Julian Lower

Electron collisions with ethylene

We have measured absolute elastic scattering and vibrational excitation cross sections for electron impact on ethylene. The experimental data have been obtained on two different crossed-beam electron spectrometers and they cover the energy range from 1 to 100 eV and scattering angles between 10° and 130°. Both differential (in angle) and energy-dependent cross sections have been measured. The differential cross sections have also been analysed using a molecular phase shift analysis technique in order to derive the integral elastic and elastic momentum transfer cross sections. Comparison is made with earlier data, where available, and also with a number of recent theoretical calculations.

Invited Article: An improved double-toroidal spectrometer for gas phase (e,2e) studies

A new spectrometer is described for measuring the momentum distributions of scattered electrons arising from electron-atom and electron-molecule ionization experiments. It incorporates and builds on elements from a number of previous designs, namely, a source of polarized electrons and two high-efficiency electrostatic electron energy analyzers. The analyzers each comprise a seven-element retarding-electrostatic lens system, four toroidal-sector electrodes, and a fast position-and-time-sensitive two-dimensional delay-line detector. Results are presented for the electron-impact-induced ionization of helium and the elastic scattering of electrons from argon and helium which demonstrate that high levels of momentum resolution and data-collection efficiency are achieved. Problematic aspects regarding variations in collection efficiency over the accepted momentum phase space are addressed and a methodology for their correction presented. Principles behind the present design and previous designs for electrostatic analyzers based around electrodes of toroidal-sector geometry are discussed and a framework is provided for optimizing future devices.

Benchmark experiment and theory for near-threshold excitation of helium by electron impact

A new experimental technique has been applied to measure absolute scattering cross sections for electron impact excitation of the n = 2, 3 states of helium at near-threshold energies. The experimental results are compared with predictions from recent state-of-the-art theoretical calculations. The calculations are performed using the R-matrix with pseudostates, B-spline R-matrix, and the convergent close-coupling methods. Generally, very good agreement is found between the experiment and the three theories.

The elastic scattering of spin-polarized electrons from xenon

Here we present experimental and theoretical results for the elastic scattering of spin-polarized electrons from xenon and the subsequent determination of the Sherman function, which is directly related to the measured spin up-down asymmetry, in the energy range between 30 and 160 eV. For our measurements, particular attention was given to minimizing the deleterious effects of finite angular resolution and background levels on the derived Sherman functions. Our theoretical results were derived from Dirac-Fock scattering calculations where, for energies above 60 eV, the effects of including a phenomenological absorption potential to account for the loss of flux into the non-elastic channels have been investigated. In general, a comparison of the new experimental results with previous data shows very good agreement, however, sharper structure is observed in some cases in the present results. A comparison of the calculated with experimental results again shows good agreement but also demonstrates that for energies above 100 eV, absorption effects must be taken into account for a detailed description of the STU parameters.

(e, 2e) collisions on xenon with spin-polarized electrons

We present measurements of the (e, 2e) cross sections of xenon for transitions leading to the and states of the residual ion using an incident polarized electron beam in coplanar asymmetric kinematics and in the intermediate energy regime. The results show that the spin up - down asymmetry for the and transitions can be understood as an analogue of the so-called fine-structure effect in inelastic scattering. However, our present measurements also point to significant contributions from relativistic effects in the electron - atom system under certain conditions. The measured spin up - down asymmetry is analysed using the density matrix formalism. This approach reveals that different exchange interactions contribute to the fine-structure effect for ionization. The contribution of relativistic interactions in the observed parameters is also investigated by using a semirelativistic DWBA model to evaluate the scattering amplitudes.

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.

Fully differential molecular-frame electron impact ionization measurements

(e,2e) measurements in the molecular frame using a new spectrometer are described. This work requires a triple coincidence experiment in which (e,2e+ion) events are detected. Preliminary results are presented for a nitrogen target, which illustrate the performance of the apparatus. The nitrogen data clearly shows that the (e,2e) cross section depends sensitively on the orientation of the molecule relative to the incident direction.

Angle-resolving time-of-flight electron spectrometer for near-threshold precision measurements of differential cross sections of electron-impact excitation of atoms and molecules

This article presents a new type of low-energy crossed-beam electron spectrometer for measuring angular differential cross sections of electron-impact excitation of atomic and molecular targets. Designed for investigations at energies close to excitation thresholds, the spectrometer combines a pulsed electron beam with the time-of-flight technique to distinguish between scattering channels. A large-area, position-sensitive detector is used to offset the low average scattering rate resulting from the pulsing duty cycle, without sacrificing angular resolution. A total energy resolution better than 150 meV (full width at half maximum) at scattered energies of 0.5-3 eV is achieved by monochromating the electron beam prior to pulsing it. The results of a precision measurement of the differential cross section for electron-impact excitation of helium, at an energy of 22 eV, are used to assess the sensitivity and resolution of the spectrometer.

A reaction microscope for positron – atom ionisation studies

A reaction microscope that will enable angle-differential positron impact ionisation measurements with atomic and molecular targets, has been recently designed and built to meet the specifications of the Australian Positron Beamline Facility. We present details of this new device, highlighting the modifications to the well-established technique that are required for compatibility with a positron beam, and report on the experimental parameters and detection scheme for the first experiments.

Low energy lepton scattering: recent results for electron and positron interactions

This report will review several new experimental developments for low energy (1-50 eV) scattering of both electrons and positrons from atoms and molecules. Experiments include an apparatus for studies of scattering from unstable molecular radicals, a position- and time-sensitive time-of-flight spectrometer for studies of near threshold electronic excitation of atoms and molecules and a trap-based positron scattering experiment for measurements of low energy positron scattering from rare gas atoms. Where possible, comparisons are also made with other experimental determinations and with contemporary scattering theory.