eV Electron Impact Ionization Research Articles

Triply differential cross sections for electron and positron impact on methane.

We here report theoretical triply differential cross sections (TDCS) for 250eV electron and positron impact ionization of the methane molecule calculated within the second-order distorted-wave Born approximation (DWBA2) for various momentum transfer conditions. The experimental data taken from Işık et al. [J. Phys. B: At., Mol. Opt. Phys. 49, 065203 (2016)] will be compared with the current theoretical predictions as well as molecular three body distorted wave (M3DW) approximation and generalized Sturmian function (GSF) theoretical models in a non-coplanar geometry. In the low analyzer scattering plane, the results obtained within the DWBA2 theory show better agreement with the experimental results compared to the GSF results. The M3DW results also exhibit agreement with the experimental results, in particular in the perpendicular plane geometry. Furthermore, significant differences between electron and positron TDCS were observed.

Triply Differential Positron and Electron Impact Ionization of Argon: Systematic Features and Scaling

Triply differential data are presented for the 200 eV positron and electron impact ionization of argon. Six electron emission energies between 2.6 and 19 eV, and for scattering angles of 2, 3, and 4 degrees cover a momentum transfer range of 0.16 to 0.31 a.u. The binary and recoil intensities are fitted using a double peak structure in both regions, which, for the present kinematic conditions, are unresolved. The fitted peak intensities and angular positions are shown to have systematic dependences as a function of the momentum transfer and kinematic emission angle, respectively, and illustrate projectile charge effects. A comparison with available theories is made where it is seen that the most notable differences include the fact that for the binary lobe, the observed intensity for emission angles around 100° is absent in the theories, and the theoretical predications overestimate the importance of recoil interactions.

Ultrafast ring-opening fragmentation dynamics of C6H63+ induced by electron-impact ionization

The fragmentation dynamics of triply charged benzene [(${\mathrm{C}}_{6}{\mathrm{H}}_{6}{)}^{3+}$] induced by 260 eV electron-impact ionization are investigated using a multiparticle coincidence momentum spectrometer. By measuring three fragment ions and one outgoing electron in quadruple coincidence, we identify the complete three-body dissociation channels of ${{\mathrm{CH}}_{2}}^{+} + {\mathrm{C}}_{2}{{\mathrm{H}}_{3}}^{+} + {\mathrm{C}}_{3}{\mathrm{H}}^{+}, {{\mathrm{CH}}_{3}}^{+} + {\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{+} + {\mathrm{C}}_{3}{\mathrm{H}}^{+}$, and ${\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{+} + {\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{+} + {\mathrm{C}}_{2}{{\mathrm{H}}_{2}}^{+}$. We determine the projectile-energy-loss spectra, Dalitz plots, Newton diagrams, momentum correlation maps, and kinetic-energy release for each fragmentation channel. The analysis of these spectra is supported by an ab initio molecular dynamics simulation, which provides a molecular movie of the dissociation process. Our study reveals sequential mechanisms for all three dissociation channels of (${\mathrm{C}}_{6}{\mathrm{H}}_{6}{)}^{3+}$ trication, i.e., an ultrafast ring-opening reaction followed by two subsequent Coulomb-explosion processes.

Automated Screening and Filtering Scripts for GC×GC-TOFMS Metabolomics Data

Comprehensive two-dimensional gas chromatography mass spectrometry (GC×GC-MS) is a powerful tool for the analysis of complex mixtures, and it is ideally suited to discovery studies where the entire sample is potentially of interest. Unfortunately, when unit mass resolution mass spectrometers are used, many detected compounds have spectra that do not match well with libraries. This could be due to the compound not being in the library, or the compound having a weak/nonexistent molecular ion cluster. While high-speed, high-resolution mass spectrometers, or ion sources with softer ionization than 70 eV electron impact (EI) may help with some of this, many GC×GC systems presently in use employ low-resolution mass spectrometers and 70 eV EI ionization. Scripting tools that apply filters to GC×GC-TOFMS data based on logical operations applied to spectral and/or retention data have been used previously for environmental and petroleum samples. This approach rapidly filters GC×GC-TOFMS peak tables (or raw data) and is available in software from multiple vendors. In this work, we present a series of scripts that have been developed to rapidly classify major groups of compounds that are of relevance to metabolomics studies including: fatty acid methyl esters, free fatty acids, aldehydes, alcohols, ketones, amino acids, and carbohydrates.

Formation of H3+ from ethane dication induced by electron impact

Hydrogen migration plays an important role in the chemistry of hydrocarbons which considerably influences their chemical functions. The migration of one or more hydrogen atoms occurring in hydrocarbon cations has an opportunity to produce the simplest polyatomic molecule, i.e. H3+. Here we present a combined experimental and theoretical study of H3+ formation dynamics from ethane dication. The experiment is performed by 300 eV electron impact ionization of ethane and a pronounced yield of H3+ + C2H3+ coincidence channel is observed. The quantum chemistry calculations show that the H3+ formation channel can be opened on the ground-state potential energy surface of ethane dication via transition state and roaming mechanisms. The ab initio molecular dynamics simulation shows that the H3+ can be generated in a wide time range from 70 to 500 fs. Qualitatively, the trajectories of the fast dissociation follow the intrinsic reaction coordinate predicted by the conventional transition state theory. The roaming mechanism, compared to the transition state, occurs within a much longer timescale accompanied by nuclear motion of larger amplitude.

In Situ Diagnostic of Supercritical Fuel Surrogates: Probing Heterogeneous Catalysis by Collision-Induced Dissociation in a Molecular Beam Tandem Mass Spectrometer

High-speed flight is limited by the amount of heat absorbed by the aviation fuel in a thermal management system. One approach to increase the thermal capacity of the fuel is via endothermic heterogeneous catalysis within the heat exchanger (HEX). To optimize such chemistry, better tools are needed to probe the chemical composition inside the HEX in situ. Toward this aim, we demonstrate the first on-line analysis of supercritical fuel surrogates (n-hexane and n-dodecane) over a zeolite catalyst (H-ZSM-5) using tandem mass spectrometry (MS). In our approach, a supersonic expansion is generated directly from the supercritical state (200–1000 °C and 400–1000 psi) of the neat fluid to capture a “snapshot” of the reactive intermediates and products inside the reactor by isolating these species in the gas phases. A molecular beam is generated, which is ionized by 10 eV electron-impact ionization (EI), and the mass spectrum is acquired using a triple quadrupole mass spectrometer. Based on precursor scans, we distinguish between EI fragments and cracking products from the furnace. Product scans from the triple quadrupole reveal a structural similarity between the EI fragments and pyrolysis products. By directly identifying the light C2 and C3 radicals from pyrolysis, our results corroborate those in our previous study [DeBlase, Energy Fuels 2018, 32, 12289], which suggested that C2 and/or C3 intermediates are the key building blocks for aromatic synthesis by supercritical alkane pyrolysis. We anticipate that the new tandem MS capability will be widely used for catalyst development and to study the endothermic chemistry of fuels.

Research and Application of Mass Spectrometry with Low Energy Electron Impact Ionization Source

Abstract The changes of concentrations of O2 and CO2 in tail gas of fermentation industry reflect some important physiological and metabolic parameters in the fermentation process. Monitoring of the tail gas of fermentation has important significance for guiding the fermentation process. However, traditional monitoring methods are time-consuming with low precision and poor versatility, and are not suitable to monitor fermentation tail gas accurately in real time. In the process mass spectrometry, the traditional 70 eV electron impact ionization source will produce a large number of fragment ions, which may overlap with different substances, thus increasing the difficulty of qualitative analysis and causing inaccurate monitoring results. In order to realize real-time, on-line accurate monitoring of fermentation tail gas, a quadrupole process mass spectrometry based on electron impact ionization was developed for on-line analysis in this study. Through investigation of the relationship between emission current of filament and the electron receiving pole at low ionization energy, the ionization efficiency of the electron impact ionization source under low ionization energy mode was improved, and CO2, O2 and N2 were tested. The results showed that under the conditions of electron energy of 22 eV, electron receiving electrode of −22 V and filament emission current of 20–25 mA, the relative errors of quantitative detection of CO2 with volume fraction of higher than 0.023% and O2 with volume fraction of higher than 0.021% were all less than 1%. The instrument and the developed method had ideal application prospect in the rapid real-time, on-line monitoring of industrial processes.

Comparison of experimental and theoretical triple differential cross sections for the single ionization ofCO2 (1πg)by electron impact

Experimental and theoretical triple differential cross sections for intermediate-energy (250 eV) electron-impact single ionization of the ${\mathrm{CO}}_{2}$ are presented for three fixed projectile scattering angles. Results are presented for ionization of the outermost $1{\ensuremath{\pi}}_{g}$ molecular orbital of $\mathrm{C}{\mathrm{O}}_{2}$ in a coplanar asymmetric geometry. The experimental data are compared to predictions from the three-center Coulomb continuum approximation for triatomic targets, and the molecular three-body distorted wave (M3DW) model. It is observed that while both theories are in reasonable qualitative agreement with experiment, the M3DW is in the best overall agreement with experiment.

Coplanar asymmetric angles and symmetric energy sharing triple differential cross sections for 200 eV electron-impact ionization of Ar (3p)

SynopsisWe have measured triple differential cross sections (TDCSs) for electron-impact ionization of the 3p shell of Ar at 200 eV incident electron energy. The experiments have been performed in coplanar asymmetric energy sharing geometry. The experimental results are compared with the theoretical models of three body distorted wave (3DW) and distorted wave Born approximation (DWBA).

Interference effects for intermediate energy electron-impact ionization of H2 and N2 molecules

We have studied electron impact ionization of H2 and N2 molecules at intermediate energies to look for possible two center interference effects experimentally and theoretically. Here we report a study of the interference factor I for 250 eV electron-impact ionization. The experimental measurements are performed using a crossed-beam-type electron-electron coincidence spectrometer and theoretical calculations are obtained using the Molecular Three Body Distorted Wave Approximation (M3DW). We found that the I-factor demonstrated strong evidence for two-center interference effects for both H2 and N2. We also found that the I-factor is more sensitive to projectile angular scans than to ejected electron energy scans which indicate that for the present set of kinematics the diffraction of the projectile from two scattering centers is more important than interference between electron waves emitted from two different centers.

Flow injection of liquid samples to a mass spectrometer with ionization under vacuum conditions: a combined ion source for single-photon and electron impact ionization

Electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photo-ionization (APPI) are the most important techniques for the ionization of liquid samples. However, working under atmospheric pressure conditions, all these techniques involve some chemical rather than purely physical processes, and therefore, side reactions often yield to matrix-dependent ionization efficiencies. Here, a system is presented that combines both soft single-photon ionization (SPI) and hard 70 eV electron impact ionization (EI) of dissolved compounds under vacuum conditions. A quadrupole mass spectrometer was modified to enable direct EI, a technique developed by Cappiello et al. to obtain library-searchable EI mass spectra as well as soft SPI mass spectra of sample solutions. An electron beam-pumped rare gas excimer lamp working at 126 nm was used as well as a focusable vacuum UV light source for single-photon ionization. Both techniques, EI and SPI, were applied successfully for flow injection experiments providing library-matchable EI fragment mass spectra and soft SPI mass spectra, showing dominant signals for the molecular ion. Four model compounds were analyzed: hexadecane, propofol, chlorpropham, and eugenol, with detection limits in the picomolar range. This novel combination of EI and SPI promises great analytical benefits, thanks to the possibility of combining database alignment for EI data and molecular mass information provided by SPI. Possible applications for the presented ionization technology system are a matrix-effect-free detection and a rapid screening of different complex mixtures without time-consuming sample preparation or separation techniques (e.g., for analysis of reaction solutions in combinatorial chemistry) or a switchable hard (EI) and soft (SPI) MS method as detection step for liquid chromatography.

Radio-Frequency Ionization of Organic Compounds for Mass Spectrometry Analysis

A new ionization technique: A radio-frequency signal was used to ionize neutral organic molecules in the ultrahigh-vacuum region of a Fourier transform ion cyclotron resonance mass spectrometer. Radio-frequency ionization (RFI) yielded signal/noise (S/N) ratios roughly six times higher than those generated by conventional 70 eV electron impact ionization (EI).

Fragmentation of14,15N2by electron impact investigated using a time-delayed spectroscopic technique

A general method to measure the energy distribution function of molecular fragments using a pulsed electron beam with time-delayed extraction coupled to a time-of-flight spectrometer is presented. The energy distributions of the homoisotopic species ${}^{14,15}$N${}^{+}$ and ${}^{14,15}$N${{}_{2}}^{2+}$, with the same mass-to-charge ratio, produced by 35--400 eV electron impact ionization of N${}_{2}$, are disentangled, showing two independent fragmentation routes for N${}^{+}$ ions with kinetic energies smaller than 1.0 eV. No measurable isotopic effect was found, indicating that predissociation does not play a role in this region of impact energies.

Low-energy electron-impact ionization of argon: Three-dimensional cross section

Low-energy (${E}_{0}$ $=$ 70.8 eV) electron-impact single ionization of a 3$p$ electron in argon has been studied experimentally and theoretically. Our measurements are performed using the so-called reaction microscope technique, which can cover nearly a full 4\ensuremath{\pi} solid angle for the emission of a secondary electron with energy below 15 eV and projectile scattering angles ranging from \ensuremath{-}8\ifmmode^\circ\else\textdegree\fi{} to \ensuremath{-}30\ifmmode^\circ\else\textdegree\fi{}. The measured cross sections are internormalized across all scattering angles and ejected energies. Several theoretical models were employed to predict the triple-differential cross sections (TDCSs). They include a standard distorted-wave Born approximation (DWBA), a modified version to account for the effects of postcollision interaction (DWBA-PCI), a hybrid second-order distorted-wave plus $R$-matrix (DWB2-RM) method, and the recently developed $B$-spline $R$-matrix with pseudostates (BSR) approach. The relative angular dependence of the BSR cross sections is generally found to be in reasonable agreement with experiment, and the importance of the PCI effect is clearly visible in this low-energy electron-impact ionization process. However, there remain significant differences in the magnitude of the calculated and the measured TDCSs.

Observation of Multiple Scattering in (e,2e) Experiments on Small Argon Clusters

A kinematically complete experiment for 100 eV electron-impact ionization of small argon clusters was realized. The triple coincidence detection of both outgoing electrons and the residual ion allows the discrimination between single ionization of atoms, dimers and non-mass-selected small clusters as well as between ionization and excitation within the same cluster. Comparison of fully and partly differential ionization cross sections for clusters with those of atoms reveal clear signatures of multiple-scattering reactions. For ionization with excitation, an almost isotropic electron emission pattern is observed.

Close-coupling calculations of 64.6 eV e-He ionization

The convergent close-coupling and the time-dependent close-coupling methods are applied to the calculation of 64.6 eV electron-impact ionization of the ground state of helium resulting in two 20 eV outgoing electrons. The results of the calculations are compared with measured fully differential cross sections in various geometries. For the generally large in-plane geometries there is good agreement between the two theories and experiment. For the out-of-plane case the cross sections are generally smaller and some differences between the two calculations are evident, as well as experiment.

Ionization of helium by 64.6 eV electrons

A comprehensive computational study using the convergent close-coupling method of 64.6 eV electron-impact ionization of the ground state of helium is presented. The kinematics considered range from the very asymmetric energy-sharing through to equal energy-sharing. The cross sections given range from the total to fully differential, with the latter being calculated for in- and out-of-plane geometries. Generally excellent agreement with available experiment is found, but some systematic discrepancies are also identified.

Triply differential measurements for 200 eV e+ – and e- – Ar collisions

Triply differential single ionization yields for projectile scattering and electron emission are presented for 200 eV positron and electron impact ionization of argon. The projectile scattering and the electron emission are observed to be different for positron and electron impact.

Fragmentation of size-selected Xe clusters: Why does the monomer ion channel dominate the Xe n and [formula omitted] ionization?

The fragmentation of the small Xe n n = 2 − 5 clusters following 70 eV electron impact ionization has been investigated in a size selective experiment and simulated using non-adiabatic dynamics. The experimental results show that the clusters strongly fragment to yield monomer Xe + (more than 90%) and dimer Xe 2 + fragments (the remaining few percent). Trimer Xe 3 + fragments first occur from the neutral pentamers Xe 5 in a very low yield of approximately 0.3%. The present results are compared with the previous ones for Kr and Ar clusters. It is shown that the Xe and Kr clusters exhibit a qualitatively similar behavior with a strong propensity for monomer fragments, while in the Ar case dimers prevail. The theoretical calculations also reveal a strong fragmentation to the dimer and monomer fragments. However, the dimer Rg 2 + is predicted to be the major product for all rare gases (Rg ≡ Ar, Kr, Xe). Possible reasons for the discrepancy between theory and experiment are discussed.

Demonstration of a VUV Lamp Photoionization Source for Improved Organic Speciation in an Aerosol Mass Spectrometer

In recent years, the Aerodyne Aerosol Mass Spectrometer (AMS) has become a widely used tool for determining aerosol size distributions and chemical composition for non-refractory inorganic and organic aerosols. All AMSs to date have used a combination of flash thermal vaporization and 70 eV electron impact (EI) ionization. However, EI causes extensive fragmentation and mass spectra of organic aerosols are difficult to deconvolve because they are composites of the overlapping fragmentation patterns of a multitude of species. In this manuscript we present an approach to gain more information about organic aerosol composition by employing the softer technique of vacuum ultraviolet (VUV) ionization in a Time-of-Flight AMS (ToF-AMS). Our novel design allows for alternation between photoionization (PI) and EI within the same instrument on a timescale of minutes. Thus, the EI-based quantification capability of the AMS is retained while improved spectral interpretation is made possible by combined analysis of the...