Table of contents

The authors present some sample results for the problem of the hydrogen atom in a magnetic field. The energies have been computed for a typical Rydberg situation of atomic physics interest using limited computer facilities. The use of the Coulomb dynamical group allows a complete description of the symmetries and a rational choice of a Sturmian type basis set. Moreover, comparison with Rayleigh-Schrodinger perturbative expansions of the energies is performed.

Measurement of shifts in the neon Lyman- alpha , - beta and - gamma lines and the He-like 1s²-1s2p line have been obtained in laser-compressed plasmas at n_e approximately 3*10²² cm-³ and T_e approximately 300 eV. Calibration of the neon line wavelengths was determined relative to the emission lines of higher Z elements present in the microballoon glass shell. A trend towards larger red shifts in the wavelengths of the lower neon Lyman series members was observed, and the neon He- alpha line was observed to be blue shifted. These trends are shown not to be the result of refractive index corrections to the dispersion of the crystal spectrometer used, and are unlikely to be the result of differential spatial or Doppler effects between the neon and the calibration lines. Assuming that the measured red shift of about 10 mAA for neon Lyman- alpha is an indication of the magnitude of the absolute shift, then the observed shift is an order of magnitude greater than predicted by current plasma polarisation shift theories at the plasma densities and temperatures obtained in the present experiment.

The emission spectrum of the GeCl molecule has been examined in the visible region between 400 and 640 nm where the A² Sigma ⁺-X² Pi _r system has been reanalysed vibrationally. Constants (cm^-1) for the A state are found to be T_e=21450.7, omega _e=250.2, omega _ex_e=0.73, those for the X² Pi _1/2 state are found to be omega _e=405.5, omega _ex_e=1.27 and those for the X² Pi _3/2 state are found to be omega _e=407.1, omega _ex_e=1.32.

Global charge transfer plus ionisation cross sections in ion-atom collisions are calculated in a quantum statistical time-dependent mean-field theory. Results from a semiclassical model based on the Vlasov equation for the C⁶⁺ and O⁸⁺ on Ne scattering systems at 1 MeV amu^-1 are compared with experiment.

An algorithm is presented for evaluating transition matrix elements between hydrogenic or hydrogen-like partial waves for arbitrary multiple orders. In general, the result can be represented by a sum of Appell's F₂ functions of two variables (1926), which reduces to a polynomial when at least one of the states is bound. To illustrate the method and study the convergence in partial waves, the authors present calculations of the total cross section for electron impact ionisation of atomic hydrogen.

Negative ion resonance formation in molecular hydrogen is investigated by monitoring the production of metastables in the c³ Pi _u ( nu =0) state and in the photon decay of the B¹ Sigma _u⁺ level for incident electron energies of 11-14.5 eV.

A VUV polarisation analyser is employed in an electron-polarised-photon coincidence experiment to determine circular/linear polarisation correlations from the differential electron impact excitation of the 2¹P state of helium. Circular polarisation correlations are presented for electron scattering angles in the range of 20-90 degrees and incident electron energies of 50, 60 and 80 eV. The sign of the orientation of the excited state was unambiguously determined in each case, thus resolving uncertainties which excited prior to this work.

For pt.I see ibid., vol.18, no.4, p.589 (1985). The transition rates for transitions among the 3s and 3p excited fine-structure levels of neutral neon are calculated and compared with experimental observations. The method of calculation involves accurate non-relativistic calculation of the wavefunctions in LS coupling followed by diagonalisation of the Breit-Pauli Hamiltonian to yield the fine structure. Particularly in the case of the 3p levels, very slight errors in the non-relativistic energies can lead to very significant errors in the relativistic level-mixing coefficients with consequently large errors in the transition rates. A method for correcting energy levels, based on inverting the eigenvalue problem using the eigenvalues (the observed level energies) as known input, allows accurate determination of level-mixing coefficients, and hence transition rates, in a test case involving levels that are strong mixtures of three terms. The paper extends earlier work by demonstrating that the method remains effective for obtaining corrected mixing coefficients in a somewhat more complicated situation involving levels with significant mixtures of four non-relativistic terms as well as in cases where only two terms are mixed. The resulting transition rates compare well with observation and some discussion of the errors implicit in the method indicates that the method is stable and accurate in the context of such transition rate calculations.

Molecular Sigma states for BaX²⁺ with X=H, Li, Na, K or Rb are calculated using a one-electron Hamiltonian model. The active electron is described for the Li atom by a model potential and for Na, K and Rb atoms by a local pseudopotential, whereas for the Ba⁺ ion the authors use a new approach that involves both these methods. They predict stable states for states that dissociate into Ba²⁺+X (ground state), with well depths of 1.46, 0.35, 0.3, 0.55 and 0.55 eV for X=H, Li, Na, K and Rb, respectively. They predict long-lived states for Ba⁺(6s)+X⁺, with well depths of 0.1, 0.6, 0.3 and 0.1 eV for X⁺=H⁺, Li⁺, Na⁺ and K⁺.

The fluorescence radiation, emitted by a two-level atom in a perturber bath, is considered. The atom is driven by a single-mode laser with a Lorentzian profile, which is brought about by a fluctuating phase. The stochastics of this phase is taken to be the Markovian random-jump process. The authors derive explicit expressions for the atomic density matrix, the temporal photon correlations and the fluorescence spectrum. The results greatly resemble the results for a non-Gaussian diffusive phase, but deviate slightly from the corresponding expressions in the Wiener-Levy limit of the diffusion process.

The enhancement of the electron collisional broadening of the 4s³S₁ and 4p³P_2,1,0 degrees levels in a helium glow discharge (n_e=10¹³-10¹⁴ cm^-3), induced by the non-resonant interaction of the 4s³S₁ and 4p³P_2,1,0 degrees levels with a CO₂ laser field, is reported. This extra broadening depends on the product of the electron density with the laser field amplitude and manifests itself on the 4713 and 3188 AA linewidths (the transitions 4s³S₁ to 2p³P_2,1,0 degrees and 4p³P_2,1,0 degrees to 2s³S₁ respectively). This effect results from the destruction, by electronic collisions, of the coherence induced by the laser field between the 4s³S₁ and 4p³P_2,1,0 degrees levels, and can be viewed as an electron-atom collisional fluorescence redistribution process at relatively large laser detuning.

The quantum defect method including spin-orbit interaction and core polarisation effects has been used to determine potassium oscillator strengths and photoionisation cross sections. Comparisons are made between a set of various theoretical and experimental results and those predicted. The model is characterized by a dipole moment cut-off factor r₀=1.70 au and an 'effective core radius' r_c=2.54 au.

The effect of a slowly receding electron on the shape and location of an Auger line is investigated for Auger transitions following photoionisation of an inner-shell electron. The theory of Niehaus and Morgenstern (1977) is reformulated and extended to take into account the time it takes for the fast Auger electron to overtake the slow electron. Because of this, much smaller shifts are found in the energies of the peaks of the Auger spectrum than those predicted by Niehaus (1977) as a result of post-collision interaction. In addition it is demonstrated that the truncation of the leading edge of the Auger lineshape associated with Niehaus' work is not a failure of the theoretical model, but is solely an artefact arising from the use of the stationary-phase approximation to evaluate the integral which follows from the model. The extended theory is generalised to treat inner-shell vacancies caused by electron impact ionisation near threshold, in which case the effect on the Auger line is caused by the two slowly receding electrons.

Multi-pseudostate close coupling is used to investigate 1s to 1s, 1s to 2s and 1s to 2p scattering at energies from 54.4 to 350 eV. Two sets of pseudostates are considered. The first is that of Fon et al. (1981); the second is an improved set. In constructing the improved pseudostates emphasis is placed upon getting the second Born term right. Target states of angular symmetries not included in the pseudostate bases are taken into account by using the distorted-wave second Born approximation of Kingston and Walters (1980). Results for elastic scattering are satisfactory; it is estimated that the present elastic differential cross sections are accurate to better than 5% at any angle at 200 and 300 eV. For the 2s and 2p excitations the theoretical picture is not so clear. Here there are significant differences between our results and the experiments of Williams (1975) and of Williams and Willis (1975). Some other theoretical calculations are in better accord with these experiments, although the authors believe the present work to be on firmer ground. Cases where the theoretical approximations are unanimously in disagreement with experiment suggest the presence of inaccuracies in the measurements and highlight the need for further experimental investigation. Of special interest are the angular correlation parameters of the 1s to 2p excitation; their results are in reasonable agreement with the main experiment at 54.4 eV in the angular range up to 70 degrees ; beyond 70 degrees there are still problems to be resolved.

The effect of distortions on the (e,2e) spectroscopy of helium is examined. In particular, it is found that distortions contribute significantly to the (e,2e) cross section for transitions to both the n=1 ground state and n=2 first excited states of He⁺ at incident energies as high as 1200 eV. This raises questions about the use of uncorrected (e,2e) cross sections at high momenta to test the accuracy of correlated wavefunctions for the ground state of helium. By numerically correcting new experimental data for distortions, it is possible to obtain improved agreement between experiment and calculated values of the momentum space overlap integral (f mod exp(iq.r)g). The helium wavefunction used in the calculations is a recent state-of-the-art CI wavefunction which gives better than 98% of the correlation energy.