Stephen Buckman

Electron-Molecule Scattering Cross-Sections. I. Experimental Techniques and Data for Diatomic Molecules

Abstract Experimental collision cross-sections for electron-molecule scattering processes at low to intermediate energies (meV– 100 eV ) are compiled and critically reviewed. Recent advances in experimental techniques are summarised. Of principle interest are differential, total and momentum transfer cross-sections for elastic scattering and rotational, vibrational and electronic excitation processes. Wherever possible, available theoretical cross-sections are also compared and discussed. Resonance effects, whilst not treated explicitly, are discussed in the context of the enhancement they produce in the various scattering cross-sections. Scattering from excited molecules is also considered.

Low-energy positron interactions with atoms and molecules

This paper is a review of low-energy positron interactions with atoms and molecules. Processes of interest include elastic scattering, electronic and vibrational excitation, ionization, positronium formation and annihilation. An overview is presented of the currently available theoretical and experimental techniques to study these phenomena, including the use of trap-based positron beam sources to study collision processes with improved energy resolution. State-resolved measurements of electronic and vibrational excitation cross sections and measurement of annihilation rates in atoms and molecules as a function of incident positron energy are discussed. Where data are available, comparisons are made with analogous electron scattering cross sections. Resonance phenomena, common in electron scattering, appear to be less common in positron scattering. Possible exceptions include the sharp onsets of positron-impact electronic and vibrational excitation of selected molecules. Recent energy-resolved studies of positron annihilation in hydrocarbons containing more than a few carbon atoms provide direct evidence that vibrational Feshbach resonances underpin the anomalously large annihilation rates observed for many polyatomic species. We discuss open questions regarding this process in larger molecules, as well as positron annihilation in smaller molecules where the theoretical picture is less clear.

Absolute elastic cross-sections for low-energy electron scattering from tetrahydrofuran

We present results for elastic electron collision cross-sections with tetrahydrofuran (C4H8O) in the energy range of 6.5–50 eV. The absolute cross-sections are determined from crossed beam measurements using the relative flow technique and cover the angular range between 10° and 130°. Integral and momentum transfer cross-sections have also been derived from these results. The data, where possible, are compared to recent experimental results and theoretical calculations.

Forward angle scattering effects in the measurement of total cross sections for positron scattering

Measurements of total scattering by positron impact have typically excluded a significant portion of the forward scattering angles of the differential cross section. This paper demonstrates the effect that this can have on measurements of the total cross section. We show that much of the apparent disagreement between experimental measurements of positron scattering from atoms and molecules may be explained by this excluded angular range. It is shown that this same effect may also lead to an anomalous energy dependence of some cross sections.

A positron trap and beam apparatus for atomic and molecular scattering experiments

An instrument has been designed and constructed to provide new insights into fundamental, low energy positron scattering processes. The design is based on the Surko trap system and produces a pulsed positron beam with an energy resolution of as good as 54 meV. The design and operation of the apparatus is explained, while the first experimental results from this apparatus have been demonstrated in recent publications.

Measurements of elastic electron scattering by water vapour extended to backward angles

Absolute differential cross sections for elastic electron scattering by water vapour have been measured at nine incident electron energies between 4 to 50 eV and over scattering angles between 10° and 180°, using a crossed-beam electron spectrometer. A magnetic angle-changing device based on the design of Read and co-workers has been used to extend the measurements to backward angles (125° to 180°). The elastic integral and momentum transfer cross sections are derived from these elastic differential cross sections. Previous experimental results at backward angles, available only up to 156°, are consistently higher than the present results. We also compare, where possible, with results from recent theoretical calculations.

Elastic electron scattering from helium: absolute experimental cross sections, theory and derived interaction potentials

The authors report new differential cross section measurements (E0=1.5-50 eV) and coupled channel optical model calculations for elastic scattering of electrons from helium. The experimental results, from a crossed electron-atomic beam apparatus are analysed via a complex phaseshift technique with the derived phaseshifts, and their associated errors, then being used to derive total elastic, total reaction, total momentum transfer and grand total cross sections, and their respective errors, in a mathematically rigorous manner. Further, the authors applied fixed-energy inverse scattering techniques to the present data in order to deduce the interaction potential between the colliding particles.

Low-energy total cross section measurements for electron scattering from helium and argon

A time-of-flight electron spectrometer has been used to measure absolute total collision cross sections for electrons scattered from helium and argon over the energy range 0.1-20 eV. The measured cross sections are in good agreement with other recent beam experiments and theoretical calculations. At energies below 1 eV modified effective-range theory has been used to enable a comparison of the present work with swarm-derived momentum transfer cross sections. Some limitations of the applicability of the MERT analysis are discussed.

Low-energy electron scattering from methane

A low-energy time-of-flight electron spectrometer has been used to measure total collision cross sections for methane in the energy range 0.1-20 eV. Agreement with two recent experiments and with the pioneering work of Ramsauer and co-workers (1930) from the 1930s is excellent.

Total, elastic, and inelastic cross sections for positron and electron collisions with tetrahydrofuran

We present total, elastic, and inelastic cross sections for positron and electron scattering from tetrahydrofuran (THF) in the energy range between 1 and 5000 eV. Total cross sections (TCS), positronium formation cross sections, the summed inelastic integral cross sections (ICS) for electronic excitations and direct ionization, as well as elastic differential cross sections (DCS) at selected incident energies, have been measured for positron collisions with THF. The positron beam used to carry out these experiments had an energy resolution in the range 40-100 meV (full-width at half-maximum). We also present TCS results for positron and electron scattering from THF computed within the independent atom model using the screening corrected additivity rule approach. In addition, we calculated positron-impact elastic DCS and the sum over all inelastic ICS (except rotations and vibrations). While our integral and differential positron cross sections are the first of their kind, we compare our TCS with previous literature values for this species. We also provide a comparison between positron and electron-impact cross sections, in order to uncover any differences or similarities in the scattering dynamics with these two different projectiles.