Table of contents

A simple analytical model of configuration interaction of ionic and covalent states is developed. The model describes the bound states of an electron in a Coulomb field and N point potentials (N=1 and 2). In this model the adiabatic ionic-covalent splitting is found as a function of the principal quantum number and the ionisation potential of the donor atom and the electron affinity of the acceptor atom.

The authors have measured the hyperfine structures of the n³S (n=4-6) and n³D (n=3-6) levels of ³He. The accuracy of their data, obtained using Doppler-free two-photon spectroscopy, is more than one order of magnitude better than the previous one. They have also observed the intercombination line 2³S-3¹D which has allowed them to measure the 3¹D-3³D singlet-triplet distance.

The results of recent relativistic calculations of Huang et al. (1979, 1980) for spin polarisation of photoelectrons ejected from outer subshells of Ar and Xe are compared with non-relativistic results. It is shown that in most cases the non-relativistic theory is capable of describing the polarisation phenomena.

Variations in asymmetry parameter, beta , and the ratio of partial photoionisation cross sections sigma (²P₃2/): sigma (²P₁2/) have been determined in the region of the 3s3p⁶4p ¹P₁⁰ resonance in Ar and the 5s5p⁶6p ¹P₁⁰ resonance in Xe. In both cases there is a considerable variation in beta through the resonance. In Ar the beta values for the spin-orbit components are similar; in Xe they are significantly different. In Xe, the branching ratio shows a modest variation through the resonance, whereas in Ar no noticeable change occurs.

The two-photon ionisation of caesium atoms in the spin-orbit coupling scheme is analysed when one uses the electric dipole plus quadrupole approximations. The electron spin polarisation is obtained on the allowed quadrupole resonances 6D_3/2-6D_5/2. Comparison with experimental results is made and a final discussion is presented.

High-resolution vacuum ultraviolet absorption spectra of H₂O are reported. The rotational structures of the bands below 1200 AA have been resolved for the first time and yield a wealth of new rovibronic information both below and above the first two ionisation potentials. The resolving power of the authors' apparatus is at least equal to the highest resolving power previously achieved at 1240 AA in studies of H₂O but our experimental method is not limited, as were the earlier ones, to wavelengths longer than 1190 AA. Surprising aspects of rotational structure in the 5s state are commented upon in the light of the group theoretical characterisation of the vacancy. Corresponding observations for D₂O have also been made.

A time-of-flight technique is described which is used to measure elastic (direct and exchange) differential cross sections for collisions of Ne(2p⁵3s ³P_0,2) with Ne(2p⁶¹S₀) at thermal energies in an experiment using two effusive beams. The angular dependence of the scattered intensity and the velocity dependence of the differential cross section in the backward scattering direction are given.

A symmetric form of the impulse approximation for charge exchange is developed. This approximation preserves the equivalence of post and prior forms of the transition amplitude. As in the usual impulse approximation the dynamics of charge transfer is seen primarily as the ionisation of a bound state. The usual impulse approximation considers ionisation of an electron from the bound state, whether initial or final, of the stronger potential. However, the symmetric form presented here includes contributions from the ionisation of an electron from both initial and final bound states.

Inner-shell excitation of molecular chlorine has been studied by the electron energy-loss technique, at an incident electron energy of 1.5 keV and with an energy resolution of 65 meV. Excitation energies of states in the vicinity of the L_2,3 ionisation edges have been accurately determined. Two series of states have been observed, and their relative energy separation and transition strength show that the inner-shell vacancy is localised at one of the chlorine atoms. Information about the 'equivalent-core' molecule ArCl has been deduced from these results.

The behaviour of the solution of the Schrodinger radial equation has been studied experimentally. From this study, a very simple and short computing program has been devised for computing the eigenvalues and eigenfunctions of the energy, and for computing the eigenvalues and eigenfunctions of any parameter that may exist in the potential expression. The number of iterations for reaching convergence to a precision of 9 significant figures is 6 (i.e. less than the previous program by a factor of ¹/₅). The total computing time is reduced by a factor of ¹/₂₀. This program is so short (about 20 statements) that presenting it, is easier than describing it. An example is given with sample data and test output. Linear extrapolation is used to find the trial values of each eigenvalue at each iteration. This method is found to be simpler, faster, and more accurate than other methods. By the use of this program it is possible to solve the 'inverse eigenvalue problem'.

The problem of finding solutions of a single-channel scattering problem in which the potential V(r) behaves as a series V(r) approximately= Sigma _nZ_nr^-n for large r is discussed. It is shown that a variational method of determining the solutions has considerable advantages over the usual asymptotic series solutions.

The Z dependence of dipole excitation channels in two-electron atoms is investigated for levels N<or=6. An SO(4) group theoretical approach is used to classify these spectra; at low Z the channels are assigned quantum numbers from a Stark representation SO(4)=SU(2)*SU(2), while at high Z there is an alternate set of quantum numbers from the symmetry-breaking chain SO(4) contains/implies SO(3). Rules are described for correlating the two sets of quantum numbers, and appear to be related to the number of nodes in the wavefunctions with respect to the interelectronic angle theta ₁₂. Due to the existence of a second constant of the motion, the dipole channel spectrum is found to contain numerous crossings. Some crossings are described near Z=1 which could lead to a breakdown of the dipole approximation, and interference in e^-+H channels originating in the short-range part of the interaction potential.

A theoretical analysis of the time development of spontaneous Raman scattering lineshapes is presented. Particular attention is paid to transients caused by the sharp turn-on of the laser fields. The author examines the breakdown of lowest order perturbative ideas, the role of induced widths and shifts and of Rabi oscillations in final-state spectra for large pump intensities.

The light statistical dependence of stationary saturated two-photon transitions is discussed. While at low intensities, in comparison with light having a stable amplitude, Gaussian amplitude fluctuations of the exciting radiation are more effective in populating the excited resonant state, light with amplitude fluctuations is less effective in saturating a two-photon transition. The amplitude fluctuations are shown to wash out the resonance curves. While at low intensities the effective Stark shift (i.e. the maximum of the dispersion curve) is larger by a factor of three in light with Gaussian amplitude fluctuations compared with radiation of constant amplitude, this enhancement factor becomes intensity dependent with increasing intensity.

For pt.I see ibid., vol.11, p.955 (1978). The adiabatic potential energy curves of the He₂²⁺ quasi-molecular system are calculated by the configuration interaction method for internuclear distance R from 0.5 to 5.0 au. The CGTO centred at each nucleus and at the nuclear charge centre are used as basis functions. The energy widths of the autoionising states are obtained from R=0.0 to 0.5 au in the Coulomb, static and static exchange approximations for the continuum electron wavefunction. The calculation is extended for the lowest ¹ Sigma _g⁺ autoionising state to R=1.0 au which corresponds to the stabilisation point of this state.

The energy dependence of the cross section for the processes K(4P_1/2)+N₂ (and Ar) to K(4P_3/2)+N₂ (and Ar) has been measured by a crossed beam experiment over the energy ranges 0.12-0.41 eV for N₂ and 0.12-0.30 eV for Ar. Absolute determination of these cross sections has been performed at 0.12 eV. Up to 0.3 eV the authors' results indicate that the internal degrees of freedom of N₂ play no important role in the fine-structure transition process. Disagreement was found with previous cell data both for N₂ and Ar. However, good agreement is observed with theoretical calculations or estimates.

In collisions of 32 MeV Sⁿ⁺ on Ar, impact parameter dependent K-shell excitation probabilities P_K(b) have been determined for n=5, 11, and absolute K-shell excitation cross sections sigma _K for n=5-13. A comparison between P_K(b) and the 2p pi -2p sigma rotational coupling model clearly shows the main contribution to the total K-shell excitation cross section sigma _K to be due to rotational coupling. This is also true for S⁵⁺ where no static L vacancies are present prior to the collision. The increase of sigma _K with projectile charge state n+ (for 5<or=n<or=13) is only 60%, suggesting a high probability for direct 2p pi excitation on the incoming part of the collision even for the lowest charge states. For impact parameters of b<or=2000 fm, which do not significantly influence the total cross section sigma _K, the contribution of direct 2p sigma excitation is found to be important.

Using the Bragg X-ray diffraction method, the linear polarisation of quasi-molecular K radiation was determined for 1.3 MeV ²⁴Mg⁺ to Mg and 1.7 MeV ³¹P⁺ to Si collisions. Polarisation values between +8% and +40% were found for X-ray energies between the separated and united atom K alpha energies. The measured values are compared with recently published calculations for N to N₂ collisions at the same scaled velocity. The anisotropy values derived from the polarisation measurements agree well with anisotropies reported in the literature.

Charge transfer reactions to the continuum lead to three-body final states, for which a Faddeev-type theory (1960) is suitable. An alternative treatment is described in terms of the R-matrix theory of final-state interactions. This provides an explicit form for the cross section in terms of spectroscopic data, namely energy eigenvalues and reduced widths. A fit is obtained to the velocity spectrum of electrons from charge transfer to protons in carbon. This agrees satisfactorily with observed 'cusp' spectra and is consistent with observed data on the excitation function for the charge transfer reaction.

A method is proposed for obtaining critical energies and angles for the scattering of electrons by atoms. The method requires phaseshifts only at three energies. The critical points have also been computed in the static-field approximation for the light atoms having atomic numbers 6 to 10. The values of the critical points obtained by the proposed method are in excellent agreement with the computed values.

The distorted-wave second Born approximation (DWSBA) has been applied to the elastic scattering of electrons by ground-state atomic hydrogen and to the electron impact excitation of the 1s to 2s and 1s to 2p transitions in the energy range 30 to 680 eV. Differential and total cross sections are given as well as the polarisation fraction of the Lyman alpha radiation resulting from the radiative transition 2p to 1s. A new method of choosing the average energy in the closure approximation to the second Born amplitude is introduced for elastic scattering. The approximate methods of Dewangan and Walters (1977) and Winters (1979) for including distortion effects in the second Born term are investigated. It is found that both approximations are viable for elastic scattering at high enough energies but unreliable for inelastic scattering.

The authors have tested the consistency of the semiclassical and the shake-down models for post-collision interaction with measured ejected-electron spectra resulting from the (2s²)¹S and the (2s2p)³P autoionising states of He excited by electrons. This was done by comparing spectra measured in two different modes of operation of the electron spectrometer with corresponding model calculations. Although differences between the calculated model spectra and the measurements are observed, this consistency appears to be fairly good. It is shown that for incident energies in the threshold region exact Coulomb wavefunctions have to be used in the shake-down model. It is proved that both models are essentially identical; differences arise from different approximations for the final-state wavefunctions and the overlap integral in the models.

The authors have calculated the collision strengths for electron impact excitation of semi-forbidden transitions 1s²2s²2p² to 1s²2s²2p², 1s²2s²2p² to 1s²2s2p³ and 1s²2s2p³ to 1s²2s2p³ in Ne V using the R-matrix method. Twelve target states having configurations 1s²2s²2p², 1s²2s2p³ and 1s²2p⁴ are included in the expansion of the total wavefunction. Configuration interaction wavefunctions are used to construct the target states. Short-range correlation terms are included in the total wavefunction. Collision strengths are averaged over a Maxwellian distribution for 31 transitions over a wide temperature range. Results are compared with earlier calculations. The oscillator strengths for the allowed transitions are also presented.

The ejected-electron spectrum of barium vapour between 0 and 20 eV has been observed at 75 degrees to an incident-electron beam of 20-500 eV kinetic energy. A total of 81 lines are reported, corresponding to the radiationless decay of Ba I and Ba II levels. Comparison is made with existing experimental and theoretical data.

For pt.I, see ibid., vol.12, p.3787 (1979). Total absolute cross section measurements are presented for electrons scattered by O₂, N₂ and NO molecules. The energy range covered spans from 100 to 1600 eV. Neglecting the systematic error introduced by the finite angular resolution of the apparatus, the experimental accuracy has been estimated to be +or-2.5%. This systematic effect could necessitate an upward correction of as much as 13% at 1000 eV in N₂. No similar evaluation was possible for O₂ and NO. A comparison is given for N₂ with existing measurements. The agreement is reasonable. The results for O₂ are compatible with the existing data in the low-energy range. No comparison was possible for NO.

Ionisation efficiency and appearance potential functions for CH₄⁺, CH₃⁺, CH₂⁺ and CH⁺ have been measured using a monoenergetic electron beam colliding with methane. The upper limit for the first ionisation potential of CH₄ is determined to be 12.71 eV. Appearance energies for the ionic fragments have yielded values for heats of formation which indicate a large reverse activation energy in the case of CH⁺. Structure in the ion yield curves for CH₄⁺, CH₃⁺ and CH₂⁺ in the energy region 19-22 eV is in qualitative agreement with the results of Marmet and Binette (1978) and may be due to highly excited states of methane. The first excited state of CH⁺ as well as four excited states of CH₂⁺ are observed. Other structure reported in various RPD measurements is not reproduced. The electron transmission function of CH₄, measured on two spectrometers over 1-20 eV, reveals a broad minimum at 7.8 eV and a resonance at 18.5 eV.