Low-energy Electron Impact Research Articles

CHARGING OF DUST GRAINS IN ASTROPHYSICAL ENVIRONMENTS BY SECONDARY ELECTRON EMISSIONS

The dominant charging processes in various astrophysical environments are considered to be by photoelectric emissions with radiation from nearby stars and secondary electron emissions (SEE) by impact of electrons in a medium with sufficiently high energies. The charging of bulk materials with planar surfaces by SEE with electron impact appears to be well understood with theoretical expressions as well as by experimental techniques. However, the charging of submicron/micron-size dust grains by SEE with sufficiently high-energy electrons is a complex process, and is a function of electron energies, the electron current, and the grain size, and the charge or the surface potential. Development of viable theoretical models and acquisition of experimental data for charging properties of micron-size dust grains are still in the early stages. This paper focuses on SEE charging properties of individual micron-size dust grains by low-energy electron impact, obtained from laboratory measurements on an experimental facility based on an electrodynamic balance. The measurements of SEE yields of positively charged dust grains indicate the yields increase with decreasing grain size and the equilibrium surface potentials showing generally linear size dependence. These experimental results are generally in agreement with several independent experimental and analytical model studies in the literature, with the exception of a recently published paper which is in fundamental conflict with our studies as well as with several other experimental and theoretical studies. The sources and causes of this conflict are critically examined and discussed in this paper.

Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact

We studied desorption of metastable atoms induced by electronic transitions in solid Ne by low energy electron impact. Time-of-flight spectra and the angular distribution of metastable atoms desorbed from the surface of an annealed sample showed increased kinetic energy, higher signal intensity, and a narrower angular distribution compared with those measured using an unannealed sample. These results were explained by considering the sample’s surface conditions in the framework of the cavity ejection mechanism. Our results for annealed solid Ne showed that when the sample temperature increased from 5 to 7 K, the width of the angular distribution increased by about 10%. A simple trajectory calculation qualitatively reproduced our experimental results.

Electronic excitation of gas-phase furan molecules by electron impact

Experiments and ab initio calculations of the differential and integral cross sections for the electronic excitation from the ground state ${}^{1}\phantom{\rule{-0.16em}{0ex}}{A}_{1}$ to the ${}^{3}\phantom{\rule{-0.16em}{0ex}}{B}_{2}$ and ${}^{3}\phantom{\rule{-0.16em}{0ex}}{A}_{1}$ states of gas-phase furan molecules by low-energy electron impact were performed. Experimental differential cross sections were measured at incident electron energies between 5 and 15 eV and for scattering angles from 10${}^{\ensuremath{\circ}}$ to 130${}^{\ensuremath{\circ}}$. The calculated cross sections were obtained using the Schwinger multichannel method implemented with pseudopotentials. The influence of channel-coupling and polarization effects is investigated through the comparison between three different models of scattering calculations, each one considering a distinct channel-coupling scheme. The comparison of experimental and calculated cross sections for electronically inelastic electron scattering by C${}_{4}$H${}_{4}$O molecules is found to be mostly reasonable. The existing discrepancies in this combined theoretical and experimental study help to illustrate difficulties in readily establishing reliable electronic excitation cross sections of polyatomic molecules by low-energy electrons.

Valence-Shell Excitations of Nitrous Oxide Studied by Fast Electron Impact

The valence-shell excitations of nitrous oxide are studied by fast electron energy loss spectroscopy. From the spectra measured at 2.5 keV and scattering angles of 3.5°–8.5°, it is found that the asymmetric peak of the transition B1Δ can be well fitted by Haarhoff-Van der Linde function, while the symmetric peaks of the transitions of C1Π and D1Σ+ can be well fitted by the Voigt function. The parameters of the peak profiles of B1Δ, C1Π and D1Σ+, i.e., their energy level positions and linewidths, are determined. With the aid of these parameters, the overlapping spectra measured at the low-energy electron impact can be deconvolved, which provides the possibility to determine the quantitative differential cross sections. The present results also show that the peak profiles of the transitions of B1Δ, C1Π and D1Σ+, are independent of the momentum transfer.

Mechanisms of the electron-impact-induced methionine molecule fragmentation

The fragmentation of a methionine (C5H11NO2S) molecule by a low-energy electron impact has been studied both experimentally and theoretically. A series of doubly-charged fragment ions have been observed for the first time. The main emphasis has been given to the mechanisms of the initial molecule fragment production. A special attention has been paid to the energy characteristics of the ionic fragment yield. The geometrical parameters of the initial molecule rearrangement have also been analyzed.

Mechanisms of the electron-impact-induced glycine molecule fragmentation

Fragmentation of the glycine (C2H5NO2) molecule by low-energy electron impact has been studied both experimentally and theoretically. The main emphasis has been given to the mechanisms of the initial molecule fragment production including formation of the doubly-charged CH2NHCO2+ fragment. A special attention has been paid to the energy characteristics of the ionic fragment yield. The geometrical parameters of the initial molecule rearrangement have also been analyzed.

The 4p6 autoionization cross section of Rb atoms excited by low-energy electron impact

The autoionization cross section of rubidium atoms was obtained by measuring the total normalized intensities of ejected-electron spectra arising from the decay of the 4p5n1l1n2l2 autoionizing levels. The electron impact energy range from the 4p6 excitation threshold at 15.31 up to 50 eV was investigated. The cross section reaches the maximum value of (2.9 ± 0.6) × 10−16 cm2 at 21.8 eV impact energy. The general behaviours of the cross section and the role of particular autoionizing configurations in its formation were considered on the basis of large-scale configuration interaction calculations of energies, cross sections, autoionization probabilities in 5snl(n ⩽ 7; l ⩽ 4) and 4d nl(n ⩽ 5; l ⩽ 2) configurations as well as the measured excitation functions for the lowest levels in 5s2 and 4d5s configurations. The resonance behaviour of the cross section between 15.3 and 18.5 eV impact energy is caused exclusively by the negative-ion resonances present close to the excitation thresholds of the (5s2)2P and (4d5s)4P autoionizing levels. At higher impact energies, the autoionization cross section is composed of contributions from the high-lying quartet and doublet levels in 4d5s, 5p and 5s5p, 5d, 6s, 6p configurations. From the comparison of the present data with available experimental and calculated ionization cross sections, the 5s + 4p6 direct ionization cross section of rubidium atoms was determined with the maximum value of (7.2 ± 2.2) × 10−16 cm2 at 36 eV. It was also found that the 4p6 excitation–autoionization is the dominant indirect ionization process contributing over 30% of the total single ionization of rubidium atoms by electron impact in the 15.3–50 eV energy range.

Dissociative recombination of BeH+in the interstellar medium

We report a theoretical study of dissociative recombination (DR) of BeH+ by low energy electron impact, using previously computed molecular data as input for a Multichannel Quantum Defect Theory (MQDT) approach. Three electronic symmetries of BeH, 2 Π, 2 Σ+ , and 2 Δ have been included in the calculations. We present cross sections and thermal rate coefficients ready to be used in the modelling of the BeH+ abundance in the interstellar medium and stellar atmosphere.

Decomposition of methionine by low energy electrons

In this work, we present the results from low energy (<12 eV) electron impact on isolated methionine, Met. We show that dissociative electron attachment is the operative mechanism for the sulfur content amino-acid fragmentation. The two most dominant fragments are attributed to the (Met-H)(-) and (C(4)NOH(5))(-) ions that are formed at energy below 2 eV. The formation of the latter anion is accompanied by the loss of neutral counterparts, which are most likely a water molecule and highly toxic methanethiol, CH(3)SH. Further fragments are associated with the damage at the sulfur end of the amino acid, producing the methyl sulfide anion CH(3)S(-) or sulfur containing neutrals. In the context of radiation induced damage to biological material at the nano-scale level, the present interest of methionine arises from the implication of the molecule in biological processes (e.g., S-adenosyl methionine for the stimulation of DNA methyltransferase reactions or protein synthesis).

Electron-impact and thermal fragmentation of amino acid molecules: Mechanisms and structure of the molecules

Fragmentation of the glycine (C2H5NO2) and methionine (C5H11NO2S) molecules by low-energy electron impact has been studied both experimentally and theoretically. The main emphasis has been given to the mechanisms of production of positively ionized fragments of both molecules with the same chemical composition. A special attention has been paid to the energy characteristics of the ionic fragment yield. The geometrical parameters of the initial molecule rearrangement have also been analyzed.

Electron impact vibrational excitation of carbon monoxide in the upper atmospheres of Mars and Venus

[1] Infrared emission from CO in the upper atmospheres of Mars, Venus and several other planets is a subject of current theoretical and experimental interest. Electron impact excitation makes a contribution that has not been included in previous studies. Given this, and recent new measurements of absolute cross sections for low-energy electron impact excitation of the vibrational levels of the ground state of CO, results from calculations are presented showing the contribution of electron impact relative to emissions by other mechanisms. It is demonstrated that emissions due to the impact of thermal, photo- and auroral electrons are generally small compared to sunlight-driven (fluorescence and photolysis) emissions, but with some exceptions. It is also shown that thermal-electron emissions may dominate over other processes at nighttime at Mars and that auroral emissions certainly do so. While measurements and other calculations do not appear to be available for Venus, the volume emission rates presented should be valuable in planning such measurements.

Negative ion formation through dissociative electron attachment to GeH4: Comparative studies with CH4 and SiH4

Negative ion formation by low-energy electron impact to germane (GeH4) has been performed in an electron energy region from 6 to 11eV with an energy resolution of ∼500meV. Anion efficiency curves of four anions have been measured. Product anions are observed mainly in the 6–11eV energy region, yielding GeHx− (x=0–3). Comparative studies with methane (CH4) and silane (SiH4) are also presented with the most intense signals observed at 14amu (CH2−), 31amu (SiH3−) and 75amu (GeH3−) from CH4, SiH4 and GeH4, respectively. Fragmentation into these negative ions is attributed to resonant dissociative electron attachment processes.

Low energy (0–4 eV) electron impact to N2O clusters: Dissociative electron attachment, ion-molecule reactions, and vibrational Feshbach resonances

Electron attachment to clusters of N(2)O in the energy range of 0-4 eV yields the ionic complexes [(N(2)O)(n)O](-), [(N(2)O)(n)NO](-), and (N(2)O)(n) (-) . The shape of the ion yields of the three homologous series differs substantially reflecting the different formation mechanisms. While the generation of [(N(2)O)(n)O](-) can be assigned to dissociative electron attachment (DEA) of an individual N(2)O molecule in the target cluster, the formation of [(N(2)O)(n)NO](-) is interpreted via a sequence of ion molecule reactions involving the formation of O(-) via DEA in the first step. The nondecomposed complexes (N(2)O)(n) (-) are preferentially formed at very low energies (below 0.5 eV) as a result of intramolecular stabilization of a diffuse molecular anion at low energy. The ion yields of [(N(2)O)(n)O](-) and (N(2)O)(n) (-) versus electron energy show sharp peaks at the threshold region, which can be assigned to vibrational Feshbach resonances mediated by the diffuse anion state as already observed in an ultrahigh resolution electron attachment study of N(2)O clusters [E. Leber, S. Barsotti, J. Bömmels, J. M. Weber, I. I. Fabrikant, M.-W. Ruf, and H. Hotop, Chem. Phys. Lett. 325, 345 (2000)].

Absolute cross sections for electronic excitations of cytosine by low energy electron impact.

The absolute cross sections (CSs) for electronic excitations of cytosine by electron impact between 5 and 18 eV were measured by electron-energy-loss (EEL) spectroscopy of the molecule deposited at low coverage on an inert Ar substrate. The lowest EEL features found at 3.55 and 4.02 eV are ascribed to transitions from the ground state to the two lowest triplet 1 (3)A(')(π→π(∗)) and 2 (3)A(')(π→π(∗)) valence states of the molecule. Their energy dependent CSs exhibit essentially a common maximum at about 6 eV with a value of 1.84×10(-17) cm(2) for the former and 4.94×10(-17) cm(2) for the latter. In contrast, the CS for the next EEL feature at 4.65 eV, which is ascribed to the optically allowed transition to the 2 (1)A(')(π→π(∗)) valence state, shows only a steep rise to about 1.04×10(-16) cm(2) followed by a monotonous decrease with the incident electron energy. The higher EEL features at 5.39, 6.18, 6.83, and 7.55 eV are assigned to the excitations of the 3 (3,1)A(')(π→π(∗)), 4 (1)A(')(π→π(∗)), 5 (1)A(')(π→π(∗)), and 6 (1)A(')(π→π(∗)) valence states, respectively. The CSs for the 3 (3,1)A(') and 4 (1)A(') states exhibit a common enhancement at about 10 eV superimposed on a more or less a steep rise, reaching, respectively, a maximum of 1.27 and 1.79×10(-16) cm(2), followed by a monotonous decrease. This latter enhancement and the maximum seen at about 6 eV in the lowest triplet states correspond to the core-excited electron resonances that have been found by dissociative electron attachment experiments with cytosine in the gas phase. The weak EEL feature found at 5.01 eV with a maximum CS of 3.8×10(-18) cm(2) near its excitation threshold is attributed to transitions from the ground state to the 1 (3,1)A(")(n→π(∗)) states. The monotonous rise of the EEL signal above 8 eV is attributed to the ionization of the molecule. It is partitioned into four excitation energy regions at about 8.55, 9.21, 9.83, and 11.53 eV, which correspond closely to the ionization energies of the four highest occupied molecular orbitals of cytosine. The sum of the ionization CS for these four excitation regions reaches a maximum of 8.1×10(-16) cm(2) at the incident energy of 13 eV.

Measurements of L-shell x-ray production cross-sections of Gd and W by low energy electron impact

Lα, Lβ x-ray production cross-sections for Gd (Z = 64) and W (Z = 74) by electron impact up to 40 keV have been measured. Thin films with thick carbon substrates were used as targets in the experiments. The effects of multiple scattering of electrons when penetrating the target film, of electrons reflected from the thick pure carbon substrate and of bremsstrahlung photons produced by the impact of incident electrons on the target were corrected by means of Monte Carlo simulation. For Gd L-shell x-ray characteristic peaks, the spectra were fitted by using spectrum-fitting program ALLFIT to extract more accurately the Lα and Lβ peak counts. The experimental data are compared with the DWBA theory and the PWBA–C–Ex theory and available experimental data from the literature.

Cross-section measurements for the fragmentation of CHClF2 by electron impact

CFC compounds present in the upper atmosphere have a significant effect on the environment, strongly contributing to the increase of the hole in the ozone layer. Recent studies show that low-energy electron impact is an important process in the dissociation of these molecules, creating atomic chlorine, which breaks down ozone molecules. In this work, the CHClF2 fragmentation by electron impact in the 40–400 eV energy range is measured. Total and partial cross sections have been obtained, showing the predominance of the release of neutral chlorine, which amounts to around 60% of the total yield. There is a strong indication that this chlorine is being released as a result of the ionization of electrons from both chlorine and fluorine orbitals.

Resonant vibrational excitation of CH3X (X = F, Cl, Br and I) by low-energy electron impact

Differential cross sections (DCSs) for excitation of the C–H stretch vibrations for CH3F, CH3Cl, CH3Br and CH3I were measured by electron impact. Resonance enhancement was observed for the C–H stretch at 6.6, 5.9, 5.5 and 4.7 eV respectively. For CH3F this is attributed to a 2A1 shape resonance formed through temporary occupation of the LUMO and for the heavier halogens to a 2E shape resonance formed through temporary occupation of the LUMO+1. DCSs for excitation of the C–X stretch were also measured for CH3F and CH3Cl. The shift in the resonance energy is discussed in context to the substitution effect of the halogens.

The vibrational excitation of hot molecules by low energy electron impact

We report vibrational excitation functions and angular distributions for electron scattering from the ground vibrational quantum (000), the bending vibrational quantum (010) and the unresolved first bending overtone (020) and symmetric stretch (100) modes of the ground-electronic state in hot (750 K) carbon dioxide (CO2) molecules. The excitation function measurements were carried out at incident electron energies in the range of 1–9 eV, and at the electron scattering angles of 30°, 60°, 90° and 120°.

Low-energy electrons and X-ray irradiation effects on plasma-polymerized allylamine bioactive coatings for stents

Radiation used in biomedical applications causes chemical changes to biomedical materials. This work is an ex situ simulation of the influence of low-energy electron (LEE) impact and X-ray irradiation on the chemical properties of plasma-polymerized allylamine (PPA) bioactive and biocompatible stent coatings. Preliminary X-ray photoelectron spectroscopy (XPS) results show that PPA coatings oxidize in contact with ambient air by the detection of C–O and C O bonds which are typical of polymer oxidation. Chemical changes after LEE and X-ray irradiation are mainly a loss of oxygen, assuming a surface deoxidizing and not a complete destruction of the surface. XPS survey analyses show that the amine groups remain stable during irradiation. LEE impact measurements by TOF mass spectrometry show that the main ionic losses are H − ions. It appears that CN groups are stable under irradiation and we observe a loss of hydrogen and oxygen as the main chemical modifications. In conclusion, these results suggest that PPA coatings are stable under biomedical radiation, and they can therefore be used for bioactive and biocompatible stent coatings.

Mass-spectrometric study of thermal and electron-impact fragmentation of glycine molecule

The process of fragmentation of the glycine of molecule (C2H5NO2), a compound that belongs to the series of amino acids, has been studied using crossed electron and molecular beams and mass-spectrometric analysis. Primary attention is focused on the formation of charged fragments of the initial molecule as a result of both the direct impact of low-energy electrons (dissociative ionization) and the thermal degradation of glycine on heating followed by the electron-impact ionization. The mass-spectrometric analysis of molecular fragments was performed using a commercial magnetic mass spectrometer of the MI-1201 type, the construction of which was subjected to significant improvement.