Electron Impact Experiments Research Articles

Updated ultraviolet fluorescence efficiencies of CS: Evidence for model discrepancies in the enhancement of NUV-derived CS abundances in comets

Observations of carbon monosulfide (CS) have a long history serving as a remote proxy for atomic sulfur, and more broadly, one of the sulfur reservoirs in cometary bodies. Recently, systematic discrepancies between NUV- and radio-derived CS abundances have been found to exceed a factor of 2–5, with NUV-derived abundances appearing enhanced for a wide array of comets. Interpretation of cometary CS emission in the ultraviolet has relied on a murky and ill-documented lineage of calculations whose accuracy can be difficult to assess. We report new fluorescence efficiencies of the CS radical, utilizing a rovibrational structure with vibrational states up to v=8 and rotational states up to N=100. The models utilize a new set of band transition rates derived from laboratory electron impact experiments. Benchmark comparisons to IUE observations of C/1979 Y1 (Bradfield) show favorable agreement with the fluorescence models. The present results affirm the accuracy of the historical record of CS abundances derived via ultraviolet CS emission in comets with IUE and HST, but do not explain the consistent enhancement of NUV-derived CS abundances relative to the radio measurements during the same apparitions. Alternative explanations of the factor of 2–5 discrepancy between NUV- and radio-derived CS abundances are discussed, as well as possible connections to sulfur reservoirs in protoplanetary disks. The model code and computed fluorescence efficiencies are made publicly available on the Zenodo service.

Absolute electron impact ionization cross-sections for CF4: Three dimensional recoil-ion imaging combined with the relative flow technique.

Here we present measurements of dissociative and non-dissociative cross-sections for the electron impact of the CF4 molecule. The present experiments are based on a Recoil Ion Momentum Spectrometer (RIMS), a standard gas mixing setup for CF4, and a reference gas. The measurements were carried out at several electron energies up to 1keV, covering the energy range of previous experiments. We apply the relative flow technique (RFT) to convert the relative cross-sections measured by the RIMS into absolute values. Using the combination of RIMS and RFT, ion collection and calibration errors were minimized. The results were compared with theoretical and experimental studies available in the literature. Previous electron impact experiments present relative cross-sections or use correction terms for the absolute cross-sections due to losses of energetic ions. We elucidate the differences between the new measurement method and the existing ones in the literature and explain why the present method can be considered reliable. Furthermore, we show how reducing correction terms affects the results.

Effect of tempering process on the mechanical properties and corrosion resistance of E690 marine steel

Abstract This study investigated the effect of the tempering process on the microstructure, mechanical properties, and corrosion resistance of E690 marine steel, aiming to replicate actual production conditions and reduce costs. Various techniques, including metallurgical microscopy, scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, impact experiments, tensile tests, and electrochemical corrosion tests, were employed to evaluate the material’s properties and performance. The results indicated that as the tempering temperature increased, the tempering degree in the tempered martensite structure improved, martensite strips coarsened, the size of precipitated carbides increased, and the proportion of large-angle grain boundaries decreased. Consequently, the tensile and yield strengths initially increased and then decreased, while the impact toughness and elongation gradually improved. At a tempering temperature of 600 °C, the steel exhibited the best overall mechanical properties, with a tensile strength of 729 MPa, a yield strength of 649 MPa, and an elongation of 18%. Furthermore, at a tempering temperature of 550 °C, the test steel showed optimal corrosion resistance, with a corrosion rate of 0.03233 mm/y and an open-circuit potential of -0.36 V.

Investigation of the valence-shell excitations of molecular carbon disulfide by high-energy electron scattering

Absolute generalized oscillator strengths of the valence-shell excitations of carbon disulfide have been determined by a joint investigation of a fast electron impact experiment and equation of motion coupled cluster with singles and doubles (EOM-CCSD) calculations. Based on the present experimental and theoretical results, the transition nature of the excitations are elucidated, and some controversies about the transition assignments are resolved. By extrapolating the generalized oscillator strengths to zero momentum transfer, the corresponding optical oscillator strengths were determined, which provide a cross-check to the previous experimental and theoretical results. Moreover, the integral cross sections of the valence-shell excitations from their excitation thresholds to 5000 eV have been obtained by means of the BE-scaling method. The oscillator strengths and integral cross-sections complement the fundamental data of carbon disulfide and supply important applications in astrophysics and atmospheric physics.

Fragmentation of SO2 q+ (q = 2-4) induced by 1keV electron collision.

We report an investigation on the fragmentation dynamics of SO2 q+ (q = 2-4) induced by 1keV electron collision utilizing an ion momentum imaging spectrometer. Six complete Coulomb explosion channels were observed using the time-of-flight correlation map. The kinetic energy release distributions for these channels were obtained and compared with those available in the literature. The fragmentation mechanisms of the three-body dissociation channels were analyzed by the Dalitz plots and Newton diagrams. Both concerted breakup and sequential fragmentation pathways were identified in the channel SO2 3+ → O+ + O+ + S+, whereas only the concerted breakup mechanism was confirmed for the channels SO2 4+ → O+ + O+ + S2+ and SO2 4+ → O2+ + O+ + S+. Using the Coulomb explosion model, we determined the molecular geometry from the concerted fragmentation channels, and the obtained bond lengths and angles from the higher kinetic energy release peaks are close to that of the neutral SO2 obtained by high-level quantum chemical calculation. The present results indicate that the electron impact experiment is a potential tool for the Coulomb explosion imaging of small molecules.

The N4,5 - OO Auger and “N3” N4,5O2,3 Coster-Kronig spectra of xenon induced by electron impact

The N4,5OO Auger electron spectrum in the kinetic energy region (14–37) eV has been measured with high resolution at electron incident energies between 71 eV and 2019 eV. The Auger and satellite lines are assigned by comparison with previous literature data. Several states which correspond to the resonant Auger transitions from the 4d3/2, 5/2 6p states are observed. The PCI effects on the N5-O2,3O2,3(1S0) Auger peak when the incident electron energy approaches to the N5 edge (67.55 eV) is investigated and it is found that the dependence of the energy shift on the excess energy, Eexc, is well represented by the function Eexcβ with β = (-1.40 ± 0.05). The “N3” N4,5O2,3 Coster-Kronig spectrum is measured at various electron incident energies. The assignments of the features are made in comparison with a similar spectrum from synchrotron radiation measured by Kivimäki et al. [J. Electron Spectrosc. Relat. Phenom. 1999, 101–103, 43–47]. The Auger spectrum in the kinetic energy regions (68–78) eV and (90–138) eV is observed for the first time in an electron impact experiment and a comparison is made with the photoionization experiment by Hikosaka et al. [Phys. Rev. A 2007, 76, 032708].

Production of Long-Lived Benzene Dications from Electron Impact in the 20–2000 eV Energy Range Combined with the Search for Global Minimum Structures

In this work, we report a systematic search of metastable C6Hn2+ (n = 1-6) dications from electron impact time-of-flight measurements of several benzene derivatives in combination with global minimum search based on the genetic algorithm. Our theoretical calculations reveal that the C6Hn2+ (n < 6) global minimum structures are completely different from that of the benzene dication, featuring linear carbon chains and/or cyclopropenylium moieties. Experimentally, the doubly charged species were investigated for a wide range of electron impact energies, from 20 to 2000 eV, for benzene and several monosubstituted compounds containing either electron-withdrawing or -donating groups. Furthermore, the C6Hn2+ production, evaluated from the yields of the dications with respect to that of the parent ion (or parent dication), was compared to those obtained from charge exchange in the doubly charged 2E spectra and electron impact experiments available in the literature. The yields of the long-lived benzene dications were contrasted to those analogues formed in chlorobenzene. Moreover, the formation of C6Hn2+ species is strongly dependent on the nature of substituent groups, with electron-withdrawing ones favoring the dication formation.

Ionization and electron capture for H+ collisions at low keV energy with CCl4 molecules

Relative total ionization cross sections and fragment target ions have been measured for proton impact on CCl4 molecules for the first time, namely associated with the formation of CCl3+, CCl2+, CCl+, Cl2+, Cl+ and C+ ions. The experiment was conducted by crossing a proton beam of 2.5 to 10 keV with a CCl4 vapor target where the fragment ions were measured employing the time of flight technique. Relative fragment cross sections were added leading to the total relative cross section. All relative cross sections seem to be independent of the collisional energy. A qualitative comparison was conducted with previous measurements of electron impact experiments. Ratios of CCl3+/CCl2+, CCl3+/CCl+, CCl3+/C+ and CCl3+/Cl+ were compared to those obtained from electron impact experiments.

Breaking up is hard to do

Dissociating hydrogen gas seems like it should be as easy as pulling apart two identical atoms. But resonant electron-impact experiments reveal that quantum interference induces a fundamental asymmetry in the process.

An experimental and theoretical investigation into the electronically excited states of para-benzoquinone

We report on a combination of experimental and theoretical investigations into the structure of electronically excited para-benzoquinone (pBQ). Here synchrotron photoabsorption measurements are reported over the 4.0–10.8 eV range. The higher resolution obtained reveals previously unresolved pBQ spectral features. Time-dependent density functional theory calculations are used to interpret the spectrum and resolve discrepancies relating to the interpretation of the Rydberg progressions. Electron-impact energy loss experiments are also reported. These are combined with elastic electron scattering cross section calculations performed within the framework of the independent atom model–screening corrected additivity rule plus interference (IAM-SCAR + I) method to derive differential cross sections for electronic excitation of key spectral bands. A generalized oscillator strength analysis is also performed, with the obtained results demonstrating that a cohesive and reliable quantum chemical structure and cross section framework has been established. Within this context, we also discuss some issues associated with the development of a minimal orbital basis for the single configuration interaction strategy to be used for our high-level low-energy electron scattering calculations that will be carried out as a subsequent step in this joint experimental and theoretical investigation.

Fragmentation and Ion Desorption from Condensed Pyrimidine by Electron Impact: Implications for Cometary and Interstellar Heterocyclic Chemistry

The desorption process induced by photons, electrons, or ions onto icy surfaces is an important mechanism for leading neutral or ionized molecular species to the gas phase in interstellar and circumstellar environments. In this work, we report the results of high energy electron impact onto a condensed pyrimidine ice by means of electron-stimulated ion desorption (ESID). Desorbed cations from the icy surface were analyzed by time-of-flight mass spectrometry (TOF-MS). The most abundant desorbed ions are H+ and H2+, although several other fragments, such as C2H2+, HCN+, C3H3+, CHN2+, C2H2N2H+, and C4H4N2H+, have also been identified. The fragmentation pattern of pyrimidine was found to be characterized by six distinct regions on its desorption spectra, containing ionic fragments bearing one to six members of the original pyrimidine ring. Five-membered fragment ions were observed for the first time in electron impact experiments. Absolute values of desorption yield for several ions were determined. Desorbed ions observed in this experiment are in agreement with observations of the comet Halley coma composition and may act as markers of the presence of pyrimidine in the interstellar medium.

(e,2e) experiments on C60

Electron impact experiments combined with the coincidence detection of the charged particles in the final state represent one of the richest sources of information on the dynamics of molecular processes and the structure of the targets, where the targets can extend from rare gases to molecules of increasing complexity. In this work the electron impact ionization of isolated C60 molecules in the gas phase has been studied via (e,2e).

Theoretical modeling of ionization energies of argon clusters: Nuclear delocalization effects

Temperature dependence of vertical ionization energies is modeled for small argon clusters (N ≤ 13) using classical parallel-tempering Monte Carlo methods and extended interaction models based on the diatomics-in-molecules approach. Quantum effects at the zero temperature are also discussed in terms of zero-point nuclear vibrations, either at the harmonic approximation level or at the fully anharmonic level using the diffusion Monte Carlo calculations. Both approaches lead to a considerable improvement of the theoretical predictions of argon clusters ionization energies and represent a realistic way of modeling of ionization energies for weakly bound and floppy complexes in general. A thorough comparison with a recent electron-impact experiment [O. Echt et al., J. Chem. Phys. 123, 084313 (2005)] is presented and a novel interpretation of the experimental data is proposed.

Dynamics of single or double ionization of small systems from coincidence electron impact experiments

In this brief review, we illustrate the potentialities and the power of electron-electron coincidence studies to investigate the dynamics of single ionization SI [(e,2e) case] or double ionization DI [(e,3e) case] processes. An example for (e,2e) SI of rare gas atoms is presented with a new insight into the behaviour of the recoil versus the binary intensity. A second example for (e,2e) SI of molecules is used to illustrate the observation of a purely molecular effect, namely the signature of interference effects due to the two-center nature of the H2 molecule. The third example discusses an unprecedented triple coincidence study involving the scattered – ejected – Auger electrons emitted from an argon target. It is shown that the method allows to disentangle the contribution of various DI mechanisms.

Multitechnique investigation of the valence and inner shell excitation, ionization and decay of halogenated pyrimidines

An extensive, multitechnique, investigation of the pyrimidine and halogenated pyrimidine molecules has been carried out by electron and photon impact experiments.

Complementary spectroscopy of tin ions using ion and electron beams

Extreme ultra-violet (EUV) emission spectra of multiply charged tin ions were measured in the wavelength range of 10-22 nm following charge exchange collisions of Snq+(q = 15-21) ions with rare gas targets at 20 keV/q or the electron impact excitation of tin ions with electron energy of 312-613 eV. In charge exchange collisions, we observed both the resonance lines and the emission lines corresponding to the transitions between the excited states. On the other hand, we observed mainly the resonance lines in the electron impact experiments. We can distinguish the resonance lines from other emission lines in the charge exchange spectrum by comparison with the emission lines in the electron impact spectrum. The comparison of the complementary experimental results can be regarded as a new method of spectroscopy of multiply charged ions.

Complex solvation of Mg atoms in 4He nanodroplets

We have determined the structure of 4He droplets doped with one or two magnesium atoms. Our results are used to interpret recent resonant two-photon-ionization (R2PI) and laser induced fluorescence (LIF) experiments. We argue that the Mg radial delocalization inside large 4He droplets may reconcile the apparently contradictory solvation properties of magnesium as provided by LIF and R2PI experiments on the one hand, and electron-impact ionization experiments on the other hand. We confirm that several Mg atoms in 4He drops form a soft "foam"-like metastable aggregate rather than coalesce into a compact metallic cluster, in agreement with R2PI experiments.

Density functional theory of the structure of magnesium-doped helium nanodroplets

We have studied the structure of $^{4}\text{H}\text{e}$ droplets doped with magnesium atoms using density functional theory. We have found that the solvation properties of this system strongly depend on the size of the $^{4}\text{H}\text{e}$ droplet. For small drops, Mg resides in a deep surface state, whereas for large-size drops it is fully solvated but radially delocalized in their interior. We have studied the $3s3p{^{1}P}_{1}\ensuremath{\leftarrow}3{s}^{2}{^{1}S}_{0}$ transition of the dopant, and have compared our results with experimental data from laser-induced fluorescence (LIF). Line-broadening effects due to the coupling of dynamical deformations of the surrounding helium with the dipole excitation of the impurity are explicitly taken into account. We show that the Mg radial delocalization inside large droplets may help reconcile the apparently contradictory solvation properties of magnesium as provided by LIF and electron-impact ionization experiments. The structure of $^{4}\text{H}\text{e}$ drops doped with two magnesium atoms is also studied and used to interpret the results of resonant two-photon-ionization (R2PI) and LIF experiments. We have found that the two solvated Mg atoms do not easily merge into a dimer, but rather form a weakly bound state due to the presence of an energy barrier caused by the helium environment that keeps them some $9.5\text{ }\text{\AA{}}$ apart, preventing the formation of the ${\text{Mg}}_{2}$ cluster. From this observation, we suggest that Mg atoms in $^{4}\text{H}\text{e}$ drops may form, under suitable conditions, a soft ``foamlike'' aggregate rather than coalesce into a compact metallic cluster. Our findings are in qualitative agreement with recent R2PI experimental evidence. We predict that, contrarily, Mg atoms adsorbed in $^{3}\text{H}\text{e}$ droplets do not form such metastable aggregates.

The Auger spectroscopy of pyrimidine and halogen-substituted pyrimidines

The C 1s and N 1s Auger spectra of pyrimidine, 2-chloropyrimidine, and 5-bromopyrimidine have been measured in an electron impact experiment at 1000 eV. In the case of the halogen-substituted pyrimidines, also the Cl 2p and Br 3d Auger spectra have been recorded. We have thoroughly analyzed and interpreted all the Auger spectra recorded here with the aid of accurate Green's function calculations with a large basis set. The spectra are extremely complex with thousands of states contributing and almost no single-state feature even near the double ionization threshold. Besides reproducing and explaining with great detail nearly all the main spectral features observed, the calculations have successfully unraveled the interplay among the different C 1s core hole chemical shifts in each molecule and how this affects some fingerprinting details in the composite C 1s Auger spectra.

Fragmentation of CH 42+ following C 1s ionisation studied by Auger electron-ion–ion coincidence experiments

The fragmentation of the CH 4 2+ dication has been studied in an electron impact experiment by Auger electron-ion and Auger electron-ion–ion coincidence measurements. Different states of the dications have been selected by changing the kinetic energy of the Auger electrons. Particular attention has been paid to the production of the CH 2 + and CH 3 + ions whose Kinetic Energy Release (KER) distributions and dissociation channels have been determined. Moreover, for the first time in an electron impact experiment, the C 2+ ion fragment has been identified.