R J Gulley

Elastic electron scattering from C6H6 and C6F6

We present absolute measurements of differential elastic electron scattering cross sections from benzene (C6H6) and hexafluorobenzene (C6F6). The measurements have been performed in our two laboratories on different crossed-beam apparatuses for scattering angles between 10° and 130°: C6H6 at the Australian National University in the energy range from 1.1 to 40 eV, and C6F6 at Sophia University from 1.5 to 100 eV. The cross sections for both molecules, which are characterized by strong forward-angle scattering and large-angle oscillations, are favourably compared with recent theoretical calculations.

Electron scattering from N2O: absolute elastic scattering and vibrational excitation

We report absolute measurements, using the relative flow technique, of low-to-intermediate energy (1.5-100 eV) differential cross sections (DCSs) for elastic electron scattering and vibrational excitation of nitrous oxide. These results, which have been undertaken independently in our two laboratories on different experimental apparatus, are compared in detail with each other and with previous experimental results and theoretical calculations. For elastic scattering the level of agreement between the DCSs from the two laboratories is generally very good. However, the agreement with recent theoretical calculations is rather poor, particularly at energies below 10 eV. For vibrational excitation DCS results have been measured at 2.4 and 8 eV - energies corresponding to the positions of two broad 2Π and 2Σ shape resonances respectively.

Low-energy electron scattering from O2

Absolute differential cross sections for elastic electron scattering from molecular oxygen have been measured at incident electron energies between 1 and 30 eV and over a range of scattering angles between 10 and 130 degrees . These results are compared with other measurements and they highlight a substantial degree of discrepancy between the various experiments at energies below 10 eV. They also indicate some interesting behaviour at low energies and forward angles, which appears to be common to a number of other diatomic molecules. The data are also compared with a recent Schwinger variational calculation, (Ribiero et al., 1994) and total elastic and elastic momentum transfer cross sections, derived from the differential measurements, are also presented and compared with a recent R-matrix calculation.

The scattering of low energy electrons from hydrogen sulphide

Absolute differential cross sections for vibrationally elastic electron scattering from hydrogen sulphide (H2S) have been measured using a crossed electronic-molecular beam apparatus at incident energies from 1.30 eV and scattering angles between -20 and 130 degrees . At each energy studied the authors have integrated their differential cross section data to obtain total elastic and elastic momentum transfer cross sections. Vibrational excitation of H2S by electron impact has also been studied at incident energies of 2 and 3 eV, in the region of the low energy shape resonance. Both their differential and integral cross sections are compared with previous experimental studies and calculated values.

Elastic scattering of low-energy electrons from ammonia

The authors report absolute differential cross-section measurements for vibrationally elastic electron scattering from NH3 at incident energies from 2-30 eV. The present results, from a crossed electron-molecular beam apparatus, represent the first comprehensive experimental attempt to quantify the elastic electron-NH3 scattering process. At each energy studied they have integrated their differential cross-section data to generate total elastic and elastic momentum transfer cross sections and a critical comparison of both their differential and integral cross sections against previous experiment and theory is provided. They also report their observation of a strong Feshbach resonance in the elastic channel at energy of 5.59+or-0.05 eV.

Elastic electron scattering from sulfur hexafluoride

Absolute differential cross sections for elastic scattering of electrons from sulfur hexafluoride (SF6) have been measured at 11 incident energies between 2.7 and 75 eV and for scattering angles between 10° and 130°. The magnetic angle-changing device of Read and co-workers has been used to extend these measurements to backward angles (130°-180°) for incident energies below 15 eV. The measurements reveal some substantial differences with several previous determinations and a reasonably good level of agreement with a recent close-coupling calculation.

Elastic scattering of low energy electrons from sulphur dioxide

Absolute differential cross sections for vibrationally elastic electron scattering from sulphur dioxide (SO2) have been measured using a crossed electron-molecular beam apparatus at incident energies from 1-30 eV. These data are compared with two previous experimental measurements and serve to resolve a discrepancy which existed in the absolute magnitude of these measurements. The differential cross section data have also been integrated to obtain total elastic and elastic momentum transfer cross sections.

Role of negative ion resonances in electron scattering from atoms and molecules

Transient negative ions (resonances) formed during the collision of an electron with an atom or molecule have been extensively studied for over thirty years. The continued interest in these states, both experimentally and theoretically, stems from the profound effects that they can have on electron scattering cross sections and the role that electron–electron correlations play in their formation and quasi-stability. A selective discussion of examples of such resonances, involving one, two and three excited electrons is given for a wide range of atomic and molecular systems.

Low Energy Electron Scattering by Polar Polyatomic Molecules

The scattering of low energy electrons from polar molecules has regularly been a central area of discussion in this series of ICPEAC satellite meetings. This is particularly true for the vibrational excitation of polar diatomic systems, such as the hydrogen halides, where there has been much uncertainty and some controversy concerning the nature of the near-threshold resonance peaks. Whilst similar stuctures have also been observed in polar polyatomic systems, the purpose of the present paper is to review a different aspect of low energy electron-polyatomic-molecule scattering, namely the measurement of absolute scattering cross sections, and the level of agreement that exists between experiment and theory for these cross sections. Based on the recent surge of publications, this field has apparently entered a new and exciting phase where theoretical calculations are offering a real prospect of reproducing the fine details of experimental differential cross sections, both in shape and absolute magnitude.