Function Of Incident Electron Energy Research Articles

Electron scattering with ethane adsorbed on rare gas multilayers: Hole transfer, coulomb decay, and ion dissociation.

Positive ion desorption following electron impact dissociative ionization of ethane adsorbed on Ar, Kr, and Xe multilayers has been studied as a function of incident electron energy from threshold to 100eV. Based on the dependence of ion yields on the identity of the rare gas, it is likely that the majority of ethane molecules undergo indirect ionization following hole transfer from the ionized underlying rare gas. This has also been corroborated by density of states calculations showing the energetic alignment of the outer valence states of ethane and the condensed rare gas ionization energies. Due to the near-resonant nature of charge transfer for single-hole states, the ethane molecular ion is excited to different final ionic states on different rare gases, which leads to differences in ion desorption yields and branching ratios. The quantitative yields increase with increasing ionization energy gap between the rare gas and ethane, in the order Ar > Kr > Xe. The large increase in yields from 25eV onwards for all rare gases is likely due to the formation and decay of two-hole states on neighboring rare gas and ethane molecules due to interatomic and intermolecular Coulomb decay (ICD) and not electron transfer mediated decay (ETMD). The ICD and ETMD pathways become accessible when the incoming electron has sufficient energy to excite the inner valence ns level of the rare gas to a Rydberg state or ionize it. The experimental findings are supported by calculations of thresholds, density of states for the final configurations of these processes, and coupling strengths for hole transfer between ethane and rare gases. The fragment ion branching ratios vary with energy from threshold to about 35eV, showing the fragmentation pattern changes with the mode of hole transfer and availability of excess energy. Sigma C-C bonds are more likely to break than C-H bonds in the mid-20eV range, and this effect is most pronounced for Xe, followed by Kr, and then Ar.

Electron attachment to tetrazoles: The influence of molecular structure on ring opening reactivity.

The electron-induced reactivity of 5-(4-chlorophenyl)-1H-tetrazole and 5-chloro-1-phenyl-1H-tetrazole was studied using a trochoidal electron monochromator quadrupole mass spectrometer experimental setup. 5-(4-chlorophenyl)-1H-tetrazole underwent dissociative electron attachment to form Cl-, [M-HCl]-, and [M-H]-. 5-chloro-1-phenyl-1H-tetrazole underwent associative electron attachment to form the parent anion and dissociative electron attachment to form Cl-, CN2Cl-, [M-N2-Cl]-, and [M-HCl]-. For each anion product, the ion yield was measured as a function of incident electron energy. Density functional theory calculations were performed to support the experimental results with estimates of the energetic thresholds for the different reaction pathways. While the tetrazole group is susceptible to electron-induced ring opening in both molecules, this process was only observed for 5-chloro-1-phenyl-1H-tetrazole, indicating that this process is influenced by the structure of the molecule.

Fragmentation and slow autoneutralization of isolated negative molecular ions of phthalocyanine and tetraphenylporphyrin.

Macrocyclic tetrapyrrolic compounds, such as naturally occurring or artificial porphyrins and phthalocyanines, have unique and highly attractive properties for applications in medicine and technology. The interaction of free-base phthalocyanine (H2Pc) and tetraphenylporphyrin (H2TPP) molecules with low-energy (0-15 eV) electrons was studied in vacuo by means of negative ion resonant electron capture mass spectrometry. Close similarities in formation and decay of negative ions of these compounds were revealed. Efficient formation of long-lived molecular negative ions (MNIs) was observed in the incident electron energy range of 0-8 eV, unprecedentedly wide for organic compounds and comparable to the range characteristic to carbon atomic clusters, fullerenes. Experiments testify to the strong persistence of MNIs of both compounds to dissociative decay, isomerization, and electron autodetachment. Lifetimes of MNIs as a function of incident electron energy were measured and it was concluded that the isolated anions may retain additional electrons in a time scale of up to hundreds of seconds at standard temperature due to the high adiabatic electron affinity of these large molecules. For the representatives of dyes and photochromic compounds comprehensively studied in terms of interaction with light, the present work highlights yet another unique property of these molecules, namely the capability to attach and durably retain an additional electron of low, pre-ionization energy.

Influence of ion implantation and electron pre-irradiation on charging of dielectrics under electron beam irradiation: Application to SiO2

We present the experimental study on the effect of preliminary ion and electron irradiation on charging kinetics of SiO2 (quartz) under medium-energy electron beam. It is shown that the charging kinetics differs for SiO2 samples implanted by ions of various chemical natures and by electrons of different energy. It is found the enhancement of charge spreading over the quartz surface pre-irradiated by electrons as a result of desorption of oxygen atoms from the surface. The charging surface potential under electron beam increases more slowly, but to higher values, than in the case of non-irradiated crystals. The observed difference of the steady state values of charging surface potential as a function of incident electron energy is explained for the case of proton pre-irradiated targets.

Out-of-plane ( e,2e ) measurements and calculations on He autoionizing levels as a function of incident-electron energy

Out-of-scattering-plane $(e,2e)$ measurements and calculations are reported for the three singlet helium $2\ensuremath{\ell}2{\ensuremath{\ell}}^{\ensuremath{'}}$ autoionizing levels, with 80, 100, 120, 150, and 488 eV incident-electron energies, and scattering angles ${60}^{\ensuremath{\circ}}, 50.{8}^{\ensuremath{\circ}}, {45}^{\ensuremath{\circ}}, 39.{2}^{\ensuremath{\circ}}$, and $20.{5}^{\ensuremath{\circ}}$, respectively. The kinematics are the same in all cases: the momentum transfer is $K=2.1$ a.u., and ejected electrons are detected in a plane that contains the momentum-transfer direction and is perpendicular to the scattering plane. The results are presented as $(e,2e)$ angular distributions energy integrated over each level. They are compared with fully nonperturbative $B$-spline $R$-matrix and hybrid second-order distorted-wave + $R$-matrix calculations.

Spur energy distributions in liquid water using a generalized oscillator strength from inelastic X-ray scattering measurements

The present paper utilizes the currently most accurate and complete generalized oscillator strength (GOS), determined experimentally using synchrotron X-rays, in constructing the spur energy distribution in liquid water. This replaces the earlier approximate evaluation using UV reflection and requiring long and uncertain extrapolation.Normalized distribution is provided as histograms of 5eV bins up to 100eV of energy loss for different incident electron energies. Certain other related quantities are also computed. These include the effective number of electrons receiving energy transfer, mean excitation potential and its effective value as a function of incident electron energy. This paper gives the results for first collision that is for the differential track, which is close to the integrated track result for high incident energy, as shown in some earlier calculations. On the whole, some earlier computations, but not all are approximately verified.

On the possibility of using low-energy electron stimulated desorption of ions as a surface probe: Analysis of Au substrates

We explore the possibility of using low-energy (0–50eV) electron stimulated desorption (ESD) of ions, analyzed by time-of-flight (TOF) mass spectrometry, as a surface analysis technique. As a first step, we measure the mass spectra from ions emanating from gold substrates prepared for molecular self-assembly by two different methods: cleaning with sulfochromic acid (CrO3/H2SO4) and exposure to UV/O3. Modifications of the yields of desorbed anions and cations as functions of incident electron energy provide information on the presence and the chemical nature of contaminants present on the Au surface, either before or after cleaning. Upon cleaning, the decrease in intensity of the yield functions of certain desorbed anions and cations associated with organic molecules confirms that both techniques are efficient in removing organic contaminants. On the other hand, other ESD signals increase or only appear by either cleaning procedure. This finding demonstrates how these cleaning techniques can contaminate Au surfaces with species, such as acid residues and nitrogenized organic molecules. Such observations have never been reported before with other methods of surface analysis. The present results strongly suggest that ESD of ions should be considered as a potential sensitive tool to complement surface analysis by other well-established methods.

Electron induced degradation of condensed Fe(CO)5 studied by electron stimulated desorption

SynopsisWe have measured the electron stimulated desorption (ESD) yields of anions and cations from thin films of Fe(CO)5 deposited onto both Xe or Pt surfaces. Measurements were made as functions of incident electron energy, incident current, and the thickness of both the Fe(CO)5 and Xe layers. The desorption of C- and O- occur exclusively via dipolar dissociation (DD), while the desorption of Fe(CO)x- (x = 0-4) proceeds via DD and dissociative electron attachment (DEA). Threshold energies for the desorption of cationic fragments C+, O+, CO+, and Fe(CO)+x (x = 0-4) were measured. Only CO+ appears to be produced by DD as opposed to dissociative ionization.

Role of Low-Energy Electrons (<35 eV) in the Degradation of Fe(CO)5 for Focused Electron Beam Induced Deposition Applications: Study by Electron Stimulated Desorption of Negative and Positive Ions

Fe(CO)5 is one of the most common precursors used in focused electron beam induced deposition (FEBID). Since high-energy electrons interacting with matter produce large amounts of secondary electrons with energies <50 eV, those generated within the substrate in FEBID are expected to play a major role in the dissociation process of the precursor molecules. The aim of this study is to identify the role of the secondary electrons in the deposition process of Fe(CO)5 and the relevant dissociation mechanisms, using an electron stimulated desorption system with 4–33 eV electrons. The desorption of charged fragments from thin films of Fe(CO)5, condensed on Xe or onto a Pt foil, was measured as a function of incident electron energy, incident current, and thickness of both Fe(CO)5 and Xe films. Both dissociative electron attachment (DEA) and dipolar dissociation (DD) are involved in the production of anions, specifically, C– and O–, and Fe(CO)x– (x = 0–4). Cations C+, O+, CO+, and Fe(CO)x+ (x = 0–4) were detected with desorption thresholds in the ∼15–25 eV range. These fragments are produced via direct dissociative processes. This study reveals the detailed dissociation mechanisms of Fe(CO)5 induced by low-energy electron impact by the direct observation of Fe(CO)x fragments under different conditions, thus confirming the frequently proposed mechanisms of deposition of Fe by FEBID.

Two-dimensional spectra of electron collisions with acrylonitrile and methacrylonitrile reveal nuclear dynamics.

Detailed experimental information on the motion of a nuclear packet on a complex (resonant) anion potential surface is obtained by measuring 2-dimensional (2D) electron energy loss spectra. The cross section is plotted as a function of incident electron energy, which determines which resonant anion state is populated, i.e., along which normal coordinate the wave packet is launched, and of the electron energy loss, which reveals into which final states each specific resonant state decays. The 2D spectra are presented for acrylonitrile and methacrylonitrile, at the incident energy range 0.095-1.0 eV, where the incoming electron is temporarily captured in the lowest π(∗) orbital. The 2D spectra reveal selectivity patterns with respect to which vibrations are excited in the attachment and de-excited in the detachment. Further insight is gained by recording 1D spectra measured along horizontal, vertical, and diagonal cuts of the 2D spectrum. The methyl group in methacrylonitrile increases the resonance width 7 times. This converts the sharp resonances of acrylonitrile into boomerang structures but preserves the essence of the selectivity patterns. Selectivity of vibrational excitation by higher-lying shape resonances up to 8 eV is also reported.

In-situ measurements of the secondary electron yield in an accelerator environment: Instrumentation and methods

The performance of a particle accelerator can be limited by the build-up of an electron cloud (EC) in the vacuum chamber. Secondary electron emission from the chamber walls can contribute to EC growth. An apparatus for in-situ measurements of the secondary electron yield (SEY) in the Cornell Electron Storage Ring (CESR) was developed in connection with EC studies for the CESR Test Accelerator program. The CESR in-situ system, in operation since 2010, allows for SEY measurements as a function of incident electron energy and angle on samples that are exposed to the accelerator environment, typically 5.3GeV counter-rotating beams of electrons and positrons. The system was designed for periodic measurements to observe beam conditioning of the SEY with discrimination between exposure to direct photons from synchrotron radiation versus scattered photons and cloud electrons. The samples can be exchanged without venting the CESR vacuum chamber. Measurements have been done on metal surfaces and EC-mitigation coatings. The in-situ SEY apparatus and improvements to the measurement tools and techniques are described.

Effects of electron incident angle on the secondary electron yield for polyimide

Relationship between secondary electron yield (SEY) and electron incident angle has been measured for a polyimide sample. SEY as a function of incident angle at different incident electron energy is measured by use of a system with a single pulsed electron beam and a special surface charge neutralization technology based on the negatively biased collector. Measured results show that the SEY may deviate from the traditional law of monotonic increase with the incident angle when the angle is higher than a certain critical value. This deviation is even more obvious at lower incident electron energy. The critical incident angle decreases with decreasing incident energy. A theoretical analysis on the deviation is given in a simplified electron elastic scattering process. The distribution of the scattering region has an important effect on the relation of SEY versus incident angles. A sector region is introduced to describe the electron scattering region. Due to the limit of sample surface, the electron scattering region will decrease if the angle between the incident direction and the sample surface is smaller than half of the central angle of the sector. Corresponding SEY might no longer increase. Based on the Rutherford’s elastic scattering formula, a formula for the critical incident angle is derived as a function of incident electron energy, which is also confirmed by our measurement results. Finally, a revised SEY computation formula is developed which can give more accurate results at high incident electron angle.

The relative cross section and kinetic energy distribution of dissociation processes of methane by electron impact

Electron-impact ionization and dissociation of methane (CH4) has been experimentally studied with a cold target recoil-ion momentum spectrometer. We report data for the formation of CHn+ (n = 0 ∼ 3) relative to that of CH4+ as a function of incident electron energy from 20 to 200 eV. Good agreements are achieved with previous studies. The kinetic energy distribution of the recoil-ions is presented and the average kinetic energy released (KER) was obtained for formation of CH3+. Taking advantages of the supersonic jet expansion, the thermal motion contribution to the kinetic energy distribution of the recoil-ions was reduced. The results agree better with theoretical data than those from previous experimental studies.

Electron impact total cross section calculations for CH3SH (methanethiol) from threshold to 5 keV

We report calculated total elastic cross sections Qel, total ionisation cross sections, Qion, summed total excitation cross sections ∑Qexc and total cross sections QT for CH3SH upon electron impact for energies from ionisation threshold to 5 keV. We have employed Spherical Complex Optical Potential (SCOP) formalism to calculate total elastic cross section Qel, and total inelastic cross section Qinel and used Complex Scattering Potential – the ionisation contribution (CSP-ic) method to extract the ionisation cross sections, Qion, from the calculated Qinel. The calculated total cross sections are examined as functions of incident electron energy and are compared with available data wherever possible and overall good agreement is observed. In this work Qel, Qion, and ∑Qexc are reported for the first time for CH3SH in this energy range.

Separate K-line contributions to fluorescence enhancement in electron probe microanalysis

The behaviour of the fluorescence enhancement correction factor in electron probe microanalysis, as a function of incident electron energy and take-off angle, is assessed for different binary samples in a wide range of compositions. Monte Carlo simulations are employed to validate Reed's correction algorithm [S.J.B. Reed, Characteristic fluorescence corrections in electron-probe microanalysis, Br. J. Appl. Phys. 16 (1965) 913-926], by means of estimating the primary excited radiation volume and the volume corresponding to secondary fluorescence generation. Then, Reed's expression for the fluorescence enhancement has been modified to account for Kα and Kβ line contributions separately. It is clearly shown that in certain cases the assignment of all fluorescent contribution to the Kα lines may be inadequate, particularly when trace element analysis imposes an accurate determination of elemental concentrations.

Low energy electron induced dissociation in condensed diallyl disulfide

Using a high sensitivity Time of Flight mass analyser we have measured the electron stimulated desorption (ESD) of anions and cations from multilayer thin films of diallyl disulfide (DADS) formed by condensation onto Pt and Kr substrates. Measurements were performed as a function of incident electron energy (Ei), film thickness and effective incident current. For Ei ≲ 15 eV the desorption yields of anions are the result of dissociative electron attachment (DEA) via several transient negative ions often associated with electron capture into C-S and S-S σ* bonds. The minimum incident energies observed for desorption of the anionic fragments H−, S−, CH2CHCH-2, and CH2CHCH2S− are compared to theoretical bond dissociation energies calculated using DFT methods. In comparison to gas-phase electron impact mass spectra, the yield of cationic fragments are dominated by desorption of low mass fragments although similar species are observed in both cases. Electron impact at higher electron energies on thicker films of DADS enhances desorption of larger fragment ions, including those formed by the scission and formation of multiple bonds.

Electron transport through an extended nano-ring in presence of magnetic field: An analytical approach

We analytically describe the influence of magnetic field on the electronic transport properties of an extended nano-ring by using Green's function technique in the nearest neighbor tight-binding approximation. We obtain exact analytic formulas for the electronic transmission coefficient, the total and local contact density of states as functions of incident electron energy, magnetic flux crossing over the ring and all the system parameters. Our formalism gives the ability to study the extended nano-rings of any size under certain conditions which provides useful tools to analyze electronic transport of these systems, much faster and more easily. The results show that the electronic transport quantities of a system consisting of a nano-ring are strongly sensitive to incoming electron energy, magnetic flux and contact hopping energies. The present approach may be useful to design nano-devices measuring magnetic field and magnetic based nano-switches.

Relation between Electron Scattering Resonances of Isolated NTCDA Molecules and Maxima in the Density of Unoccupied States of Condensed NTCDA Layers

The empty-level structure of the 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) molecule is characterized by means of dissociative electron attachment (DEA) experiments in the gas phase coupled with DFT calculations. Distinct maxima in the anion currents generated by electron attachment to NTCDA, as a function of incident electron energy, are ascribed to capture of incident electrons into empty orbitals, i.e., the process referred to as shape resonance. The empty orbital energies of gas-phase NTCDA shifted to 1.2 eV lower energy reproduce satisfactorily the maxima of the unoccupied electronic states of a multilayer NTCDA film measured by means of the very low energy electron diffraction method and the total current spectroscopy measurement scheme. The present results indicate that the empty levels of individual NTCDA molecules are stabilized in the solid state, but their relative energies remain nearly unaltered. The stabilization energy in multilayer film of NTCDA molecules is likely due to attractive polarization forces. Fragmentation of the gas-phase NTCDA temporary parent anions via the DEA mechanism, the other issue of the present investigation, leads to the rupture of the bonds between the end carbonyl groups and the naphthalene core, and occurs at incident electron energies above 2 eV. Possible chemical changes in condensed NTCDA molecules initiated by the DEA mechanism under conditions of electron transport through the film are discussed.

Resonance electron attachment and long-lived negative ions of phthalimide and pyromellitic diimide

Resonance attachment of low energy (0-15 eV) electrons to imide-containing molecules, phthalimide (PTI) and pyromellitic diimide (PMDI), was investigated in the gas-phase by means of Electron Transmission Spectroscopy (ETS) and Dissociative Electron Attachment Spectroscopy (DEAS). Among a variety of low intensity negatively charged fragments formed by DEA, in both compounds the dominant species was found to be a long-lived (μs) parent molecular anion formed at zero energy. In addition, in PMDI long-lived molecular anions were also observed at 0.85 and 2.0 eV. The experimentally evaluated detachment times from the molecular anions as a function of incident electron energy are modeled with a simple computational approach based on the RRKM theory. The occurrence of radiationless transitions to the ground anion state, followed by internal vibrational relaxation, is believed to be a plausible mechanism to explain the exceptionally long lifetime of the PMDI molecular anions formed above zero energy.

Approximation of Range in Materials as a Function of Incident Electron Energy

A simple composite analytic expression has been developed to approximate the electron range in materials. The expression is applicable over more than six orders of magnitude in energy ( ; MeV) and range ( 10-9-10-2 m), with an uncertainty of ≤ 20% for most conducting, semiconducting, and insulating materials. This is accomplished by fitting data from two standard NIST databases [ESTAR for the higher energy range and the electron inelastic mean free path (IMFP) for the lower energies]. In turn, these data have been fit with well-established semiempirical models for range and IMFP that are related to standard material properties (e.g., density, atomic number, atomic weight, stoichiometry, and bandgap energy). Simple relations between the IMFP and the range, based on the continuous-slow-down approximation, are used to merge results from the two databases into a composite range expression. A single free parameter, termed the effective number of valence electrons per atom Nv, is used to predict the range over the entire energy span.