Low-energy Electron-impact Ionization Research Articles

Absolute triple differential cross sections for low-energy electron impact ionization of biochemically relevant systems: Water, tetrahydrofuran, and hydrated tetrahydrofuran

An experimental procedure is reported, which provides the absolute triple differential cross sections (ATDCSs) for electron-impact ionization of large (bio)molecules. This type of measurements represents the most stringent tests for new or existing theoretical models. We will use this procedure to test the accuracy of the best currently available theoretical models for the problems of electron-impact (65 eV) ionization of the molecules water $({\mathrm{H}}_{2}\mathrm{O})$, tetrahydrofuran $({\mathrm{C}}_{4}{\mathrm{H}}_{8}\mathrm{O})$, and their hydrogen-bonded dimer ${\mathrm{H}}_{2}\mathrm{O}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{C}}_{4}{\mathrm{H}}_{8}\mathrm{O}$. The cross sections were calculated using the molecular three-body distorted-wave (M3DW) model, the multicenter three-distorted-wave (MCTDW) approach, and the multicenter three-distorted-wave using the Ward-Macek approximation (MCTDW-WM). When compared to the new experimental ATDCS results which cover almost the full solid angle of the ejected electron and a broad range of ejected electron energies and projectile scattering angles, it is found that the data for water are generally well reproduced by the M3DW model, while strong deviations in the absolute magnitude of the cross sections are found for the MCTDW. The MCTDW-WM model provides improved agreement over the MCTDW. These theoretical models, however, become less adequate for the ATDCS of ${\mathrm{C}}_{4}{\mathrm{H}}_{8}\mathrm{O}$, in particular concerning the absolute magnitude. Furthermore, we find that a water environment can play a noticeable role for the ionization dynamics in the case of hydrated molecules.

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.

Analysis of multiple scattering contributions in electron-impact ionization of molecular hydrogen

We report a combined experimental and theoretical study on the low-energy (E0 = 31.5 eV) electron-impact ionization of molecular hydrogen (H2). Triple differential cross sections are measured for a range of fixed emission angles of one outgoing electron between and −130° covering the full 4π solid angle of the second electron. The energy sharing of the outgoing electrons varies from symmetric ( eV) to highly asymmetric (E1 = 1 eV and E2 = 15 eV). In addition to the binary and recoil lobes, a structure is observed perpendicular to the incoming beam direction which is due to multiple scattering of the projectile inside the molecular potential. The absolutely normalized experimental cross sections are compared with results from the time-dependent close-coupling (TDCC) calculations. Molecular alignment dependent TDCC results demonstrate that these structures are only present if the molecule axis is lying in the scattering plane.

A theoretical study on collision mechanisms for low energy electron impact ionization of helium in the perpendicular geometry

Under the condition of ten different incident energies ranging from 3 eV to 80 eV above the ionization potential of helium and the outgoing electrons having equal energies, by making use of 3C model and modified 3C model, the triple differential cross sections of electron-impact single ionization of the ground state of helium in the perpendicular geometry are calculated. The result is compared with corresponding experimental result to systematically investigate the influences of various screening effects on the triple differential cross sections for helium. The collision mechanisms of the triple differential cross sections are explored. The result shows that the effects of dynamic screening in the final state can directly affect the structures of the triple differential cross sections at lower incident energy, which will unavoidably affect the angular distribution and relative amplitude of side peaks at angles =90 and =270. The screening effects of residual electron in the final state of He+ have a similar significant effect on the amplitude of triple differential cross section of helium and angular distributions and relative amplitudes of side peaks at angles =90 and =270. When the incident energy is over 84.6 eV, the screening effect of residual electron in the final state of He+ has a slight effect on the amplitude of triple differential cross section, which can be overlooked. But the effects of dynamic screening in the final state on side peaks at angles =90 and =270 need considering. In addition, by taking advantage of DS3C-Z model, the results of collision mechanism of various peaks at angles =180, =90 and =270 show that the middle peak at angle =180 is produced by a process called triple scattering mechanism and then the side peaks at angles =90 and =270 are produced by a process called double scattering mechanism. Such a collision mechanism has a direct influence on the generation and variation law of triple differential cross sections.

Low energy electron-impact ionization of hydrogen atom for coplanar equal-energy-sharing kinematics in Debye plasmas

Low energy electron-impact ionization of hydrogen atom in Debye plasmas has been investigated by employing the exterior complex scaling method. The interactions between the charged particles in the plasma have been represented by Debye-Huckel potentials. Triple differential cross sections (TDCS) in the coplanar equal-energy-sharing geometry at an incident energy of 15.6 eV for different screening lengths are reported. As the screening strength increases, TDCS change significantly. The evolutions of dominant typical peak structures of the TDCS are studied in detail for different screening lengths and for different coplanar equal-energy-sharing geometries.

Experimental and theoretical triple-differential cross sections for tetrahydrofuran ionized by low-energy 26-eV-electron impact

We report an experimental and theoretical study of low-energy electron-impact ionization of tetrahydrofuran, which is a molecule of biological interest. The experiments were performed using an advanced reaction microscope specially built for electron-impact ionization studies. The theoretical calculations were performed within the molecular three-body distorted-wave model. Reasonably good agreement is found between experiment and theory.

Kinematically complete study of low-energy electron-impact ionization of argon: Internormalized cross sections in three-dimensional kinematics

Kinematically complete study of low-energy electron-impact ionization of argon: Internormalized cross sections in three-dimensional kinematics

Calculation of FDCS for the low and intermediate energy electron impact ioni-zation of water molecules

SynopsisFully differential cross sections (FDCS) results for the electron impact ionization of water molecules taking place from 1b1, 1b2 and 2a1 orbitals are presented. The FDCS calculations have been performed in second Born approximation.

Erratum: Low-energy electron-impact ionization of argon: Three-dimensional cross section [Phys. Rev. A 85, 032702 (2012)

Erratum: Low-energy electron-impact ionization of argon: Three-dimensional cross section [Phys. Rev. A 85, 032702 (2012)

Low-energy (E0 = 65 eV) electron-impact ionization of neon: Internormalized triple-differentical cross sections in 3D kinematics

SynopsisWe present a combined experimental and theoretical study on the low-energy (E0 = 65 eV) electron- impact ionization of neon. The experimental data are compared to predictions from a hybrid second-order distorted-wave Born plus R-matrix approach (DWB2-RM), the distorted-wave Born approximation with inclusion of post-collision interaction (DWBA-PCI), a three-body distorted-wave approach (3DW), and a B-spline R-matrix (BSR) with pseudostates approach. Excellent agreement is found between experiment and the 3DW and BSR theories. The importance of PCI effects is clearly visible in this low-energy electron-impact ionization process.

Kinematically complete study of low-energy electron-impact ionization of neon: Internormalized cross sections in three-dimensional kinematics

As a further test of advanced theoretical methods to describe electron-impact single-ionization processes in complex atomic targets, we extended our recent work on $\mathrm{Ne}(2p$) ionization [X. Ren, S. Amami, O. Zatsarinny, T. Pfl\uger, M. Weyland, W. Y. Baek, H. Rabus, K. Bartschat, D. Madison, and A. Dorn, Phys. Rev. A 91, 032707 (2015)] to $\mathrm{Ar}(3p$) ionization at the relatively low incident energy of ${E}_{0}=66$ eV. The experimental data were obtained with a reaction microscope, which can cover nearly the entire $4\ensuremath{\pi}$ solid angle for the secondary electron emission. We present experimental data for detection angles of 10, 15, and ${20}^{\ensuremath{\circ}}$ for the faster of the two outgoing electrons as a function of the detection angle of the secondary electron with energies of 3, 5, and 10 eV, respectively. Comparison with theoretical predictions from a $B$-spline $R$-matrix (BSR) with pseudostates approach and a three-body distorted-wave (3DW) approach, for detection of the secondary electron in three orthogonal planes as well as the entire solid angle, shows overall satisfactory agreement between experiment and the BSR results, whereas the 3DW approach faces difficulties in predicting some of the details of the angular distributions. These findings are different from our earlier work on $\mathrm{Ne}(2p$), where both the BSR and 3DW approaches yielded comparable levels of agreement with the experimental data.

An (e, 2e + ion) study of low-energy electron-impact ionization and fragmentation of tetrahydrofuran with high mass and energy resolutions.

We study the low-energy (E0 = 26 eV) electron-impact induced ionization and fragmentation of tetrahydrofuran using a reaction microscope. All three final-state charged particles, i.e., two outgoing electrons and one fragment ion, are detected in triple coincidence such that the momentum vectors and, consequently, the kinetic energies for charged reaction products are determined. The ionic fragments are clearly identified in the experiment with a mass resolution of 1 amu. The fragmentation pathways of tetrahydrofuran are investigated by measuring the ion kinetic energy spectra and the binding energy spectra where an energy resolution of 1.5 eV has been achieved using the recently developed photoemission electron source. Here, we will discuss the fragmentation reactions for the cations C4H8O(+), C4H7O(+), C2H3O(+), C3H6(+), C3H5(+), C3H3(+), CH3O(+), CHO(+), and C2H3(+).

Comparison of Experiment and Theory for Electron Impact Ionization of Isoelectronic Atoms and Molecules

Experimental and Theoretical Triply Differential Cross sections will be presented for low energy electron impact ionization of Ne, CH4, and NH3. The collision mechanisms responsible for the various structures found in the cross sections will be discussed.

Electron impact ionization and excitation studies of laser prepared atomic targets

A review of current work at Manchester is given, with emphasis on low energy electron impact ionization & excitation experiments from laser-prepared targets. Three different methods are considered: super-elastic scattering from targets prepared by laser radiation in an optical enhancement cavity, (e,2e) studies of laser-aligned atoms, and electron impact ionization studies from laser cooled atoms in a new type of atom trap - the AC-MOT. The status of each experimental programme is detailed, and new data presented. Future directions and techniques are then discussed.

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.

(e,2e) measurements using a magnetic angle changer

While measurements of triple differential cross sections (TDCS) for electron impact ionization have been performed for almost 40 years, the angular range of available data has generally been restricted due to physical constraints of the apparatus. We have overcome these constraints by incorporating a magnetic angle changer (MAC) into a conventional (e,2e) spectrometer. Use of a MAC in an (e,2e) experiment allows access to regions where as yet unobserved physical effects occur. One such example is where the scattered and ejected electrons emerge at the same angle, a region where the Coulomb repulsion between the two should be largest. This ‘post-collision interaction’ (PCI) is thought to play an important role in low energy electron impact ionization. We discuss recently published data for TDCS of the inner valence shell 3s ionization of argon measured over the backward scattering direction. We also present new data for outer shell 3p ionization of argon over the full angular range in the scattering plane, including the region where scattered/ejected electrons emerge at the same angle. To our knowledge, this is the first reported TDCS over the complete 360° angular range using a conventional (e,2e) spectrometer.

Direct Measurement and Theoretical Calculation of the Rate Coefficient for Cl + CH3 in the Range from T = 202−298 K

The rate coefficient has been measured under pseudo-first-order conditions for the Cl+CH3 association reaction at T=202, 250, and 298 K and P=0.3-2.0 Torr helium using the technique of discharge-flow mass spectrometry with low-energy (12-eV) electron-impact ionization and collision-free sampling. Cl and CH3 were generated rapidly and simultaneously by reaction of F with HCl and CH4, respectively. Fluorine atoms were produced by microwave discharge in an approximately 1% mixture of F2 in He. The decay of CH3 was monitored under pseudo-first-order conditions with the Cl-atom concentration in large excess over the CH3 concentration ([Cl]0/[CH3]0=9-67). Small corrections were made for both axial and radial diffusion and minor secondary chemistry. The rate coefficient was found to be in the falloff regime over the range of pressures studied. For example, at T=202 K, the rate coefficient increases from 8.4x10(-12) at P=0.30 Torr He to 1.8x10(-11) at P=2.00 Torr He, both in units of cm3 molecule-1 s-1. A combination of ab initio quantum chemistry, variational transition-state theory, and master-equation simulations was employed in developing a theoretical model for the temperature and pressure dependence of the rate coefficient. Reasonable empirical representations of energy transfer and of the effect of spin-orbit interactions yield a temperature- and pressure-dependent rate coefficient that is in excellent agreement with the present experimental results. The high-pressure limiting rate coefficient from the RRKM calculations is k2=6.0x10(-11) cm3 molecule-1 s-1, independent of temperature in the range from 200 to 300 K.

Double- and triple-differential cross sections for the low-energy electron-impact ionization of hydrogen

Double- and triple-differential cross sections are presented for the electron-impact ionization of ground-state hydrogen at incident electron energies of 15.6 and $17.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The time-dependent close-coupling method is used to calculate the differential ionization cross sections, and comparisons are made with previous theoretical calculations and with experimental measurements. Excellent agreement is obtained between our calculations and previous work.

Low-energy electron-impact single ionization of helium

A study is made of low-energy electron-impact single ionization of ground-state helium. The time-dependent close-coupling method is used to calculate total integral, single differential, double differential, and triple differential ionization cross sections for impact electron energies ranging from 32 to $45\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. For all quantities, the calculated cross sections are found to be in very good agreement with experiment, and for the triple differential cross sections, good agreement is also found with calculations made using the convergent close-coupling technique.

An ICR Study of Ion-Molecules Reactions Relevant to Titan’s Atmosphere: An Investigation of Binary Hydrocarbon Mixtures up to 1 Micron

Results are reported for studies of binary mixtures of hydrocarbons exposed to low-energy electron impact ionization. A variety of experimental methods are used: conventional ICR mass spectrometry, the standard double resonance in an ICR for determination of the precursor ions, and the modulated double resonance ejection in an ICR for the determination of the daughter ions. A flowing afterglow-selected ion flow tube experiment (FA-SIFT) was used for validation and examination of termolecular reactions. An extensive database of reaction kinetics already exists for many of these reactions. The main point of this study was the determination of the accuracy of this database and the identification of missing reactions and reaction channels. An effort was made to extend the study to the highest pressures possible to determine if any important termolecular reaction channels were present that were not recognized in earlier investigations. A new approach was used here. In the binary mixtures of hydrocarbon gases, mass spectra were obtained as a function of independent pressure changes of both gases. All the mass peaks in the spectra were fitted using existing kinetic data as a starting point. A model of the ion abundances was then produced from the solution of the partial differential equations derived from the kinetics in terms of reaction rate coefficients and initial abundances. The model was fitted to the data for all of the pressures by a least-squares fit to the reaction rate coefficients and initial abundances. The kinetic parameters were then adjusted if required.