M R C McDowell

Electron impact excitation of H and He+. I. 1s to ns transitions

A polarized orbital distorted wave model is used to calculate total and differential cross sections for 1s to ns(n=2,3,4,5) electron impact induced transitions in H and He+ from threshold to 20 Ryd. The total cross sections for the 1s to 2s transitions are in good agreement with experiment, but cannot in this model show resonances. The differential cross sections at energies above 100 eV agree closely with those of Geltman and Hidalgo (1971). The model reduces to the Born-Oppenheimer (z=0) or CBO I(z=0) when the polarization and static potentials are omitted.

Elastic scattering of slow electrons by helium

The optical potential model of Pu and Chang (1966) is extended to include EPV diagrams. Calculations are carried out for the s, p and d wave phase shifts at energies up to 17 eV. The results for s and p waves are in close agreement with those of other recent calculations and, with a phase shift analysis of experimental data. The d wave results lie between the full polarized orbital and the Green function expansion results. It is shown that third order diagrams are required for an accurate evaluation of the static polarizability, yielding alpha =1.372 a03, and estimates are made of the third order corrections to the d-wave phase shift, which brings the optical potential values into good agreement with the full polarized orbital results.

Positron collisions with helium at intermediate energies

Calculations are reported of total and differential cross sections for elastic scattering and excitation of the 11S to 21S and 11S to 21P transitions in helium by positrons with energies between 60 and 400 eV. Results are given in three-state close-coupling, Born, Glauber, eikonal Born series, optical model and a distorted-wave Born model. They are compared with electron impact results. Excitation cross sections are summed and an estimate of the ionisation cross section derived from the total cross section measurements of Twomey et al. Cross sections for 21S to 21P and coincidence parameters for 11S to 21P transitions at 80 eV are briefly discussed.

Electron impact excitation of H and He+. IV. Orientation and alignment of the 2p state

For pt.III see ibid., vol.8, no.7, p.1053 (1975). Orientation and alignment effects observable in coincidence measurements of Ly alpha photons and scattered electrons are discussed. Results, in the DWPO II model are presented for the Fano-Macek parameters. Computer generated maps of the coincidence rate as a function of scattering electron angle and photon emission angle are presented at two energies, and show interesting, energy-dependent, symmetries.

Electron impact excitation of hydrogenic systems in a distorted-wave model

A generalisation of the two-potential distorted-wave model of Shelton and others (1973, 1976) is presented which allows more fully for exchange. It is applied to excitation of the n=2 levels of H and He+, the results being compared with earlier theoretical work and with experiment.

Electron impact excitation of one- and two-electron positive ions

The distorted-wave polarised-orbital (DWPO) model is applied to the evaluation of electron impact excitation cross sections for the lowest s to s and s to p transitions in one-electron positive ions (2<or=Z<or=10) and in helium-like ions (3<or=Z<or=10 and Si12+, Ca18+ and Fe24+). The results for He+(2P) are compared with experiment, and the other results with available theoretical models, from excitation threshold to high energies.

Laser-assisted electron impact ionisation of helium at 256.5 eV

Reports calculations on the triple differential ionisation cross section for the above process for laser energies of 0.12 and 1.17 eV. The two cases considered are when the laser field is (i) perpendicular to the momentum transfer, and (ii) parallel to it. The fast electrons are described by plane waves while the target initial and final states are described in 1s, 2s, 2p close coupling. The results are shown to be determined by the behaviour of the Bessel functions occurring in a Kroll-Watson-type formula.

Electron impact excitation of the 21P and 23S levels of helium

A distorted-wave model is used to calculate electron impact excitation of the 21P and 23S states of helium from the ground state at energies from 30 to 200 eV. Total and differential cross sections are calculated and compared with the results of other workers and with experiment. Similar comparisons are made with recent absolute measurements for lambda (21P) and mod chi (21P) mod . The discrepancy between theory and experiment for the total 12S to 23S cross section at energies above 100 eV is discussed and an explanation proposed.

Electron impact excitation of the n1P(n=2-5) and 23P states of helium at intermediate energies

Calculations are reported on the electron impact excitation of the n1P (n=2,3,4,5) states of He from the ground state. The incident energy range covered is from threshold to 500 eV, and the calculations are carried out in the distorted-wave polarized-orbital model. Total cross sections for all n<or=5, total differential cross sections differential cross sections for individual magnetic sub-levels, and orientation and alignment parameters for n=2,3 are presented and compared with other theoretical studies and with experiment. Excitation of He (23P) is briefly discussed.

Electron impact excitation n1S and n3S states of He at intermediate energies

Total and differential cross sections are calculated for electron impact excitation of the 21S, 23S and 31S states of helium in a distorted wave model at impact energies between 30 and 300 eV. The results are compared with other theoretical models and with recent absolute experimental measurements.