eV Impact Energy Research Articles

Excited atomic fragments following electron dissociation of pyrimidine

A crossed electron–gas beam system coupled to a VUV spectrometer has been used to investigate the dissociation of pyrimidine into excited atomic fragments in the impact energy range from threshold to 375 eV. Data have been made absolute using Lyman-α from H2 as a secondary standard. The main features in the spectrum are the H Lyman series lines. The emission cross section of Lyman-α is measured to be (2.44 ± 0.25) 10−18 cm2 at 100 eV impact energy. The probability of extracting C or N atoms from the ring is shown to be very small. Possible dissociation channels and excitation mechanisms in the parent molecule are discussed.

Positron scattering from O2

We report on recent cross section results for positron scattering from molecular oxygen (O2). Total cross sections (TCSs) were measured with a positron spectrometer in the energy range from 0.1 to 50 eV and with an energy resolution of the positron beam of ∼0.25 eV. In addition, TCSs as well as elastic and inelastic integral cross sections were computed within the independent atom model and screening corrected additivity rule approach, with both dipole and dipole plus quadrupole polarization potentials, between 1 and 1000 eV impact energy. An overall fair level of accord is found between the experimental TCS and that calculated with the model that includes the quadrupole term. Comparison to earlier measurements shows very good agreement with the present measured TCS and fair agreement with the TCS computed with our most physical model above ∼8 eV. Conversely, we find only a marginal level of accord when comparing our experimental TCS with previous computations, while the present TCS calculated with the dipole plus quadrupole potentials appears to agree reasonably well with most of the existing theoretical results above ∼100 eV.

Differential cross sections and electron impact coherence parameters for elastic electron scattering from laser-excited 138Ba

We have measured and calculated differential cross sections and the P3 collision Stokes parameter for elastic electron scattering from the laser-excited (…6s6p 1P1) and cascade-populated (…6s5d 3D1, 2) levels of barium at 5, 10, and 20 eV impact energy. Measurements were performed using a crossed-beam apparatus, where excited-state atomic targets were prepared with resonant laser radiation incident on the electron–atom interaction region. Calculations were performed in the convergent close-coupling (CCC), relativistic CCC, and optical-model potential approximations. We present measured and calculated partial differential cross sections, which relate to differential cross sections and electron impact coherence parameters through the target state anisotropy introduced by the laser geometry.

Elastic and inelastic cross sections for low-energy electron collisions with pyrimidine

We present theoretical elastic and electronic excitation cross sections and experimental electronic excitation cross sections for electron collisions with pyrimidine. We use the R-matrix method to determine elastic integral and differential cross sections and integral inelastic cross sections for energies up to 15 eV. The experimental inelastic cross sections have been determined in the 15-50 eV impact energy range. Typically, there is quite reasonable agreement between the theoretical and experimental integral inelastic cross sections. Calculated elastic cross sections agree very well with prior results.

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.

HIGH-RESOLUTION ELECTRON-IMPACT EMISSION SPECTRA AND VIBRATIONAL EMISSION CROSS SECTIONS FROM 330–1100 nm FOR N2

ABSTRACT Electron-impact emission cross sections for N2 were measured in the wavelength range of 330–1100 nm at 25 eV and 100 eV impact energies. Cross sections of several molecular emission bands of the first positive band system B 3Π g +(ν′) → A 3Σ g +(ν″) and the second positive band system C 3Π u (ν′) → B 3Π g (ν″) of N2, the first negative band (1NB) system B 2Σ u +(ν′) → X 2Σ g +(ν″) and Meinel band system A 2Π u (ν′) → X 2Σ g +(ν″) of N2 + ions as well as line emissions of N (N i) and N+ (N ii) in the visible–optical–near-IR wavelength range reported in this work were measured for the first time in a single experimental setup at high spectral resolving power (λ/Δλ ≈ 10000) under single-collision-scattering geometry and optically thin conditions. Rotational emission lines of N2 and N2 + were observed for strong emission bands at a gas temperature of about 300 K. The absolute cross section of the strongest (0,0) vibrational band at 391.43 nm of 1NB was determined using the standard Hα emission cross sections of H2 by electron impact at both 25 eV and 100 eV electron-impact energies, and the cross sections for the remainder of the emissions were determined using (0,0) 1NB value. A comparison of the present emission cross sections with the earlier published data from both electron energy loss and electron-impact-induced fluorescence emission is discussed.

Kinematically complete experiments for positron-impact ionization of helium atoms at the NEPOMUC facility

Positron impact ionization of helium is studied with fully resolved momentum vectors of all continuum particles. An imaging multi-particle momentum spectrometer (reaction microscope) detecting all final state particles over the full solid angle was applied. This apparatus was connected to the NEPOMUC facility delivering intense positron beams tuneable over a large energy range with good beam quality. At 80 eV impact energy about 5000 triple coincidence events were collected. Cross sections as function of the longitudinal particle momentum show strong differences compared to respective electron impact ionization data most likely originating from the reversed post collision interaction in both cases. Calculations using the 3 Coulomb wavefunction method show clear discrepancies from the experimental results.

Advanced SIMS quantification in the first few nm of B, P and As ultrashallow implants

AbstractThis paper presents an investigation on SIMS profile quantification for ultra‐shallow profiles. New configuration for the Cesium and Oxygen sources on the CAMECA IMS Wf tool provides SIMS profiling capability at 150 eV impact energy with a sputter rate of 1 and 2 nm/min for the Cs+ and O2+ primary beams, respectively. Results for as‐implanted B, P and As profiles using extremely low impact energy (EXLIE) sputtering conditions are compared with HR‐RBS and ERDA profiles. Copyright © 2010 John Wiley & Sons, Ltd.

Influence of nuclear exchange on nonadiabatic electron processes in H$^+$++H$_2$2 collisions

H(+)+H(2) collisions are studied by means of a semiclassical approach that explicitly accounts for nuclear rearrangement channels in nonadiabatic electron processes. A set of classical trajectories is used to describe the nuclear motion, while the electronic degrees of freedom are treated quantum mechanically in terms of a three-state expansion of the collision wavefunction. We describe electron capture and vibrational excitation, which can also involve nuclear exchange and dissociation, in the E = 2-1000 eV impact energy range. We compare dynamical results obtained with two parametrizations of the potential energy surface of H(3)(+) ground electronic state. Total cross sections for E > 10 eV agree with previous results using a vibronic close-coupling expansion, and with experimental data for E < 10 eV. Additionally, some prototypical features of both nuclear and electron dynamics at low E are discussed.

Differential cross sections for electron-impact excitation of laser-excited 174Yb (…6s6p 3P1)

We have measured and calculated electron-impact excitation cross sections from the laser-excited Yb (…6s6p 3P1) level to the higher-lying Yb (…6s6p 1P1), (…6s5d 3D1, 2, 3) and (…6s7s 3S1) levels at 20 eV impact energy. Measurements were performed using a crossed-beam apparatus, with resonant laser radiation incident on the electron–atom interaction region, providing excited-state atomic targets. Calculations were performed in the relativistic convergent close-coupling (RCCC), convergent close-coupling (CCC), and relativistic distorted-wave (RDW) approximations. We present measured and calculated partial differential cross sections (), which relate to differential cross sections and electron-impact coherence parameters through the target state anisotropy introduced by the laser geometry. Using theoretical predictions of the h electron-impact coherence parameter, we present the differential cross section for the 3P1–3S1 excitation.

Reaction dynamics in double ionization of helium by electron impact

We present theoretical fully differential cross sections (FDCS) for double ionization of helium by $500$ eV and $2$ keV electron impact. Contributions from various reaction mechanisms to the FDCS were calculated separately and compared to experimental data. Our theoretical methods are based on the first Born approximation. Higher-order effects are incorporated using the Monte Carlo event generator technique. Earlier, we successfully applied this approach to double ionization by ion impact, and in the work reported here it is extended to electron impact. We demonstrate that at $500$ eV impact energy, double ionization is dominated by higher-order mechanisms. Even at $2$ keV, double ionization does not predominantly proceed through a pure first-order process.

ChemInform Abstract: Electron Impact Excitation of Coronene.

A preliminary study of the electron‐impact excitation of thermally evaporated coronene at 550° C was carried out using electron‐energy‐loss spectroscopy. Measurements of the energy‐loss spectra of coronene at high (100 eV) and low (5–20 eV) impact energies are presented. One of the high‐energy spectra was converted to an apparent generalized oscillator strength spectrum and compared to the photoabsorption spectrum of coronene. Observations concerning vibrational excitation of coronene by electron impact are also presented and discussed.

Atomistic Insights into the Running-in, Lubrication, and Failure of Hydrogenated Diamond-Like Carbon Coatings

The tribological performance of hydrogenated diamond-like carbon (DLC) coatings is studied by molecular dynamics simulations employing a screened reactive bond-order potential that has been adjusted to reliably describe bond-breaking under shear. Two types of DLC films are grown by CH2 deposition on an amorphous substrate with 45 and 60 eV impact energy resulting in 45 and 30% H content as well as 50 and 30% sp3 hybridization of the final films, respectively. By combining two equivalent realizations for both impact energies, a hydrogen-depleted and a hydrogen-rich tribo-contact is formed and studied for a realistic sliding speed of 20 m s−1 and loads of 1 and 5 GPa. While the hydrogen-rich system shows a pronounced drop of the friction coefficient for both loads, the hydrogen-depleted system exhibits such kind of running-in for 1 GPa, only. Chemical passivation of the DLC/DLC interface explains this running-in behavior. Fluctuations in the friction coefficient occurring at the higher load can be traced back to a cold welding of the DLC/DLC tribo-surfaces, leading to the formation of a transfer film (transferred from one DLC partner to the other) and the establishment of a new tribo-interface with a low friction coefficient. The presence of a hexadecane lubricant leads to low friction coefficients without any running-in for low loads. At 10 GPa load, the lubricant starts to degenerate resulting in enhanced friction.

Advanced secondary ion mass spectroscopy quantification in the first few nanometer of B, P, and As ultrashallow implants

This article presents investigation on secondary ion mass spectroscopy (SIMS) profile quantification for ultrashallow profiles. New configuration for the cesium and oxygen sources on the CAMECA IMS Wf tool-provides SIMS profiling capability at 150 eV impact energy with sputter rates of 1 and 2 nm/min for the Cs+ and O2+ primary beams, respectively. Results for as-implanted B, P, and As profiles using extremely low impact energy (EXLIE) sputtering conditions are reported. They are compared with high resolution Rutherford backscattering spectroscopy and elastic recoil detection analysis profiles. The overall results confirm that the use of EXLIE conditions minimizes near surface (depth &amp;lt;5 nm) artifacts but data quantification still requires dedicated postanalysis data treatment to take into account matrix effects between Si and SiO2.

Electron scattering from excited-state ytterbium

We present results of our recent experimental investigation of electron scattering from the 6s6p 3P1 laser excited state of the Yb atom. Measurements of the (partial) differential cross sections for the excitations 6s6p 3P1 → 5d6s3D1,2,3 + 6s6p1P1 (unresolved) and 6s6p3P1 → 6s7s3S1 at 20 eV impact energy are presented. P3 Stokes parameters for the corresponding, time-inverse, de-excitations are also given.

Light-particle single ionization of argon: Influence of the projectile charge sign

The ionization of the 3p orbital of argon by incident electrons and positrons is studied by means of the post version of the continuum distorted wave-eikonal initial-state model. Results are presented at both 200 and 500 eV impact energies for conditions amenable to present experiments. Differences in the fully differential cross sections (FDCSs) are analyzed and the influence of the projectile charge sign on the emission dynamics is discussed. The FDCSs are found to display the classic binary plus recoil peak structure at 500 eV, but transition to a more complicated four-lobed structure at the lower impact energy.

Hyperspherical partial wave theory with two-term error correction

Hyperspherical partial wave theory has been applied to calculate $T$-matrix elements and single differential cross-section (SDCS) results for electron-hydrogen ionization processes within the Temkin-Poet model potential. We considered three different values of step length to compute the radial part of final state wave function. Numerical outcomes show that $T$-matrix elements and SDCS values depend on the step length $h$. Here, we have presented $T$-matrix elements and the corresponding SDCS results for 0.0075, 0.009, and 0.01 a.u. values of $h$ and for 27.2, 40.8, and 54.4 eV impact energies. With the help of the calculated data for three different step lengths, we have been able to evaluate a two-term error function depending on the step length $h$. Finally, two-term error corrected $T$-matrix elements and the corresponding SDCS values have been computed. We fitted our two-term error corrected SDCS results by a suitable curve and compared with the benchmark results of Jones et al. [Phys. Rev. A 66, 032717 (2002)]. Our fitted curves agree very well with the calculated results of Jones et al. and two-term error corrected SDCS results show fair agreement with the benchmark results. Two-term error corrected SDCS results are significantly better than the calculated SDCS results of different step lengths.

The role of projectile double scattering in positron–atom collisions

We present the doubly differential cross sections of the ionization for positron and hydrogen atom collisions at 50 eV impact energy. We treat the collision problem in the framework of the full and reduced three-body classical trajectory Monte Carlo (CTMC) model. The role of projectile double scattering in light projectile and atom collisions are discussed.

Experimental and Theoretical Investigation of Valence Orbitals in1,4-Dioxane by Electron momentum Spectroscopy

The binding energy spectrum of all valence orbitals and the momentumdistributions of highest occupied molecular orbital (HOMO: 8ag),7bu+7ag, 4bu, 2bg+4ag and 2au in 1, 4-dioxaneare investigated by electron momentum spectroscopy (EMS) with 600 eV impactenergy. The experimental results are consistent with theoreticalcalculations of C2h chair conformation using the Hartree–Fock method anddensity functional theory with 6-311++G** and AUG-CC-PVTZbasis sets.

Thermal and Ion-Induced Surface Reactions of 1,1-Difluoroethylene on Si(111)7×7 and Vitreous SiO2

Thermal and ion-induced reactions of 1,1-difluoroethylene (1,1-C2H2F2 or iso-DFE) on Si(111)7 x 7 and vitreous SiO2 surfaces have been investigated by vibrational electron energy loss spectroscopy and thermal desorption spectrometry. Like ethylene, iso-DFE predominantly chemisorbs via a [2 + 2] cycloaddition mechanism onto the 7 x 7 surface as a di-sigma-bonded difluoroethane-1,2-diyl adstructure, which undergoes H abstraction and defluorination, producing hydrocarbon fragments and SiF(x) (x = 1-3) upon annealing to >700 K. Ion irradiation of Si(111)7 x 7 in iso-DFE at 50 eV impact energy appears to substantially enhance the production of hydrocarbon fragments and SiF(x)(), leading to stronger SiF4 desorption products over an extended temperature range (400-900 K). The observed SiC and SiF(x) produced on the 7 x 7 surface by ion irradiation in iso-DFE are found to be similar to those obtained by ion irradiation in the fluoromethane homologues, CF4 and CH2F2. The production of higher relative concentrations for the larger SiF(x) and C2-containing fragments is evidently favored on the 7 x 7 surface. On a vitreous SiO2 surface, ion irradiation in iso-DFE, unlike that in CF4 and CH2F2, appears to produce less SiF(x) than that on the 7 x 7 surface, which indicates that surface O does not interact strongly with the C2-containing fragments. The presence or absence of a C=C bond and the relative F-to-C ratio of the sputtering gas could therefore produce important effects on the resulting surface products obtained by low-energy ion irradiation.