Detachment Cross Sections Research Articles

Anion Photoelectron Imaging Spectroscopy of C6F5X- (X = F, Cl, Br, I).

The photoelectron (PE) spectra of C6F5X- (X = Cl, Br, I) and computational results on the anions and neutrals are presented and compared to previously reported results on C6F6- [McGee, C. J. J. Phys. Chem. A 2023, 127, 8556-8565.]. The spectra all exhibit broad, vibrationally unresolved detachment transitions, indicating that the equilibrium structures of the anions are significantly different from the neutrals. The PE spectrum of C6F5Cl- exhibits a parallel photoelectron angular distribution (PAD), similar to that of the previously reported C6F6- spectrum, while the PE spectra of C6F5Br- and C6F5I- have isotropic PADs, and also exhibit a prominent X- PE feature due to photodissociation of C6F5X- resulting in X- formation. Identification of the C6F5X- detachment transition origins, which is equivalent to the neutral electron affinity (EA), in all three cases is difficult, since the broadness of the detachment feature is accompanied by vanishingly small detachment cross section near the origin. Upper limits on the EAs were determined to be 1.70 eV for C6F5Cl, 2.10 eV for C6F5Br, and 2.00 eV for C6F5I, all significantly higher than the 0.76 eV upper limit determined for C6F6 with the same experiment. The broad detachment transitions are consistent with computational results, which predict very large differences between the neutral and anionic C-X (X = Cl, Br, I) bond lengths. Based on differences between the MBIS atom charges in the anions and neutrals, the excess charge in the anion is on the unique C atom and X, in contrast to the nonplanar C2v structured C6F6- anion, for which the charge is delocalized over the molecule. In C6F5Cl-, the C-Cl bond is predicted to be bent out of the plane, while both C6F5Br- and C6F5I- are predicted to be planar on average. The impact of the interruption of the symmetry in the hexafluorobenzene neutral and anion on the molecular and electronic structure of C6F5X/C6F5X- is considered, as well as the possible dissociative state leading to X- (X = Br, I) formation, and the nature of the C-X bond.

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−.

Threshold photodetachment spectroscopy of the astrochemical anion CN.

Threshold photodetachment spectroscopy has been performed on the molecular anion CN- at both 16(1) K and 295(2) K in a 22-pole ion trap and at 295(2) K from a pulsed ion beam. The spectra show a typical energy dependence of the detachment cross section yielding a determination of the electron affinity of CN to greater precision than has previously been known at 31 163(16) cm-1 [3.864(2) eV]. Allowed s-wave detachment is observed for CN-, but the dependence of the photodetachment cross section near the threshold is perturbed by the long-range interaction between the permanent dipole moment of CN and the outgoing electron. Furthermore, we observe a temperature dependence of the cross section near the threshold, which we attribute to a reduction of the effective permanent dipole due to higher rotational excitation at higher temperatures.

Multiple Photodetachment of Carbon Anions via Single and Double Core-Hole Creation.

We report on new measurements of m-fold photodetachment (m=2-5) of carbon anions via K-shell excitation and ionization. The experiments were carried out employing the photon-ion merged-beams technique at a synchrotron light source. While previous measurements were restricted to double detachment (m=2) and to just the lowest-energy K-shell resonance at about 282eV, our absolute experimental m-fold detachment cross sections at photon energies of up to 1000eV exhibit a wealth of new thresholds and resonances. We tentatively identify these features with the aid of detailed atomic-structure calculations. In particular, we find unambiguous evidence for fivefold detachment via double K-hole production.

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.

Slow photoelectron velocity-map imaging of cold and .

High-resolution anion photoelectron spectra of cryogenically cooled C7 - and C9 - clusters obtained using slow photoelectron velocity-map imaging are presented, providing insight into the vibronic structure of neutral C7 and C9. These spectra yield accurate measurements of vibrational frequencies for the neutral clusters as well as electron affinities of 3.3517(4) and 3.6766(14) eV for C7 and C9, respectively. In the C7 - spectrum, transitions involving the previously unreported v 1 and v 2 symmetric stretching modes, as well as the v 9, v 10, and v 11 asymmetric bending modes, are assigned. Spin-orbit splitting is observed for several transitions in this spectrum, giving an energy difference of 28(6) cm-1 between the and spin-orbit levels of the C7 - anion. In the spectrum of C9 -, transitions involving the previously unreported symmetric stretch v 1 and the asymmetric bend v 11 are observed. In both spectra, several features are assigned to Franck-Condon forbidden transitions involving the doubly degenerate v 10 and v 11 modes of C7 and the v 13 and v 14 modes of C9. The appearance of these transitions is attributed to Herzberg-Teller coupling between the electronic states of the neutral clusters. Additional FC-forbidden transitions to states previously observed in gas-phase infrared experiments are observed and attributed to vibronic coupling between the electronic states of the anion, resulting in non-totally symmetric character in the anion's full vibrational ground state. Finally, consideration of the energy dependence of detachment cross sections and Dyson orbital analyses reveal that addition of more carbon atoms to the linear chain results in photodetachment from delocalized molecular orbitals with increasing nodal structure, leading to threshold photodetachment cross sections that differ considerably from simple symmetry considerations.

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.

Radiative Rotational Lifetimes and State-Resolved Relative Detachment Cross Sections from Photodetachment Thermometry of Molecular Anions in a Cryogenic Storage Ring.

Photodetachment thermometry on a beam of OH^{-} in a cryogenic storage ring cooled to below 10K is carried out using two-dimensional frequency- and time-dependent photodetachment spectroscopy over 20min of ion storage. In equilibrium with the low-level blackbody field, we find an effective radiative temperature near 15K with about 90% of all ions in the rotational ground state. We measure the J=1 natural lifetime (about 193s) and determine the OH^{-} rotational transition dipole moment with 1.5% uncertainty. We also measure rotationally dependent relative near-threshold photodetachment cross sections for photodetachment thermometry.

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.

Prominent role of multielectron processes inK-shell double and triple photodetachment of oxygen anions

The photon-ion merged-beams technique was used at a synchrotron light source for measuring absolute cross sections of double and triple photodetachment of O$^{-}$ ions. The experimental photon energy range of 524-543 eV comprised the threshold for K-shell ionization. Using resolving powers of up to 13,000, the position, strength and width of the below-threshold 1s 2s2 2p6 2S resonance as well as the positions of the 1s 2s2 2p5 3P and 1s 2s2 2p5 1P thresholds for K-shell ionization were determined with high precision. In addition, systematically enlarged multi-configuration Dirac-Fock calculations have been performed for the resonant detachment cross sections. Results from these ab-initio computations agree very well with the measurements for the widths and branching fractions for double and triple detachment, if double shake-up (and -down) of the valence electrons and the rearrangement of the electron density is taken into account. For the absolute cross sections, however, a previously found discrepancy between measurements and theory is confirmed.

One- and two-photon detachment ofO−

Cross sections for one- and two-photon detachment of ${\mathrm{O}}^{\ensuremath{-}}(1{s}^{2}2{s}^{2}2{p}^{5}\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{P}^{\mathrm{o}})$ have been determined in a joint experimental and theoretical study. The absolute measurement is based on the animated-crossed-beam technique, which is extended to the case of pulsed lasers, pulsed ion beams, and multiphoton detachment. The ab initio calculations employ $R$-matrix Floquet theory, with simple descriptions of the initial bound state and the residual oxygen atom which reproduce well the electron affinity and ground-state polarizability. For one-photon detachment, the measured and computed cross sections are in good mutual agreement, departing significantly from previous reference experiments and calculations. The generalized two-photon detachment cross section, measured at the Nd:YAG laser wavelength, is in good agreement with the $R$-matrix Floquet calculations. Long-standing discrepancies between theory and experiment are thus resolved.

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.

Photodetachment of a model molecular system by an elliptically polarized field

The differential cross section for one-photon molecular detachment by an elliptically polarized field is analyzed for a one-electron molecular model comprised of an electron in the field of two (generally nonequivalent) attractive zero-range potentials (ZRPs) separated by the distance . A phenomenological parametrization of the photodetachment cross section for a fixed-in-space molecular system in terms of two scalar dynamical parameters is presented and circular dichroism effects are discussed. Analytic results for the dynamical molecular parameters within the ZRP molecular model are used to analyze interference phenomena (including two-center interference) and dichroic effects in the detached electron angular distributions and their dependence on the interatomic distance and on the orientation of the molecular axis with respect to the polarization plane. Numerical ZRP results for angular distributions are presented for both symmetric and asymmetric molecules in an elliptically polarized field.