Electron Detachment Cross Sections Research Articles

Absolute total electron detachment and relative ionisation cross sections for O2 − collisions with O2 in the energy range (10–1500 eV)

Absolute total electron detachment cross for O2 − collisions with O2 are reported for impact energies ranging from 10 to 1600 eV as measured with a transmission-beam experimental apparatus. The primary anionic beam projectile is produced in a hollow cathode discharge-induced plasma, and the collisions with the neutral molecular target occur in a gas cell at a well-known constant pressure. The accurate determination of the gas pressure (within 8%) in the scattering chamber allowed us to clarify the important discrepancies (by about a factor 4) prevailing between previous total electron detachment cross-section measurements. Additional local maxima in the cross-section values, not previously observed, have been detected around 50 and 150 eV.

Total Electron Detachment and Induced Cationic Fragmentation Cross Sections for Superoxide Anion (O2-) Collisions with Benzene (C6H6) Molecules.

In this study, novel experimental total electron detachment cross sections for O2− collisions with benzene molecules are reported for the impact energy range (10–1000 eV), as measured with a transmission beam apparatus. By analysing the positively charged species produced during the collision events, relative total ionisation cross sections were derived in the incident energy range of 160–900 eV. Relative partial ionisation cross sections for fragments with m/z ≤ 78 u were also given in this energy range. We also confirmed that heavier compounds (m/z > 78 u) formed for impact energies between 550 and 800 eV. In order to further our knowledge about the collision dynamics governing the fragmentation of such heavier molecular compounds, we performed molecular dynamics calculations within the framework of the Density Functional Theory (DFT). These results demonstrated that the fragmentation of these heavier compounds strongly supports the experimental evidence of m/z = 39–42, 50, 60 (u) cations formation, which contributed to the broad local maximum in the total ionisation observed from 550 to 800 eV. This work reveals the reactivity induced by molecular anions colliding with hydrocarbons at high energies, processes that can take place in the interstellar medium under various local conditions.

Electron loss of CH− and [formula omitted] induced by interactions with N2 and O2 at keV energies

Relative electron detachment cross sections for the projectiles CH− and CH2− colliding with N2 and O2 target gases were measured. Two different methods to measure the cross sections were used, the signal growth rate and the beam attenuation. The cross sections measured with the beam attenuation technique were higher than those measured with the signal growth rate method. An analysis based on the differences of the cross sections measured with the two methods showed consistency with the time-dependent processes of metastable auto-detaching state populations undergoing decay as a function of its time of flight. These metastable processes seem to be target dependent. When the target was O2, a decaying process characterized by a lifetime of the order of 0.40 ± 0.16 μs was found. For the case of the N2 as a target, it was not possible to derive a characteristic lifetime.

Electron detachment cross section of H− induced by collisions with O2

We present measured electron detachment cross sections from hydrogen anion induced by collisions with O2 in the energy range of 1 to 9 keV with two methods, the beam attenuation and the signal growth rate. Processes involved in the interaction are discussed in terms of the cross section differences as a function of the time of flight of the H− through an analysing electric field. As a consequence, an ansatz value of the life-time for the auto-detaching states of H− population of 39 ± 12 ns is derived that is consistent with predictions for the lifetime of the (2p 2)3P state of H−.

Experimental study on electron detachment cross sections of Cl– in collisions with inert gas atoms

The single-electron and double-electron detachment cross sections of Cl– in collision with inert gas atoms (He–Xe) have been measured by the growth rate method. The incident negative ions’ energies are from 5 to 30 keV. It is found that the single-electron detachment cross sections become greater and greater when the target atoms change from helium to xenon. However, double-electron detachment cross sections show a more complicated relationship with the target atom changing. The results of this work have been compared with the previous data, and a model based on energy division is used to interpret the trends of cross sections.

Experimental electron-detachment cross sections for collisions of O2− with N2 molecules in the energy range 50–7000 eV

Experimental electron-detachment cross sections for collisions of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow><mml:mo>−</mml:mo></mml:msup></mml:math> with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">N</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math> molecules in the energy range 50–7000 eV

Charge-state distributions and charge-changing cross sections and their impact on the performance of AMS facilities

The charge state distributions of ions passing through gases or foils have a significant influence on the performance of AMS facilities. Most important is the stripping process in the terminal of the accelerator. Published data on charge state distributions have been collected as well as associated charge-changing cross sections for various projectiles and targets relevant to AMS. Based on these data, the stripping yields as function of the stripper thickness and ion energy are calculated. This allows to evaluate the stripping thickness required for reaching charge state equilibrium, as well as mass fractionation effects depending on energy and stripper thickness. The significantly different behavior of helium compared to other gases leads to very high stripping yields for specific charge states at low energies. The cross sections for electron detachment of negative ions give a tool for estimating beam losses in the low energy acceleration tube. Further, electron capture and loss cross sections allow the estimation of backgrounds caused by charge exchange processes in the acceleration tubes. The effects discussed here are illustrated with calculations and simulations for Carbon beams. The data are useful for the design of new AMS facilities or improving existing ones.

Electron detachment of hydrogen anion in collisions with hydrogen molecule studied by surface hopping classical trajectory calculations.

We employ the on-the-fly surface hopping classical trajectory algorithm to study the electron detachment process in low-energy H- + H2 collisions. The ground-state and the first-excited-state Local Complex Potentials (LCPs) calculated by the generalized diatomics-in-molecule method are used for the full three-dimensional nonadiabatic nuclear dynamics. Two kinds of nonadiabatic effects are taken into account: discrete-discrete transitions and discrete-continuous transitions. Discrete-discrete nonadiabatic transition probabilities are calculated by means of the adiabatic-potential-based formula within the Landau-Zener model for each individual trajectory computed along real parts of the LCPs. Discrete-continuous (electron detachment) nonadiabatic transition probabilities are calculated via quasi-stationary widths which are related to the imaginary parts of the LCPs of both the electronic states of the H3- anion. Two mechanisms of the electron detachment process are treated and discussed: the direct mechanism based on quasi-stationarity of the ground state and the indirect mechanism based on both nonadiabatic transitions from the ground state to the first excited state and quasi-stationarity of the excited state. It is shown that the direct mechanism prevails at low collision energies, while the indirect mechanism makes a substantial contribution at relatively high collision energies, roughly higher than 5 eV. At collision energies higher than 2 eV, the electron detachment probability has rather high values and this affects noticeably other inelastic processes in these collisions. The electron detachment cross section in H- + H2 collisions is calculated for the collision energy range from 1 to 100 eV and a reasonable agreement with available experimental data is obtained.

Collision induced electron detachment cross sections of the H2CC− anion below 10 keV on O2 and N2

Measurements of the total electron detachment cross section induced by collisions from the vinylidene molecular anion H2CC− have been performed below 10keV kinetic energy on N2 and O2 target gases. The process of electron detachment for this molecular anion is studied with two methods, the signal growth rate and the beam attenuation techniques. The results are analyzed based on the differences of the cross section measurements with the two methods and on the application of the geometrical scale model.

Multiple electron capture from isolated protein poly-anions in collision with slow highly charged ions.

Collisions of 375 keV Xe25+ ions with trapped mass/charge selected poly-anions of the cytochrome C protein (∼12.5 kDa) were studied by coupling a linear quadrupole ion trap with low-energy ion beam facility. Tandem mass spectra were recorded for the protein precursor charge states ranging from -9 to -17. The present work reports the first study of slow highly charged ion collisions with poly-anions. A high signal to noise ratio allowed the study of the intensity of single and multiple electron removal by a projectile, as well as associated neutral losses, as a function of the target charge state. Relative single and double electron detachment cross sections were found to increase with increasing charge state of the precursor anion. The experimental findings are supported by the calculations of the total electron capture cross sections, based on the classical over-the-barrier model, restricted to a simple uniformly charged linear protein structure and a near-end electron capture.

Collisional induced double electron loss of NO− and CH[formula omitted] anions below 10 keV energies

Double electron detachment cross sections for methane and nitric oxide negative ions passing through oxygen and nitrogen gas targets have been studied in the energy ranges of 2.0–10.0keV for CH4− and 4.0–10.0keV for NO−. The signal growth rate method was employed. Fragmentation was only observed for NO− anions colliding with both targets. In the case of CH4− anions no fragmentation was observed. The double electron detachment cross sections for methane ions is almost one order of magnitude larger than the one for nitric oxide ions. The fragmentation cross-sections for NO− is larger than the double electron detachment cross sections.

Theoretical investigation of electron transfer and detachment processes in low energy H− + Li and Li− + H collisions

The charge exchange and collisional detachment processes in H− + Li and Li− + H collisions have been studied by using the quantal molecular orbital close-coupling (QMOCC) method in the energy ranges of about 0.12–1000 eV u−1 and 0.1 meV–1000 eV, respectively, and the inelastic collision cross sections and rate coefficients have been computed and presented. It is found that the electron transfer process in the H− + Li and Li− + H collisions is due to the Demkov coupling between the 12Σ+ and 22Σ+ states at internuclear distances of about 15a0. The collisional electron detachment in the considered collision system is due to the excitation of the remaining six states, which are all unstable against autodetachment. These states are populated through a series of Landau–Zener couplings of the 22Σ+ state with upper 2Σ+ states and by the rotational 2Σ+–2Π couplings at small internuclear distances. The cross sections for electron transfer in H− + Li and Li− + H collisions in the energy range of 10–1000 eV u−1 attain values in the range of 10−16–10−15 cm2 (reaching their maximum values of about 5 × 10−15 cm2 at 500–600 eV u−1), while the values of the corresponding electron detachment cross sections in this energy range attain generally smaller values.

Electron detachment of NO− in collisions with O2 and N2 below 10 keV

Measurements for the total electron detachment cross-sections with the beam attenuation technique and for the single electron detachment cross-sections with the growth rate method of NO− in collisions with O2 and N2 are reported. Differences of one order of magnitude between the measuring techniques were found. Possible contributions of auto-detachment processes and an application of the geometrical model are discussed.

Experimental study of single-electron detachment for [formula omitted], [formula omitted], [formula omitted], [formula omitted], and [formula omitted] in collisions with Kr and Xe

The single electron detachment (SED) cross sections for Cu−, Ag−, Au−, Co−, and Ir− in collisions with Kr and Xe in the energy range of 10keV≤E≤30keV are measured by using the growth rate method in our experiments. The SED cross sections increase as the incident velocity increases. These data are compared with other available experimental data for the corresponding negative ions colliding with Ar. The affinities of the negative ions and the mass relation between the projectile and the target seem important in the electron detachment process.

Electron detachment cross sections of colliding with O2 and N2 below 10 keV energies

Negative methane is an interesting molecular anion from a fundamental point of view and also because it can be a source of thermal electrons in low temperature hydrocarbon plasma. Measurements of collisional electron detachment from methane anions colliding with O2 and N2 in the energy range of 0.3 to 10.0 keV are presented. Single electron detachment was measured with the signal grow-rate method and total electron detachment was measured with the beam attenuation technique. Both techniques were employed in order to study the process of electron detachment over the present energy range. The validity of the geometrical model is discussed.

Electron-detachment cross section forCN−andO2−incident on N2at intermediate velocities

The electron-detachment cross section was measured for the molecular negative ions ${\mathrm{CN}}^{\ensuremath{-}}$ and ${\mathrm{O}}_{2}^{\ensuremath{-}}$ incident on N${}_{2}$, in the 0.08\char21{}1.2 a.u. velocity region. Our setup uses the gas stripper of a tandem accelerator as a gas chamber. All cross sections present a maximum near 0.4 a.u., a velocity corresponding to the well-known shape-resonance that appears in the electron-N${}_{2}$ collisions. A semiclassical model is employed to analyze the data. The role of this shape resonance is discussed in this article.

Absolute photo-destruction and photo-fragmentation cross section measurements using an electrostatic ion beam trap

We describe a technique to measure absolute photo-induced cross sections for cluster anions stored in an electrostatic ion beam trap (EIBT) with a central deflector. The setup allows determination of total photo-destruction cross sections as well as partial cross sections for fragmentation and electron detachment. The unique properties of this special EIBT setup are investigated and illustrated using small Al(n)(-) clusters.

Absolute electron detachment and fragmentation cross sections for laser excited Al−n (2 ≤ n ≤ 10)

The absolute cross sections for photo-absorption followed by electron detachment or fragmentation of Aln− (2 ≤ n ≤ 10) were measured using an electrostatic ion beam trap. The competition between prompt fragmentation and electron detachment was determined and quantified by the branching fractions.

Negative ions and their behavior in collisions

AbstractWe present a review of electron detachment cross sections of atomic, molecular, and cluster anionic projectiles colliding with atoms and molecules, the main focus being on reporting systematic experimental studies. These studies use an experimental method developed at our laboratory, which uses the gas cell of standard tandem accelerators as a collision target. The experimental data show that the velocity dependence of electron detachment cross sections can be described modeling negative ions as a quasi‐free electron bound to a neutral core. Using this model, a velocity‐independent contribution to cross sections is associated with the neutral projectile core, while a velocity‐dependent curve is associated to the projectile quasi‐free electron. This latter curve is the convolution of experimental free‐electron cross sections, for the same target, with the momentum distribution of the quasi‐free projectile electron. This model is useful to obtain cross section estimates, as exemplified by its application in several collision systems relevant for inertial fusion and plasma diagnostics. It is also a powerful tool for verifying the accuracy of anion structure calculations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Single-Electron Detachment for Ti−, Fe−, Co−, Ni− and Cu− in Collision with Ar

By using the micro-channel plate position sensitive detector with the delay-line anode we measure the single electron detachment cross sections for some transition elements in collision with Ar in the energy region 10–30 keV. These single electron detachment cross sections perform as velocity and electron affinity dependences. The experiments are carried out using the growth rate method.