Table of contents

Calculations of oscillator strengths for the 3s²3p(²P⁰) to 3s3p²(²S^e) and 3s²3p(²P⁰) to 3s3p²(²D^e) transitions of Al I have been carried out. Variational atomic Bethe-Goldstone calculations reported here give a very good account of the empirical excitation energy. Oscillator strengths are computed using both length and velocity formulae. The agreement between these two formulae and with experiment is excellent. The results are compared with previous calculations which introduced electronic correlations by configuration interaction. The present calculations indicate that, when electronic correlations are properly introduced, length and velocity formulae give equivalent results.

The 584.3 AA photoelectron spectrum of Ba has been measured in a crossed-beam experiment with about 30 meV energy resolution using a 127 degrees cylindrical analyser. In the single-ionization part of the spectrum, population of Ba⁺ ions in the states 6s, 5d, 6p, 7s, 6d, 4f, 7p, 5f, 8s, 7d, 8p and 11s is clearly detected, the double-ionization spectrum mainly consists of three lines ('natural' widths <15 meV) at 5.935(4), 5.852(4) and 5.798(4) eV together with their detected counterparts at 0.067, 0.150 and 0.204 eV. The (lowest) 5p ionization potential is deduced to be 21.014(4) eV.

The classical trajectory method has been used for investigating electron-hydrogen-atom excitation and ionization regions close to the ionization threshold. Final energy and angular momentum distribution functions have been computed with the total angular momentum equal to zero, both below and above the ionization threshold. Combining the analytical result of Vinkalns and Gailitis (1967) with the computed energy distribution function, the threshold behaviour of the high n excitation function has been derived, which is subsequently confirmed by ab initio numerical calculations. The results show that the Wannier theory (1953) is applicable below the threshold, too. The numerical results show that each electron acquires large angular momentum in the threshold vicinity, as predicted by Fano (1974).

Total elastic cross sections and differential elastic cross sections were calculated by the method of partial waves for very low energy (2-10 eV) electrons incident on Li. The spin polarization P was found to be less than 10^-4 at all angles and energies. The total cross section was found to be insensitive to the exact form of the potential at less than five Bohr radii. The partial-wave results were in much better agreement with experimental values than the corresponding first Born and eikonal predictions.

A model for K-vacancy production in ion-atom collisions is proposed. The model represents a simplification of the work of Briggs and Macek (1972) for symmetric collisions. It is based on a perturbed united-atom approach and applied also to asymmetric collisions. The agreement with experiment is as good as for the Briggs-Macek theory for Ne⁺-Ne collisions.

The Ne K Auger electron cross sections for N^4+,5+,6+,7+ bombardment at 14 and 19 MeV energy are measured. Ne K fluorescence yields are derived using the reported Auger electron cross sections combined with previous X-ray cross section measurements. These fluorescence yields are compared with semi-empirical yields derived from high resolution X-ray spectra.

It is shown that several relationships valid for exact or variational-perturbation theories hold also, under quite general conditions, for much simpler (local potential) theories based on variations of an ad hoc functional. Equations of motion, formulae for the f-sum rule, frequency-dependent polarizability- and shielding-tensors are derived. Scaling procedures are discussed. Numerical examples, with a Slater exchange potential, are given. It is shown that the use of a cut-off potential, as in previous calculations, is detrimental to the optimization procedure.

The construction of model Hamiltonians for atoms with a potential use in molecular theory is examined. The Dirac-van Vleck vector model is expressed in second quantization and energy expressions are related to central field orbitals obtained from an approximation formalism for open shells defined through an ensemble average construction. Term splittings and ionization energies are studied for atoms from the second and third period.

The Coulomb-projected Born approximation proposed by Geltman (1971) has been generalized by allowing only part of the nuclear charge to act directly on the incident particle. Analytic expressions are derived for elastic scattering cross sections. Results are presented for the 1s-1s, 1s-2s and 1s-2p transitions in H and the 1s-2s transitions in He⁺. Various criteria for the energy at which the Born approximation becomes valid are discussed.

The results of a pseudostate expansion for low-order partial waves (0<or=L<or=3) are combined with the distorted-wave polarized-orbital method for higher partial waves in order to obtain cross sections for excitation of the 2s and 2p levels of atomic hydrogen by electron impact for incident energies in the range 10-30 eV. Both total and differential cross sections are calculated. Results are also presented for Ly alpha polarization and for the total cross section for all scattering processes.

For pt.II see ibid., vol.8, no.10, p.1683 (1975). Absolute differential elastic cross sections have been measured for the scattering of electrons from atomic hydrogen over the electron energy range from 20 to 680 eV and over the angular range from 10 to 140 degrees. The experimental techniques and method are those associated with crossed electron and modulated atom beams with momentum analysis of the incident and scattered electrons. The measured relative angular distributions were made absolute within 11% by means of the following sequence of measurements, (i) a phaseshift analysis around the (1s2s²)²S resonance elastic scattering in helium, (ii) relative beam densities of helium and hydrogen beams and (iii) absolute calibration of all apparatus functions.

The total wavefunction (target plus incident electron) is represented by a close-coupling expansion. For complex atoms such as atomic helium and oxygen, the effects of configuration-interaction target states are discussed and analysed, in part. For atomic helium, the ²S partial-wave elastic cross section obtained using a He configuration-interaction ground state is virtually identical with the corresponding cross section obtained using a Roothaan-Hartree-Fock ground state. For atomic oxygen, the ⁴P partial-wave elastic phaseshift can increase to such an extent as to cause the ⁴P partial-wave cross section to go through zero at k² approximately=0.2 Ryd. Finally, a second procedure is discussed for including correlation states when CI target states are used in the close-coupling expansion.

Cross sections for the excitation of bound states of atoms by electron impact have been found to contain structures at incident energies near to the energies of short-lived autoionizing states of the target atoms. These structures look superficially like resonances, but in fact they seem to be caused by a post-collision interaction between inelastically scattered and ejected electrons. Examples are given of these structures in He, Ne and Ar. A quantum-mechanical model, which is termed 'shake-down', is advanced to explain these structures and comparison is made with the experimental data.

It is pointed out that with some additional assumptions the Barker and Berry model (1966) for post-collision interaction between the scattered electron and an electron ejected from an autoionizing state predicts the profile of the structure on excitation functions and in the spectra of the ejected electrons. The profiles are affected by the fact that in the experiments these profiles are measured at a constant value of the final energy of the scattered electron. Several features of the predicted profiles are confirmed by experiment. The remaining deviations between the model predictions and the experimental results display the inadequacy of a classical description of electrons at low energy.

For pt.I see ibid., vol.8, no.10, p.1670 (1975). Measurements of the absolute differential cross sections for the elastic scattering of electrons from neon, krypton and xenon for a wide range of electron energies and scattering angles and with high angular resolution are presented. The range of electron energies is 20 to 400 eV, the range of scattering angles from 20 to 150 degrees and the angular resolution of both the incident and scattered electron beams is 1.5 degrees. The experimental method is based on the crossed electron and modulated atom beams techniques. Measured angular distributions have been made absolute by carrying out phase shift analyses over the 19.35 eV (1s2s²)²S resonant state of helium and the 11.10 and 11.27 eV (3s²3p⁵4s²), ²P_1/2, ²P_3/2 resonance doublet in argon coupled with accurate determinations of the beam density ratios of neon, krypton and xenon to helium and argon. The absolute differential cross sections are not consistently in agreement with any previous absolute determinations.

The impact parameter version of the classical finite characteristic collision time approximation of Bates and Snyder (1973) is formulated and applied to H(1s)-H⁺ ionizing collisions. The results are in only modest agreement with those obtained by the Born approximation. It is possible that this is partly because the angular momentum (which is important for ionization probabilities though not for ionization cross sections) is incorrectly described in the classical model of the H(1s) atom.

Cross sections for intermultiplet transitions in Li(2p²P)-He(1¹S) collisions are calculated for impact energies E between 6*10^-5 eV and 6*10^-2 eV by a close-coupled wave treatment. A special study is made of the onset of spin uncoupling as E is increased. The total cross sections indicate that the spin-uncoupled limit (in which the spin of the valence electron is unaffected by the collision) is reached at 3*10^-4 eV. However partial-wave analysis indicates that there are significant departures from the limit up to 3*10^-3 eV. A correlation between peaks in the calculated cross sections and resonances in elastic scattering by the ² Pi molecular potential is studied, and is interpreted as showing the essentially molecular nature of the collision.

The broadening and shift of the caesium 4593 AA and 4555 AA lines have been measured when the lines are formed in absorption at 400K under pressures of Ne, Ar and Xe up to 150 Torr. Neon-broadened profiles were symmetrical over the entire pressure range whereas argon- and xenon-broadened profiles became asymmetric as the pressure increased. These observations are shown to be compatible with a collision theory which includes radiation during collisions. Compared to the 4555 AA line, the 4593 AA line shows slightly greater broadening and a significantly smaller shift. Combination of the present results with data obtained by other workers for the 4593 AA line at 4500K reveals a different temperature dependence for argon- as compared to neon-broadening.

The rotational excitation cross sections for Delta j=+or-1 and +or-2 transitions for HD⁺ by electron and positron impact have been calculated by the semi-classical first-order time-dependent perturbation theory. The trajectory is assumed to be determined by the spherical Coulomb interaction and the transitions are induced by the various non-spherical terms arising from the dipole moment, quadrupole moment and anisotropy of polarizability of HD⁺. The energy ranges covered are 0.04 eV to 5 eV for electron-HD⁺ and 0.1 eV to 5 eV for positron-HD⁺ collisions. For electron-HD⁺ collisions the cross section for rotational excitation decreases with the increase of energy. The reverse is the case of positron-HD⁺ collisions.

Inelastic cross sections for rotational transitions in He-TlF collisions have been calculated in the semi-classical strong-coupling approximation for the initial rotational states j₀=0, 1 and 2. Since He-TlF represents a typical strongly polar, heavy diatom-atom interaction, the use of a realistic potential function which included the various long- and short-range anisotropies was necessary. The theoretical cross sections Q_{2,0 to 3,0} for small-angle scattering were of the same order of magnitude as the measured values, the significant contribution to these cross sections coming from the dominant coupling region of small impact parameters.

Emission spectra in the range 10-250 AA from plasmas produced by picosecond laser pulses from a variety of solid targets are presented. These spectra provide some useful information about the state of ionization of the plasma and other plasma parameters such as temperature. Some of the results obtained are compared with the predictions of detailed numerical calculations. Ionization in these plasmas proceeds rapidly before reaching a limit. Depending on the laser energy input this limit generally occurs at a closed shell where further ionization is inhibited by the large jumps in the binding energy of the closed-shell electrons.