R. J. Allan

Triple differential cross sections for the electron-impact ionization of argon and neon

Cross sections have been measured for the electron-impact ionization of argon and neon in a coplanar symmetric geometry. Results are presented for impact energies of 115.8, 85.8 and 50 eV for argon and 50 eV for neon. A comparison is made with distorted-wave Born approximation calculations which include initial channel polarization and final channel post-collisional interactions. The lowest-energy argon data appear to indicate that polarization effects are very significant, and, in order to explore this, additional measurements have been performed in the coplanar constant geometry.

(e,2e), effective charges, distorted waves and all that!

An overview of the theory of (e,2e) processes is presented. Effective charges are introduced and the Peterkop relation discussed. The distored wave Born approximation is considered and applied to the calculation of triple differential cross-sections. A derivation of the distorted wave impulse approximation is given and the difficulties encountered in the choice of off-shell Coulomb T-matrix highlighted. It is shown that 3 body effects are significant in both the initial and final channels for the ionisation of He at energies of 50eV and below.

Electron-helium scattering in a Nd-YAG laser field at collision energies near the He(1s2l) thresholds

The R-matrix Floquet method is used to study electron-helium scattering in a Nd-YAG laser in the energy range from 0.65 to 0.78 Hartree. The effect of strong AC Stark mixing between the 1s2s 3S and 1s2p 3Po states is discussed. Integrated cross sections for elastic scattering and excitation from the ground state into the triplet states are presented. The He-(1s2s2 2S) resonance is shown to be a dominant feature below the field-free threshold.

(e, 2e) on hydrogen minus,

For the first time theoretical predictions for (e, 2e) on a hydrogen minus, , target are presented. This study shows that the size and shape of the TDCS for this process is extremely sensitive to the form of the approximation used for the wavefunction. In particular, the TDCS is enormously model sensitive in the Bethe ridge region (when the recoil ion momentum is zero).

processes with positive-ion targets in coplanar symmetric geometry

We present results for the electron impact ionization of a sequence of hydrogenic ions and the first five ions in the He isoelectronic sequence. These are obtained in coplanar symmetric geometry for a wide range of energies, from near threshold into the intermediate range. We have concentrated the analysis on the role of the ion in the incident and final channel. All calculations are performed in a distorted-wave Born approximation (DWBA) which allows for elastic scattering in both the incident and final channels. The role of repulsion between the final state electrons is discussed in a simple model. Comparisons are also shown with DWBA calculations that include a polarization potential, and with a Coulomb-Born approximation.

An Explanation of the Structure Observed in Out-Of-Plane Symmetric Measurements on Helium

The study of (e,2e) processes on Helium at impact energies of 100eV and below has yielded a wealth of experimental data and has contributed greatly to the understanding of the Coulomb few body problem. The great advantage of coincidence measurements is that one can manipulate the geometry of the experiment to study delicate processes which yield striking structures in a particular arrangement, but are largely masked by stronger effects in virtually all other setups. Quite recently Murray1 identified a very strong structure in the ionization of Helium at an impact energy of 64.6eV in a very specific out of plane geometry. In this paper we will show that this structure is well represented in a Distorted Wave Born Approximation (DWBA) calculation and we will interpret it as an interference, that is to say a pure quantum, effect. We predict that similar structures will be present in the Triple Differential Cross Section (TDCS) for some targets but not for others.

The Normalisation of the Experimental Triple Differential Cross Section of Noble Gas Atoms in Extreme Asymmetric Geometry

Electron-electron coincidence experiments, usually known as (e,2e) experiments, are the most powerful tool to study the process of the electron impact ionizationl,2. Despite the extended activity in the field in the last years, the understanding of the process for targets heavier than H and He still presents many serious challenges for both, theoreticians and experimentalists alike. As far as the experiments are concerned, the challenge consists in (a) providing a large body of experimental data to explore the various zones of the Bethe surface3, (b) determining the absolute scale of the triple differential cross-section (TDCS).

Exact versus local exchange in distorted-wave Born calculations of electron impact ionisation

The distorted-wave Born approximation (DWBA) is applied to the electron impact ionization of the ground state of hydrogen and a comparison is made between calculations of the triple differential cross section (TDCS) performed using the exact non-local exchange integral and a local potential exchange approximation. The exact exchange calculations are in slightly better agreement with experiment than the local exchange approximation calculations, retaining more structure as the energy approaches threshold. However the general conclusion of the earlier work is essentially unaltered - one needs to include both the effects of polarization in the incident channel and post-collisional Coulombic interactions between the three charged particles for a good agreement with experiment. Atomic units are used ħ = me = e = ao = 1.

TRIPLE DIFFERENTIAL CROSS SECTIONS FOR THE ELECTRON IMPACT IONIZATION OF HELIUM, NEON AND ARGON FROM 0.1 TO 1 KEV. THEORY AND EXPERIMENT COMPARED

The triple differential cross section (TDCS) for electron impact ionization of atoms is proportional to the momentum density of the ejected electron under conditions when the colliding electrons can be considered to be free and the interactions between the incident, scattered and ejected electrons with the residual ion can be neglected. These are the conditions of the plane wave impulse approximation (PWIA). Under such circumstances, the measured “momentum densities” can be used to investigate single electron atomic wave functions and correlate these with the chemical and physical properties of the atoms. For these studies to be meaningful, the data should have a relative precision of 5 to 10%, (see for example the discussion of experimental precision in (e, 2e) collisions by Moore et al.1) To attain this degree of precision at incident electron energies of 1 keV or greater, the typical energies for such experiments, requires data acquisition times of several hours to days with current technologies. An alternative is to decrease the incident energy in order to increase the absolute value of the TDCS. While this is an attractive alternative, it is essential to recognize that as the incident energy is decreased the interactions between the incident, scattered and ejected electrons and the residual ion become relatively more important, until the point is reached where the TDCS is not even approximately proportional to the single electron momentum density.

Absolute Experimental Cross Sections for Symmetric Coplanar (e,2e) Collisions of 45 to 500 eV Electrons With Helium

Electron impact ionization of helium in a symmetric coplanar energy-sharing geometry at 45° angle is investigated in the incident energy range 45 eV to 500 eV. The results of coincidence experiments are put on an absolute scale by normalisation to known elastic scattering cross sections. Comparison is made with calculations in the plane-wave and distorted-wave Born and Impulse approximations.