Table of contents

A simple yet accurate technique is described for fitting Buttle corrections as functions of the energy.

Many-body perturbation theory has been applied to compute accurate g_J factors for the ground states of Al, Si, P, S and Cl. The effects considered are the relativistic (Abragam-Van Vleck) corrections to the Lande g factors, and the computations are complete to third order in the perturbation expansion. In the case of Cl, agreement between theory and experiment was obtained to about the experimental accuracy of 2 p.p.m., whereas an irreconcilable discrepancy between experiment and theory still exists for Al and Si.

Multiphoton ionisation of xenon is treated by a theory involving Volkov final states. Comparisons are made with recent above-threshold-ionisation experiments using circularly polarised radiation. Qualitative agreement is very good, especially at higher field intensities and photon orders.

A beam-gas method is used to measure cross sections, Q, for ion-pair production in collisions of Li-Na and Li-Na* in a projectile energy range from 1000 to 5500 eV. The Na*, which represents Na in the 3p²P_3/2 resonance state, is generated with a laser. For the same relative kinetic energy of the reactants, the Q for producing a given set of ionic products is larger for Li-Na* than for Li-Na collisions. Qualitative explanations are given which show why laser excitation of Na is responsible for this.

Differential cross section for the double charge transfer process in C⁴⁺-He collisions are studied using a quantal two-channel molecular orbital close coupling expansion method. The calculated results for C⁴⁺ at incident energies of 1520 and 500 eV are found to be in good agreement with recent measurements. It is also shown that the quantal calculations and the experimental results are well reproduced by a semiclassical approximation. Comparison with the recent semi-empirical study of the same system by Barany et al. (1986) is also discussed.

Using positron-gas collisions in a short scattering cell it is demonstrated that, at certain impact energies, approximately 4% of the scattered positrons can be detected as o-Ps atoms collimated in a 6 degrees cone about the incident positron direction.

The momentum distribution of positronium (Ps) in the free space between silica aerogel grains filled with gaseous O₂ has been measured as a function of O₂ pressure. It reveals that para-Ps with kinetic energy >or approximately=1 eV is efficiently slowed down. This is attributed to successive triplet to or from singlet conversions of the Ps through inelastic collisions with O₂. The cross section for this process is estimated to be at least two orders of magnitude larger than that for the conversion through elastic collisions.

The authors reconsider the analytic expression of classical electrodynamics for the bremsstrahlung intensity component I_y in the case of a Coulomb field. They show that I_y, known in the literature as a parametric integral, can be expressed compactly with a Hankel function and its derivatives with respect to variable and order or, alternatively, with hypergeometric functions. The authors express the total intensity I_t in the same manner. The new analytic expressions of I_y and I_t are used for precise tabulations of the spectrum sigma and the particle parameter a₂, the two quantities relevant for classical dipole approximation of the bremsstrahlung angular distributions. Two simple analytic approximate formulae are found to satisfactorily describe the majority of the angular distributions. A comparison with the quantum results is also presented.

The integral I= integral Y_lm(P-hat)(A+2L.P)^-ivd Omega _p is evaluated analytically in terms of Gauss hypergeometric functions and the result has been checked by comparison with the known results corresponding to v=0, +or-i. Two applications of this result are given: (i) analytic evaluation of the zeroth-order strong potential Born amplitude for electron capture into arbitrary hydrogenic states, and (ii) expansion of (P-K)^2iv-2 in spherical harmonics and evaluation of a partial-wave expansion of Coulomb continuum wavefunctions in momentum space. By analytic continuation it has been shown that the radial part of the partial-wave expansion in case (ii) gives the corresponding bound-state wavefunction in momentum space obtained by Podolsky and Pauling (1929).

The (np)^-1 and (ns)^-1 photoelectron spectra of the alkali atoms were calculated using a CI restricted to a few quasi-degenerate configurations and the more extended 2h-1p CI. A satisfactory description of the spectra is obtained with only the minimal CI calculation, although the results deteriorate for K in the (np)^-1 case. Different correlation effects for the two ionisations are pointed out. No significant improvement is obtained with the 2h-1p CI, which confirms the dominance of the quasi-degeneracy effects already included, suggesting, however, the importance of other kinds of correlation.

Intra- and inter-shell correlation effects have been analysed in detail for a series of Li-like ions in their first excited state. This (1s²2p alpha )²P state was described at the correlated and Hartree-Fock levels by the wavefunctions of Weiss. Coulomb holes and partial 'holes' were determined in position space for the individual electronic shells and, for the K alpha K beta shell, the authors also evaluated several statistical correlation coefficients. For comparability with the ²S ground state, the discussion centred mainly on Li. When comparing different states, the partial Coulomb holes were particularly useful in providing insight into the changing characteristics of electron correlation within a specific shell. It was found that excitation of the outer electron not only influenced the correlation effects in the K alpha L alpha and K beta L alpha inter-shells but, equally noteworthy, a marked shift occurred in the radial and angular components of correlation in the intra-shell. The analysis was also capable of highlighting a weakness in the correlated description of the ²P state inter-shells.

By taking the actual chemical environment explicitly into account in modelling the electron relaxation and the cascading de-excitation of a multiply ionised Si atom, as a representative example of a heavy-ion-bombarded target atom, these post-collision processes are found to have a universal non-discriminating nature provided that the valence electron supply of neighbouring atoms does not curtail the compensating interatomic valence charge transfer. This finding is in obvious contradiction to the picture commonly presupposed in the literature, according to which the massive electron rearrangement occurring prior to X-ray emission fully accounts for the pronounced chemical effects on heavy-ion-induced X-ray satellite spectra. Thus one is led to reconsider a modification of the primary multiple-vacancy distribution through its projectile charge state dependence. The correlation of the effects with bulk chemical properties is now explained by the influence of the average valence electron density on the equilibrium charge state of energetic ions travelling in solids.

Using a well known semiclassical theory the authors calculated the width and shift of the Na D lines as well as the cross sections for fine-structure mixing and depolarisation on the basis of the Na-Xe potentials from Czuchaj and Sienkiewicz (1979). The results of this model were found to be in good agreement with the fully quantal values obtained by de Vries for this system. The authors discuss possible sources of error in the cross sections due to this physical model, the adopted interaction potentials and the numerical calculations. The broadening, shift and asymmetry of the Na D lines due to collisions with Ne and Xe atoms at 720 and 990 K were measured in a fluorescence cell. From the authors experimental data and results available in the literature on broadening, shift, fine structure and depolarisation, the authors evaluated several Na-Ne, Xe interaction potentials with this semiclassical model. They conclude that the best Na-Ne potentials in the internuclear region between about 5 and 13 AA are given by the X Sigma potential of Masnou-Seeuws et al and the A Sigma and B Sigma of Peach (1982). For Na-Xe the situation in the internuclear region between about 6 and 18AA is less satisfactory. Although the X Sigma potential is well known (Van Deventer and van der Valk), the A Sigma and B Sigma potentials that are currently available in the literature could not fully reproduce the experimental cross sections. However, a best set of potentials is given.

For pt.I see ibid., vol.20, p.487 (1987). The far wings of the Na D lines, perturbed by Ne and Xe atoms in a fluorescence cell at 990 K, were obtained using the fluorescence excitation method. The authors measured the red wing of the Na D₁ line and the blue wing of the Na D₂ line at detunings between 2.5 and 350 AA from the line centres. The wings were normalised by making use of the authors previously determined line core profiles. Existing potentials for the Na-Ne, Xe system were re-examined by applying the quasistatic theory for calculating the far-wing spectra from the potentials. In a separate section the authors arrive at a conclusion about the most reliable X Sigma , A Pi and B Sigma potentials in these collisional systems.

Excitation transfer between 6s6d Hg fine-structure states, induced by collisions with Hg, Ar and N₂, was studied using methods of fluorescence spectroscopy in a vapour cell. Hg vapour, pure or mixed with Ar or N₂, was excited to the 6¹D₂ or 6³D₂ state by two-photon absorption and the relative intensities of the fluorescence components emitted from the radiatively and collisionally populated states were determined using a monochromator and a photomultiplier. Measurements of the fluorescent intensity ratios in relation to the density of the collision partner yielded the total thermally averaged cross sections for the transfers ¹D₂ to ³D₂, ¹D₂ to ³D₃³D₂ to ³D₃, and ³D₂ to ¹D₂, with Hg, Ar and N₂ as collision partners.

Capture cross sections from H(2s) by protons were calculated in the intermediate energy region from 1 to 200 keV. Two methods were applied: the close coupling of atomic states with translation factors, and the classical trajectory Monte Carlo method. Detailed comparisons between these two methods are presented. The theories fill the gap between the molecular treatment at low energies and high-energy approximations.

For pt.I see ibid., vol.16, p.4427 (1983). Simplified versions of the second-order Born and Faddeev-Watson approximations are applied to the charge transfer process. The theories are compared with experimental measurements of the differential cross sections for the capture of electrons from helium by protons with energies 2.82, 5.42 and 7.40 MeV. The author's results are consistent with previous work on the second-order Faddeev-Watson approximation by Alton and on the free-particle second Born by McGuire et al. confirming that the former approximation gives a better description of the Thomas peak than does the latter.

The derivative electron transmission spectrum is presented for gaseous TiCl₄ between 0 and 10 eV. The spectral features are consistent with a high scattering cross section just above threshold and with a weak shoulder in the cross section around 6 eV. Multiple-scattering X_alpha molecular orbital calculations establish that addition of an electron to the e or t₂ crystal field orbital gives a stable anion. Calculated vertical electron affinities for these orbitals are in good agreement with experimental values from studies of donor-acceptor complexes. Addition of an electron to any of the higher-energy sigma * orbitals is calculated to yield an unstable negative ion. Multiple-scattering X alpha continuum calculations show very large elastic scattering cross sections just above threshold, near the energies calculated for these sigma * orbitals. The calculated cross section shows a weak local maximum at about 7 eV. Correspondences are drawn between the high stability of the crystal-field orbital-occupied anions and the high ultraviolet and X-ray term energies of these orbitals.

Cross sections are calculated for the scattering of slow electrons by the methane molecule. A parameter-free treatment of the interaction is employed, whereby the static, exchange and polarisation contributions are obtained from an ab initio SCF wavefunction expanded over the heavy atom (OCE wavefunction), making use of the electron gas model for the (N+1)-electron system. Functional forms of exchange and polarisation interactions are examined and discussed in order to assess their relative reliability and importance over the whole range of computed collision energies. Except in the low-energy regions (E<2 eV) the present parameter-free model gives satisfactory qualitative agreement with recent measurements and other calculations. Excitation processes are also considered and rotationally inelastic differential cross sections at 10 eV compare favourably with recent measurements and other calculations.

With the aid of translational energy spectroscopy the authors have studied the double ionisation of Ar by single electron impact at collision energies between threshold and 600 eV. On the one hand the authors have determined the total cross section for double ionisation, sigma _t, and on the other hand they have measured, for the first time, apparent partial cross sections for production of Ar²⁺ in specific long-lived states: ³P, ¹D₂, ¹S₀, ⁵D^degrees ₄ and ³F^degrees ₄. The experimental setup, the principle of the applied method and the evaluation procedure are described. Where possible the results are compared with other experimental results, yielding rather good agreement.

Relative ionisation cross section of Li₂ molecules, excited by optical pumping into vibrational quantum states up to v"=21, have been measured in a molecular-beam laser experiment as a function of the electron impact energy. The vibrational distribution of the excited Li₂ beam is fully characterised by an experimentally tested exact calculation based on the known transition dipole moment of Li₂. The experimental results can be largely explained by a simple photoionisation model, which indicates a maximum increase of the ionisation cross section of about 70% for v"=20 at an impact energy of 12 eV. The best fit requires an additional 5% increase in the cross section at v"=20, which is attributed to a dynamic effect not predicted by the model.

The problem of a two-level atom interacting with an external, time-dependent field is solved approximately using Light's perturbation theory. The solutions obtained are compared with accurate numerical solutions for three specific time profiles. The agreement is found to be excellent except for a certain range of detunings, and even in this range the agreement is good. The validity of an adiabatic approximation is also assessed.

Collective effects in resonant Raman scattering in the presence of intense incident and scattered light waves are investigated. The analytical expressions for the steady-state spectrum and normalised intensity correlation functions for components of the spectra are given. In the cooperative limit N to infinity the relative intensities and normalised intensity correlation functions for each component of the spectra have a discontinuous behaviour reminiscent of a typical non-equilibrium first-order phase transition.