High energy electron impact ionization for elemental analysis by energy dispersive X-ray spectrometry

A method has been demonstrated in which the emission of characteristic X-rays following impact ionization by electrons of several MeV energy is used for quantitative elemental analysis. Because of the unique properties of MeV electrons with respect to their X-ray production yield and range in solids, the method has comparable sensitivity over a wide range of elements and relatively less stringent sample preparation requirements. By using energy dispersive techniques to detect the X-rays, a simultaneous determination of the weight fraction of the elements in an unknown sample is possible from a procedure based entirely on empirically determined relative X-ray yields. The method was tested by analyzing three standard metal alloy samples obtained from the National Bureau of Standards. Elements reaging from iron (Z=26) to lead (Z=82) with concentrations from 0.06% to 88% were quantitatively determined. In general, good approximate agreement was found wit the reported assays even for the minor constituents.     

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.     

Differential elastic electron scattering cross sections for CCl4 by 1.5-100 eV energy electron impact

We report absolute elastic differential, integral and momentum transfer cross sections for electron interactions with CCl(4). The incident electron energy range is 1.5-100 eV, and the scattered electron angular range for the differential measurements varies from 15°-130°. The absolute scale of the differential cross section was set using the relative flow technique with helium as the reference species. Comparison with previous total cross sections shows good agreement. Atomic-like behaviour in this scattering system is shown here for the first time, and is further investigated by comparing the CCl(4) elastic cross sections to recent results on the halomethanes and atomic chlorine at higher impact energies [H. Kato, T. Asahina, H. Masui, M. Hoshino, H. Tanaka, H. Cho, O. Ingólfsson, F. Blanco, G. Garcia, S. J. Buckman, and M. J. Brunger, J. Chem. Phys. 132, 074309 (2010)].     

Relativistic calculation of Stokes' parameters for intermediate energy electron impact excitation of cesium atoms

The integrated and differential Stokes' parameters of the light emitted in the decay 6p ² P _1/2,3/2→6s ² S _1/2 fore-Cs scattering in the intermediate energy range are presented. These have been calculated using scattering amplitudes obtained by a relativistic distorted-wave method.     

Cross sections and ion kinetic energy analysis for the electron impact ionization of acetylene

Using a Nier-type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene are measured for electron energies up to 1000 eV. Discrimination factors for ions are determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover a charge separation reaction, the Coulomb explosion of the doubly charged parent ions C2H2++ into the fragment ions C2H+ and H+, is investigated and its mean kinetic energy release (KER=3.88 eV) is deduced.     

Absolute partial cross sections and kinetic energy analysis for the electron impact ionization of ethylene

We present absolute partial electron impact ionization cross sections for ethylene in the electron energy range between threshold and 1000 eV measured with a two sector field double focusing mass spectrometer. Ion kinetic energy distribution functions have been measured at all electron energies by applying a deflection field method. Multiplication of the measured relative cross sections by the appropriately determined discrimination factors lead to accurate relative partial cross sections. Normalization of the sum of the relative partial cross sections to an absolute total cross section gives absolute partial cross section values. The initial kinetic energy distributions of several fragment ions show the presence of two or more contributions that exhibit different electron energy dependencies. Differential cross sections with respect to the initial kinetic energy of the ions are provided and are related to specific ion production channels. The electron threshold energies for the direct and numerous other dissociative ionization channels are determined by quantum chemical calculation and these allow the determination of the total kinetic energy release and the electron energy loss for the most prominent dissociative ionization channels.     

Spatially resolved energy electron loss spectroscopy studies of iron oxide nanoparticles.

The oxidation state of iron oxide nanoparticles co-generated with soot during a combustion process was studied using electron energy-loss spectroscopy (EELS). Spatially resolved EELS spectra in the scanning transmission electron microscopy mode were collected to detect changes in the oxidation state between the cores and surfaces of the particles. Quantification of the intensity ratio of the white lines of the iron L-ionization edge was used to measure the iron oxidation state. Quantitative results obtained from Pearson's method, which can be directly compared with the literature data, indicated that the L3 /L2-intensity ratio for these particles changes from 5.5 +/- 0.3 in the particles' cores to 4.4 +/- 0.3 at their surfaces. This change can be directly related to the reduction of the iron oxidation state at the surface of the particles. Experimental results indicate that the cores of the particles are composed of gamma-Fe2O3, which seems to be reduced to FeO at their surfaces. These results were also supported by the fine structure of the oxygen K-edge and by the significant chemical shift of the iron L-edge.     

Theoretical studies of intramolecular electron transfer reactions: Distance and free energy dependences

Electron tunneling through a square potential energy barrier is used to calculate the distance-dependent factors of electron transfer (ET) processes in metal-monolayer-metal junctions, donors and acceptors dispersed in rigid organic glasses, intramolecular ET in rigid donorbridge—acceptor species in solution and redox centers attached to electrodes through adsorbed monolayers. This tunneling model of distancedependent non-adiabatic factors is incorporated in the intersecting state model (ISM). The result is a simple semiclassical theory which is used to calculate the rates of non-adiabatic ET reactions. When the electron is originally located in a π* molecular orbital of the donor and the reaction free energy is no lower than approximately −50 kJ mol⁻¹, no adjustable parameters are necessary to calculate the intramolecular ET rates from a donor, through a rigid bridge, to an acceptor. Such calculated rates are within an order of magnitude of the experimental values. The model can also account for the ET rates of more exothermic reactions provided that the value of an empirical parameter, which is constant for structurally related reactants and solvents of similar polarity, is estimated. The physical meaning of this parameter is related to the dynamics of the reactions. The profiles of the distance and free energy dependences of photoinduced ET rates are closely reproduced. The occurrence of distance-dependent non-adiabatic factors in intermolecular σ*-d ETs is rationalized.     

Combined vibrational,conformational energy and electron diffraction studies of trans-2-decalone

The structure of trans-2-decalone was investigated by combined gas phase electron diffraction, conformational energy and vibrational analyses. In this study first the minimum energy conformations for trans-2-decalone were calculated by molecular mechanics techniques using a force field described in the literature; the same field and the minimum energy conformations were then used in subsequent vibrational analyses to calculate the mean amplitudes of vibration for each minimum energy conformation of trans-2-decalone; these mean amplitudes and the corresponding internuclear distances were then used to calculate theoretical electron diffraction radial distribution curves which were compared to the experimental curves. Four conformational energy minima were investigated with one or both rings in a chair or non-chair form. The results of the combined investigations indicate that the molecule exists in the conformation which has both rings in a distorted chair form.     

Low energy electron diffraction and cyclic voltammetry studies of flame-annealed platinum single crystals

Low energy electron diffraction (LEED) and cyclic voltammetry were used to examine the surface structure of flame-annealed platinum (100), (110), and (111) electrodes. Well-defined LEED patterns were obtained for these surfaces but only after heat treatment in vacuo at 400°C in order to remove surface contaminants. The cyclic voltammograms correspond to those expected for these low index platinum surfaces from prior studies.     

Semi-empirical models for auger electron energy calculations

Three approaches to the semi-empirical calculation of Auger electron energies developed by Shirley et al., Larkins and Hoogewijs et al. respectively are critically discussed using a common notation.     

Further studies of the continuous UV emission produced by electron impact on CF4

The intense continuous UV emission which extends from 200 nm to beyond 500 nm produced by electron impact on carbontetrafluoride, CF₄, has been investigated in a crossed electron-beam — gas-beam apparatus as well as in a gas-target apparatus under single collision conditions with special emphasis on the regime of impact energies below 50 eV. Systematic measurements of excitation functions (relative emission cross sections) at various wavelengths between 225 nm and 420 nm revealed the same onset of 23.5±2 eV and the same energy dependence for all excitation functions independent of the selected wavelength indicating that the continuous emission consists of a single fluorescence contribution. Evidence of a second onset around 40 eV, roughly 16 eV above the first onset which is an energy close to the ionization energy of atomic fluorine (17.4 eV), was found — more or less prominent — in all excitation functions. The presence of the second onset lends strong support to the assignment of the excited (CF ₃ ⁺ )* fragment ion as the emitting source of the emission continuum. We identify the lower onset at 23.5 eV with the electron-impact induced breakup of the parent CF₄ molecule into an excited (CF ₃ ⁺ )* ion and a ground state fluorine atom (neutral channel), whereas the second onset around 40 eV indicates the presence of an ionic fragmentation channel in which an excited (CF ₃ ⁺ )* fragment ion is produced together with a fluorine ion, F⁺. Wavelength scans taken at impact energies above and below the second onset revealed no significant difference in the envelope of the observed emission continuum which further supports the notion that the emission continuum consists of a single fluorescence contribution.     

Translational energy distribution of the excited hydrogen atom produced by controlled electron impact on HCl

The translational energy distribution of an atom can be calculated by differentiating the Doppler line shape of its emission line taken at a high optical resolution. The Balmer-β line of the excited hydrogen atom (n = 4) produced by electron impact on HCl has been measured at a high resolution (0.033Å) and at two angles (55° and 90°) with respect to the electron beam. The translation energy distribution depends on the electron energies and has almost two groups of components: ≈ 5 eV (fast) and ≈ eV (slow). Anisotropy is imporant for the slow component. The excitation function shows the corresponding structures. It is concluded that Rydberg states converging to the ²Π state of HCl⁺ produce the fast component and Rydberg states converging to the repulsive HCl⁺ states which cross the ²Σ⁺ state produce the slow component.     

Soft electron (low energy electron) processing of foods for microbial control

“Soft-electron” is a new term we have created referring to electrons with energies of 300 kV or lower. Homogenous irradiation of surfaces with soft electrons can decontaminate dry food ingredients such as gains, pulses, spices, dehydrated vegetables and tealeaves without detrimental effects. Treatment of soybeans with electrons of acceleration voltages at 170 kV reduced their microbial count to an undetectable level. Pre-treatment of soybeans with soft electrons enabled the extension of soymilk without sterilization process at a high temperature (120°C). The gelatinized property of soymilk from soft electron-treated beans was better than that of high-temperature sterilized soymilks. These results indicate that soft-electron sterilization improved the quality of soybeans for the processing of soymilk and Tofu(soymilk curd).     

Anisotropy parameters for molecules excited by electron impact

Rotationally resolved excitation of the a¹Δ_g and b¹∑ _g ⁺ states of O₂ is considered. Results for the relevant state multipoles are given. The anisotropy produced by electron impact is visualised by figures of the angular distribution of the molecular axes in the excited states.     

Polymeric arrays for directional energy and electron transfer

Conformationally rigid block copolymers prepared by alkyllithium-initiated polymerization of vinyl arenes, by molybdenum amide catalyzed ring-opening metathesis polymerization of chromophore-labeled norbornenes, and by nickel catalyzed polymerization of functionalized isonitriles are described. Steady state and time-resolved spectroscopic methods have been employed as tools for the structural characterization of these polymers and for studying the kinetics of long distance electron transfer.     

Dissociation of ethane by electron impact

The absolute total dissociation cross section for ethane is reported for electron energies between 10 and 600 eV. A maximum value of 7.6 × 10^?16 cm² occurs at 80 eV while the apparent threshold is ≈ 10 eV. Dissociative ionization is more probable than dissociation into neutral fragments at all energies except in the threshold region. The data indicates that fragmentation involving methane elimination (c^? + C₂ H₆ → e^? + CH₄ + CH₂) occurs in less than 2% of the dissociative events for 50 < E < 600 eV. Arguments are presented which suggest that some of the lower excited states of ethane are stable against dissociation.     

Threshold electron impact excitation of acetylene

Threshold electron energy loss spectra for acetylene are presented and assignments suggested for the excitation processes occuring.