Don H. Madison

Electron impact excitation of rare gases: differential cross sections and angular correlation parameters for neon, argon, krypton and xenon

Distorted-wave Born approximation results for the differential cross sections and various angular correlation parameters for electron impact excitation of the rare gases Ne, Ar, Kr and Xe are presented and compared with recent experimental data and other theoretical calculations. The sensitivity of the results to different static and optical potentials in the calculation of the distorted waves is analysed and the importance of relativistic effects both in the description of the target states (intermediate coupling) as well as in the wavefunction for the continuum electron is investigated. The overall agreement with the available experimental data is very satisfactory over a wide range of incident-electron energies.

Scattering of Spin-1/2 Particles from Unpolarised Targets

A general method for describing the scattering of polarised spin-1/2 particles from unpolarised targets is presented. The reduced density matrix formalism is used to define a new set of parameters sigma u, SP, SA, Ty, Tx, Tz, Uxz, and Uzx, which contain the maximum possible information about a scattering process, in which the target is unpolarised and only the scattered particles and their polarisation are observed. For the special case of scattering from targets with vanishing total angular momentum, the number of independent variables can be further reduced to the standard set of observables sigma , S, T and U given, for instance, by Kessler (1985). Numerical results for the new parameters are presented for inelastic electron scattering from xenon and mercury atoms.

Angular correlation parameters for 1s-2p excitation of hydrogen calculated in the distorted-wave approximation

An uncoupled distorted-wave formalism is presented. Source parameters necessary for obtaining the coincidence rate of Ly- alpha photons and scattered electrons for electron excitation of the 2p state of hydrogen at 54, 100, and 200 eV are calculated in this formalism. These results are compared with source parameters obtained in other theoretical models.

The sign of the orientation parameter in electron-photon coincidence experiments

Steph et al. (1980) have proposed a model for the orientation parameter which attributes positive transfer of angular momentum to long-range attractive atomic polarisation potentials and negative transfer to electron-electron repulsion. This model is based on a classical grazing picture for the collision and has received considerable attention due to its intuitive appeal. The authors show that quantal calculations reveal that the physical effects of this model are not of primary importance in determining the sign of the orientation parameter.

An exact second-order theory and its application to electron-hydrogen excitation

Recent technological advances have produced increasingly detailed experimental measurements. These results have often revealed problems associated with the standard first-order perturbation series theoretical approaches. For several scattering problems, it is becoming evident that a good second-order theory is needed. However, theoretical calculations of second-order amplitudes typically resort to several simplifying approximations to make the calculation tractable. A new approach is developed which reduces the exact evaluation of a second-order amplitude to a tractable form. The utility of the method is demonstrated through a practical calculation of the exact second-order distorted-wave amplitude for electron excitation of the 2p state of hydrogen. For this case, the second-order results are in significantly better agreement with experimental data than the first-order results.

A second-order distorted-wave model for the excitation of the 21P state of helium by electron and positron impact

The second-order distorted-wave amplitude is evaluated in a high-energy approximation. In this approximation, the non-local second-order distorting potential is replaced by a local second-order potential obtained in the closure approximation. It is shown that for the case of electron excitation of the 21P state of helium, the second-order effects are fairly large and improve agreement with experimental data. It is also noted, however, that the best agreement with experimental data may be obtained through a hybrid first-order calculation. Results for positron excitation of helium are also given and discussed.

Differential cross sections and angular correlation parameters for electron impact excitation of the 33P, 31D and 33D states of helium

Distorted-wave Born approximation results for the differential cross section and various angular correlation parameters for electron impact excitation of the 33P, 31D and 33D states of helium are presented and compared with recent experimental data and other theoretical calculations. The connection between the Stokes parameters, the scattering amplitudes and the transferred angular momentum is given and discussed. The results from different distorted-wave models are given and the accuracy of various theoretical approaches for allowed and forbidden transitions is discussed.

Inelastic Electron Scattering from Mercury Atoms at Intermediate Energies (8-50 eV)

Distorted-wave Born approximation results for the scattering asymmetry for (polarised) electron impact excitation of the (6s6p)1P1 and (6s6p)3P1,2 states of mercury are presented and compared with recent experimental data. The sensitivity of the results to different atomic wavefunctions and static potentials as well as to various optical potentials which simulate electron exchange, absorption and polarisation effects is examined in detail. For the 1P1 and 3P1 states, the influence of intermediate coupling is discussed.

Third-order effects of electron-hydrogen scattering

Recent comparisons of experimental and theoretical angular correlation parameters have revealed that no theoretical calculations are in good agreement with the experimental data. If the scattering is viewed from the perspective of the conventional Born series, it is now clear that second- and perhaps higher-order terms will have to be evaluated before agreement with experiment can be achieved. The authors investigate the size of some third- and higher-order contributions to the Born series. It is shown that these third- and higher-order effects are small.

Theoretical (e, 2e) cross sections for ionisation of the 3s state of argon?

Triple differential cross sections for electron impact ionisation of the 3s shell of argon are examined for incident electron energies between 100 and 400 eV. Theoretical distorted-wave results are presented for different models of the scattering process and comparison is made with existing experimental data. The sensitivity of the theoretical results to atomic wavefunctions, atomic potentials and optical potentials is examined. An attempt to obtain an elementary parametrisation for the cross section is also reported.