuremath{\sigma}$ electrons becomes significant with respect to that of $2\ensuremath{\pi}$ electrons with stretching bonding lengths, and the relative contributions of the ionization from $5\ensuremath{\sigma}$ and $2\ensuremath{\pi}$ electrons change as a function of internuclear separations, which gives rise to the above distinct oriented ionization of HCl molecules.