D L Moores

Ionisation from the 3p and 3d sublevels of highly charged ions

Coulomb-Born-exchange calculations of the scaled cross sections for ionisation from the 3p and 3d sublevels of hydrogenic ions in the limit Z to infinity are made for eight impact electron energies. In addition, similar calculations are made for ionisation from the lower sublevels considered in earlier work (see ibid., vol.11, p.3235 (1978)). The results are fitted to a simple form which is readily integrated over a Maxwellian distribution function to obtain collisional ionisation rates. The results can be easily used to obtain approximate results for ionisation from the 3d and all lower sublevels of highly charged complex ions with finite Z. Results obtained in this way agree well with the Coulomb-Born-exchange results of Rudge and Schwartz (1966) obtained using the Hartree-Fock target wavefunctions.

Electron impact ionisation of Li-like and Be-like ions

Electron impact ionisation cross sections for positive ions in the Li and Be isoelectronic sequences are calculated in the Coulomb-Born exchange and distorted-wave exchange approximations - using close-coupling wavefunctions to represent the initial and final states of the target system. The corresponding ionisation rate coefficients are also computed as a function of temperature. The results obtained are compared with experiments, with the infinite-Z method of Golden and Sampson (1979) and with the Lotz semiempirical formula. For C, N, O and Ne ions the mutual agreement is better than 25% and in some cases is of the order of a few per cent. For Fe23+ and Fe22+ the Lotz formula gives rates up to 40% higher than the Coulomb-Borm at high temperatures (>108K), indicating that the formula probably gives an inaccurate estimate of the high-energy behaviour of the cross section for these ions.

The ionization of Li+ and Mg+ by electron impact

Cross sections for the single ionization of Li+ and Mg+ by electron impact are calculated in the Coulomb-Born approximation (neglecting exchange), and compared with recent experimental results. In the case of Li+ it is shown that the Bethe-Born approximation is not valid until the incident electron energy is much greater than previously supposed. For Mg+, it is shown that ionization from inner shells provides an important contribution to the cross section. Contributions from autoionization are also shown to be significant but do not introduce large irregularities in the total cross section.

Electron impact ionization of positive ions with configurations 1s22s22pq

Cross sections for ionization of positive ions with configuration 1s22s22pq by electron impact are calculated in the no-exchange Coulomb-Born approximation, contributions from ejection of both 2s and 2p electrons being included. Reduced cross sections for one and twice ionized systems, and for the limiting case of infinite nuclear charge are evaluated, enabling cross sections for other systems to be obtained by interpolation. Comparison with experiment is made where possible, and agreement is good in all cases at high incident energies. In some cases there is evidence that the effects of autoionization, not included in this work could be important at low energies.

The scattering of electrons by potassium atoms

A three-state (4s-4p-3d) close-coupling approximation is applied to the scattering of electrons with energies of up to 5 eV by potassium atoms. Amplitudes for elastic scattering and 4s to 4p excitation have been computed. Good agreement with the behaviour of the elastic differential cross section at fixed angle observed by Eyb and Hofmann (1975) in the immediate vicinity of the first excitation threshold is obtained. The threshold structure is analysed in detail. For elastic scattering below the excitation threshold good agreement is obtained with absolute differential measurements. Above the excitation threshold and in its vicinity however, some differences between the calculations are apparent. Good agreement is obtained except for the case of elastic scattering at 3 eV.

Electron impact ionisation of Li-like and Be-like carbon and nitrogen ions

Electron impact ionisation cross sections of the Li-like ions C IV and N V and the Be-like ions C III and N IV are calculated in a no-exchange Coulomb-Born approximation. Inner-shell ionisation is included but not autoionisation. Comparison with available theoretical and experimental data has been made. In C IV, the results agree with the recent calculations of Golden and Sampson (1977) and with exchange-classical impact-parameter calculations, which are about 30% higher than the crossed-beam experimental results of Crandall and co-workers (1978). In N V the results agree with those of Golden and Sampson but are now in close agreement with Crandall et. al.

Electron impact ionization of highly-charged uranium ions

Cross sections for electron impact ionization from the 1s and 2s shells of He-like, Li-like and Be-like U ions have been calculated by the relativistic distorted wave method at incident electron energies up to five times the ionization energy. Differences between the results obtained using the Moller and Coulomb interactions are attributed to differences in the contribution from interference between direct and exchange ionization. Excellent agreement is obtained between the Moller result and an EBIT experimental result for He-like U at an incident electron energy of 198 keV.

The use of two-centre expansions in the scattering of electrons by N2 molecules

The use of a two-centre prolate spheroidal coordinate system in ab initio calculations of scattering of slow electrons by N2 molecules is studied. The speedier convergence of the resulting multipole expansion of the static potential near the nuclei, compared with that of its one-centre form, is demonstrated. Eigenphase sums calculated using two-centre coordinates also converge more rapidly.

Electron impact excitation of highly charged hydrogenic ions

Electron impact excitation cross sections of the 2s and 2p states of hydrogen-like ions with nuclear charge Z>or=26 have been calculated in a fully relativistic distorted wave approximation. The importance of including the Moller interaction for the more highly charged ions is demonstrated. Excellent agreement, of the order of a few per cent, is obtained with the older calculations of Walker (1975) and results by Fontes (1992).

Scattering of low-energy electrons by neutral lithium atoms: five-state close-coupling calculations

Results of the five-state (2s-2p-3s-3d) close-coupling calculations, using the published program IMPACT, of the scattering of 2-10 eV electrons by neutral lithium atoms are reported. Cross sections for all upward transitions between these five states are given. For the 2s-2p transition, the excitation cross section, the singlet-triplet asymmetry factor and the polarisation of the line radiation are compared with experimental data and with previous two-state close-coupling calculations.