Low-energy Electron Collisions Research Articles

Ab initiostudy of low-energy electron collisions with ethylene

We present the results of an investigation of elastic electron scattering by ethylene ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ with incident electron energies ranging from 0.5 to 20 eV, using the complex Kohn variational method. These fully ab initio calculations accurately reproduce experimental angular differential cross sections at energies below 3 eV. Low-energy electron scattering by ethylene is sensitive to the inclusion of electronic correlation and target-distortion effects. We therefore report results that describe the dynamic polarization of the target by the incident electron and involve calculations over a range of different geometries, including the effects of nuclear motion in the resonant ${}^{2}{B}_{2g}$ symmetry with an adiabatic nuclei treatment of the C-C stretch mode. The inclusion of dynamic polarization and the effect of nuclear motion are equally critical in obtaining accurate results. The calculated cross sections are compared with recent experimental measurements.

Dissociative attachment and vibrational excitation in low-energy electron collisions withchlorine molecules

In a combined experimental and theoretical effort, we have investigated dissociative attachmentand vibrational excitation in low-energy electron collisions with chlorine molecules. Using the laserphotoelectron attachment method, we have measured the energy dependence of the cross sectionσDA(E) for dissociativeelectron attachment (Cl− formation) over the range 0–195 meV with an energy width of 1–3 meVand for Rydberg electron transfer at high principal quantum numbers(n>67). Nearzero energy, the cross section shows a behaviour compatible with the threshold law for p-wave attachmentvia the 2Σu+ resonance, reaches a maximum around 50 meV and declines towards higherenergies. These findings are in good agreement with the results of semi-empiricalR-matrix calculations. Measured rate coefficientsknl forCl− formation due toelectron transfer from K**(nl) Rydberg atoms were found to be nearly constant for high principal quantum numbers(n>67) incontrast to the behaviour expected within the quasi-free electron model for p-wave attachment. TheR-matrix calculations are extended to describe electron attachment through the2Πg and 2Πu resonances, and recommended absolute cross sections for dissociative attachment to chlorinemolecules at room temperature are provided over the energy range 0–9 eV. Furthermore, wepredict cross sections for vibrationally inelastic electron scattering through the2Σu+,2Πg and 2Πu resonances.

Low-energy electron scattering by methylsilane

We report calculated elastic and inelastic cross sections for low-energy electron collisions with methylsilane, CH3SiH3, obtained using the Schwinger multichannel method. The elastic cross sections, obtained within the static-exchange approximation, are compared with elastic results for C2H6 and Si2H6. Electron-impact excitation cross sections were computed for sixteen electronic states arising from excitation out of the two highest-lying valence orbitals. The dissociation of the lowest few states was examined through limited electronic-structure calculations, which indicated that the 2 1,3A1 states dissociate to CH3SiH+H2 while the 1 1,3E states dissociate to CH3+SiH3.

Fluorescence emission of carbon disulfide in low-energy electron collisions

Emission spectra of carbon disulfide (CS 2) excited by electron collisions have been measured in the wavelength region from 235 to 575 nm at the incident electron energies of 9.7 and 15 eV. It has been found that the most intense emission bands in the observed fluorescence come from excited CS (A 1Π) radicals produced in the dissociative excitation of carbon disulfide.

Electron molecule collisions calculations using the R-matrix method

The R-matrix method provides a complete theoretical framework for the treatment of low energy electron collisions. Recent results obtained with the UK R-matrix codes are presented focusing on electron impact electronic excitation of water and the CF radical, electron impact dissociation of molecular hydrogen and its isotopomers, and the dissociative recombination of the CO 2+ dication. Examples of other processes studied in recent calculations are also given.

Trapped metastable anions in low-energy electron scattering from C20 clusters

Calculations are reported on scattering resonances in low-energy electron collisions with the fullerene carbon cluster C20. The quantum treatment of the scattering process is carried out using a single-center expansion of the total (bound + scattering electronic) wave function and with the electron–molecule interaction represented by a set of adiabatic multipolar effective potential curves. All resonant wave functions with scattering energies less than 20 eV are analyzed. In some of the resonant states the scattered electron density is seen to remain partly trapped inside carbon cage, although in all cases the resonant, continuum orbital density in the anionic state is primarily distributed near the surface of the cluster cage.

Low Energy Electron Collisions with Molecular Hydrogen

We discuss the theory of electron-molecule excitation processes of importance in fusion plasmas. We consider elastic scattering, rotational, vibrational, and electronic excitation in turn and also dissociative attachment and impact dissociation.

Electron Interactions With BCl3

In this paper we review and assess the cross sections for collisions of low-energy electrons with boron trichloride (BCl3). The only available experimental cross section data are for partial and total ionization and electron attachment, and the electron attachment cross sections are uncertain. Calculated values are available for the total elastic, differential elastic, and momentum transfer cross sections, and derived cross sections have been published for vibrational excitation and dissociation. Other than some rather uncertain data on electron attachment rate constants and some measurements of electron drift velocities in BCl3/Ar and BCl3/He mixtures, there are no measurements of the electron attachment, ionization, or transport coefficients for this gas. Analysis of the experimental data on the electron affinity, electron attachment, and electron scattering, enabled identification of negative ion states of BCl3 at about −0.3, 1.0, 2.8, 5.2, 7.6, and 9.0 eV. Because the existing electron collision data are few and uncertain, relevant data are provided for photon impact on BCl3..

Low-energy electron collisions in nitrogen oxides: a comparative study

Cross-sections for low-energy electron scattering in nitric oxide (NO), nitrous oxide (N2O) and nitrogen dioxide (NO2) are discussed. We compare earlier and recent cross-sections derived from beam and swarm experiments for elastic, vibrational, electronic excitation, ionization, electron attachment phenomena. The importance of resonant scattering at low energies is stressed for all three targets.

Electron collision cross sections for tetraethoxysilane

We describe the development of a consistent set of low-energy electron-collision cross sections for tetraethoxysilane, also known as tetraethylorthosilicate. Ab initio calculations are used to obtain the elastic and momentum-transfer cross sections. These are combined with measured values of the ionization cross section and with model vibrational and electronic excitation cross sections to form a preliminary cross section set, which is then adjusted to achieve consistency with measured swarm parameters.

High resolution study of cluster anion formation in low-energy electron collisions with molecular clusters of CO 2, CS 2, and O 2

Using a high resolution (Δ E=1–2 meV) laser photoelectron attachment (LPA) method, we have studied the formation of (CO 2) q − ( q=4–32), (CS 2) q − ( q=1, 2, 3, 5), and (O 2) q − ( q=5–14) cluster anions in collisions of low-energy electrons (0–200 meV) with molecular clusters of CO 2, CS 2, and O 2 molecules in a skimmed supersonic beam. For CO 2 clusters, the attachment spectrum is dominated by rather narrow vibrational Feshbach resonances (VFRs) of the type [(CO 2) N−1 CO 2( ν i )] − which involve a vibrationally excited molecular constituent ( ν i ≥1 denotes excited vibrational mode). The size-dependent values of the observed redshifts of the VFRs (about 12 meV per added CO 2 unit for small q) in conjunction with their narrow widths and with simple model calculations of the electron binding energies suggest that the size q of the observed (CO 2) q − anions is essentially the same ( q= N or N−1) as the size N of the neutral precursor (CO 2) N which captures the electron. For cluster sizes q above 24, a doubling of the VFR peak structure is observed which we tentatively attribute to the transition from icosahedral (low N) to bulk cubic (high N) cluster structure, previously predicted to occur for N around 30. The size distribution in the (CO 2) q − anion mass spectrum, due to attachment of electrons with well-defined energies, shows undulatory structure which reflects the presence or absence of VFRs at that particular energy. In contrast, the energy-integrated cluster anion mass spectrum shows a rather smooth dependence on cluster size q. The yield spectra for (CS 2) q − cluster anions exhibit an s-wave attachment peak near 0 eV and a smoothly falling cross section towards higher energies without significant structure. In the yield for (O 2) q − anions we observe a rise towards 0 eV, indicative of s-wave electron capture, and a slowly varying and essentially structureless attachment continuum between 10 and 200 meV. The attachment characteristics of the different clusters systems are discussed in the light of the basic properties of the monomers.

Electron impact collisionstrengths and rates for neutral sulphur using the B-splineR-matrix approach

Our previous calculations based on the use of the B-splinebasis in the R-matrix approach for low-energy electroncollisions with atomic sulphur (Zatsarinny O and Tayal S S 2001J. Phys. B: At. Mol. Opt. Phys. 34 3383) have beenextended to determine electron collisional excitation strengthsand rates for transitions between the 3s23p4 3P,1D and 1S LS states and from these states to the 24states of the excited configurations. The close-couplingexpansion consists of the 27 spectroscopic bound andautoionizing target states. The collision strengths arerepresented by simple parametric functions of scaled energy.Their values in a very wide energy range are easily obtainedusing the tabulated parameters. The same parameters can be usedto determine effective collision strengths over a wide electrontemperature range. Branching ratios are presented that can beused to estimate cascade contributions and to determine emissioncross sections for transitions between low-lying states.

Low-energy electron collisions with tetrafluoroethene, C2F4

We report calculated cross sections for elastic and inelastic collisions of low-energy electrons with tetrafluoroethene, C2F4. The elastic cross section shows a number of resonance features, which we classify according to symmetry and analyze in relation to available experimental data. Electron-impact excitation cross sections are obtained for the 1 3B1u (T) and 1 1B1u (V) states arising from the π→π* transition, as well as for eight other low-lying excited states arising from excitations out of the highest occupied molecular orbital. As expected, the T and V states make the largest individual contributions to electron-impact excitation at low energies; however, the other states are shown to contribute significantly to the total excitation cross section at impact energies from 10 to 25 eV.

Resonances and Threshold Phenomena in Low-Energy Electron Collisions with Hydrogen Halides: New Experimental and Theoretical Results

Low-energy electron collisions with HCl and HBr and the deuterated compounds have been investigated by experimental and theoretical methods. New experimental results have been obtained on relative differential cross-sections for vibrational excitation and dissociative electron attachment. Measurements with high energy resolution for rotationally cooled molecules have revealed, in addition to shape resonance, threshold peaks and Wigner cusps, the existence of surprisingly sharp oscillatory structures in the elastic and v = 0 → 1 vibrational excitation cross-sections in a narrow range below the dissociative attachment threshold. The theoretical analysis is based on an improved nonlocal resonance model which has been constructed on the basis of ab initio fixed-nuclei scattering phase shifts for HCl and HBr and accurate ab initio calculations of the bound part of the HCl− and HBr− potential-energy functions. The high degree of agreement which has been obtained between experiment and theory for all channels indicates that the mechanisms responsible for the rich threshold structures in the collision cross-sections are completely understood.

Resonant features of inelasticelectron scattering off CF3Cl in the low-energy region

We report on ab initio calculations of resonant cross sectionsfor dissociative attachment and vibrational excitation processesin low-energy electron collisions with the CF3Cl molecule.Collision energies up to 3 eV are considered. The results arecompared with available experimental data and other theoreticalstudies. For gas temperatures of 300 K the present results agreesatisfactorily with the experimental findings. For higher gastemperatures the influence of non-resonant effects is discussed.

Energy threshold and ion yield for H − ions ejected from Si( [formula omitted]):H(1×1) during low energy electron collisions: study of ESD process in relation with atom manipulation in STM

This paper deals with an experimental study of H − ions desorption out of a well organised silicon crystal surface Si(1 1 1):H(1×1). It mainly concerns the resonant desorption process attributed to a dissociative electron attachment below 12 eV. In situ access to the work function provides an accurate reference of the incident electron energy. The resonant peak for desorption is centred at 8 eV with a corresponding threshold of about 6 eV. For the resonance energy, a probability 2.0×10 −6 ion H − per electron is deduced from both the incident electron density and the spectrometer calibration. Variation in desorption yield with electron dose suggest that hydrogen leaves the surface as neutral also with a probability about 1.1×10 −3 atom per electron. Evaluation of the image charge influence provides a distance 1.2 Å between external silicon atoms and hydrogen. This determination was based on the measured-desorption threshold and the H − ion kinetic energy, as well as on the hydrogen electron affinity and the SiH binding energy deduced from data tables. This distance fairly agrees with ab initio simulations once the jellium extension is included. Comparing most ESD features for H − ions desorption with hydrogen atom manipulation in scanning tunnel microscopy out of the same substrate, we assume that similar energy threshold and yields strongly suggest a common initial electron excitation process. Projected ESD experiments might enlighten the understanding of such surface atom manipulation.

Low-energy electroncollisions with atomic sulfur: R-matrix calculation withnon-orthogonal orbitals

The low-energy electron collision with neutral sulfur has beeninvestigated using a modified R-matrix method based on theB-spline representation of the scattering orbitals. Integralcross sections for elastic scattering and for excitation of the3s23p4 1D, 1S, 3s23p34s 3,5So, 1,3Po, 1,3Do, 3s23p33d 3Do and 3s3p5 3Po states in the LS coupling scheme arepresented in the energy region from threshold to 50 eV. Accuraterepresentation of target wavefunctions has been obtained on the basis of the non-orthogonal orbitals. The close-couplingexpansion consists of the 27 spectroscopic bound andautoionizing target states. Calculations in three-, five- and 15-stateclose-coupling approximations and in the Born approximation havealso been carried out to check the convergence of excitationcross sections. The large influence of inclusion of the 3s3p5 3Po state in the close-coupling expansion wasfound to be due to large exchange interaction at the 3s-3pexcitation. The present cross sections differ considerably fromthe existing results obtained in less elaborate calculations.

Electron cross section set for CHF3

We describe the development of a consistent set of low-energy electron collision cross sections for trifluoromethane, CHF3. First-principles calculations are used to obtain key elastic and inelastic cross sections. These are combined with literature values of the ionization cross section and with vibrational excitation cross sections obtained from the Born approximation to form a preliminary set, which is then adjusted to achieve consistency with measured swarm parameters.

Inelastic low-energy electron collisions with the HBr and DBr molecules: Experiment and theory

Low-energy electron collisions with the HBr and DBr molecules are addressed from the experimental and theoretical points of view. Relative differential cross sections for the excitation of vibrational levels of HBr and DBr up to $v=6$ have been measured as a function of the incident electron energy in the range 0--4 eV. In addition to the shape resonance near 2 eV collision energy, intense and narrow threshold peaks are found for the excitation of the $v=1$ level of HBr and the $v=1$ and $v=2$ levels of DBr. Measurements with high resolution for rotationally cooled molecules have revealed the existence of sharp oscillatory structures in the elastic and $v=\stackrel{\ensuremath{\rightarrow}}{0}1$ cross sections in a narrow range below the dissociative-attachment threshold. The dissociative-attachment cross section has been measured with high resolution of the incident electrons in the range 0.2--1.4 eV. The theoretical analysis is based on an improved nonlocal resonance model, which has been constructed on the basis of existing fixed-nuclei electron-HBr scattering phase shifts and accurate ab initio calculations of the bound part of the ${\mathrm{HBr}}^{\ensuremath{-}}$ potential-energy function. This purely ab initio-based model is used to calculate integral electron-scattering and dissociative-attachment cross sections for HBr and DBr. The theoretical cross sections agree very well with the experimental data. The observed threshold peaks and Wigner cusp structures in the vibrational excitation functions are correctly reproduced. The sharp structures in the $v=\stackrel{\ensuremath{\rightarrow}}{0}0$ and $v=\stackrel{\ensuremath{\rightarrow}}{0}1$ cross sections below the dissociative-attachment threshold, consisting of a superposition of boomerang-type oscillations and quasibound levels of the outer well of the ${\mathrm{HBr}}^{\ensuremath{-}}$ potential-energy function, are quantitatively described by the theory. The high degree of agreement between experiment and theory indicates that the essentials of low-energy electron-HBr collision dynamics are completely understood.

Electron collisions with octafluorocyclobutane, c-C4F8

We present calculated cross sections for elastic and inelastic collisions of low-energy electrons with octafluorocyclobutane, c-C4F8. The integral elastic cross section displays a rich resonance structure, which we analyze in terms of temporary trapping in virtual valence orbitals. The differential elastic cross sections compare well with recent measurements at energies where the approximations used in the calculations are expected to be valid. Integral and differential cross sections for electron-impact excitation of the lowest singlet and triplet excited states were obtained. We relate the small magnitude of the inelastic integral cross sections and the unusual form of the inelastic differential cross sections to the symmetries of the electronic states involved in the transition.