Table of contents

The total cross section for the double photoionization process is calculated for the first elements of the He isoelectronic series using the dipole approximation and highly correlated forms for the initial state wavefunction. The final state is approximated by a symmetrized product of Coulomb scattering wavefunctions. This allows the establishment of simple trends in the magnitude of the cross section and the importance of the angular correlations of the initial state. It is suggested that the double photoionization can be used as a tool for measuring excess charges on atoms in various chemical environments.

The authors have performed coupled-channel computations of the cross section for excitation of the 2s2p ¹P resonance in collisions between ground-state helium atoms and protons, in the range of impact energies 0.5<or=E_p<or=3 MeV. For the first time in such computations, the coupling of the doubly-excited state to the continuum has been taken into account. Comparison with the equivalent first-order calculations confirms that the Born approximation is not valid for E_p<0.5 MeV.

In highly dissociated hydrogen plasmas the imprisonment of resonance radiation can significantly increase the effective lifetime of the n=2 state to such an extent that gas phase collision involving H(n=2) become important. Under these conditions the excitation of the H₂(³ Sigma _u) repulsive state which leads to H atom production occurs with a higher probability through excitation transfer collisions between H(n=2) and ground state H₂ than through direct electron impact collisions with ground state H₂.

Projectile K-shell ionization cross sections were measured for 80-200 MeV u^-1 H- and He-like Bi ions on thin C, Al and Ni targets. The results are compared with predictions of first-order perturbation theories, RSCA and PWBA, as well as with experimental results published for Xe and U ions at similar ratios eta of projectile velocity to K-shell electron velocity. All experimental data lie a factor of 1.5 above theory for 0.6< eta <1.5.

The authors have measured the radiative stabilization probability in double-electron capture collisions for the systems 10 keV u^-1 Kr^q+ (q=13-34) and Ar^q+ (q=6-17) on Ar, and examined its velocity dependence for Ar⁸⁺ on Ar. The measured probability exhibits pronounced structures as a function of q. A model incorporating the initial population, the collision velocity, and the core structure of the incoming projectile is proposed, and the gross features are fairly accounted for. The results support the importance of populating asymmetric Rydberg states and reveal the crucial role played by the projectile core structure. The velocity dependence measurements favour a post-collision interaction mechanism for the population of the asymmetric Rydberg states, such as that proposed by Bachau et al. (1992).

Differential cross sections for the elastic scattering of electrons by N₂ molecules are calculated in the first Born approximation. Configuration interaction wavefunctions constructed from a large basis set of Slater-type functions are used to describe the target. These wavefunctions recover up to 83% of the empirical valence correlation energy, predict accurate quadruple moments, and cross sections apparently converged to 5% with respect to one-particle basis set effects and valence correlation. However there is a persistent discrepancy between the calculations and recent 29 keV experiments. Further experimental work is needed to determine whether the experimental data contain systematic errors.

A reduced intermediate energy R-matrix (IERM) approach has been developed which allows more rapid calculation of accurate cross sections at intermediate energies. This approach has been applied to electron-hydrogen atom scattering for L=0, S=0, and even parity to give the first accurate cross sections for n=1 and n=2 to n=3 transitions at intermediate energies. The results have important implications for earlier work which overestimates these cross sections.

The alignment A₂₀ of Mg⁺(L₃) ions after electron impact ionization has been measured for the range of impact energy E₀=300 eV down to 59.8 eV; the latter value is close to the threshold energy E(L₃)=57.54 eV. The experimental alignment values agree reasonably with the results of a BA calculation for E₀>100 eV but disagree with DWBA results in the full range of energy. From the alignment value obtained near threshold, A₂₀=-0.18(5), it is concluded that two-electron partial waves with L=0 and L>0 contribute to the Wannier threshold cross section.

The distorted-wave approximation is applied for the first time to study the electronic excitation of a non-linear polyatomic target by electron impact. Specifically, differential and integral cross sections for the X¹A₁ to ³A₁(3a₁ to 3sa₁) transition H₂O are reported in the energy range 12-30 eV. The comparison with the available experimental and theoretical results is encouraging.

The authors report results concerning the L₁ atomic fluorescence yield, omega ₁, of Yb and Ir, determined by comparing the L X-ray spectrum with the K and gamma spectra, emitted after disintegration of ¹⁷⁰Tm and ¹⁹¹Os, respectively. The experimental results are: Yb, omega ₁=0.131+or-0.013; Ir, omega ₁=0.152+or-0.014. These results are in general agreement with experimental results from other authors in the same Z region, enhancing the need for new theoretical calculations in this region of atomic numbers.

New experiments have been carried out on a velocity selected beam of H(D)*(2s₁2/) atoms using a longitudinal Stern-Gerlach atomic interferometer. A large number of fringes of the interference pattern has been observed. The results are in good agreement with theoretical predictions. An application to a topological phaseshift of the Aharonov-Anandan type is given.

Perturbation theory in the lowest non-vanishing order in interelectron interaction has been applied to the theoretical investigation of double-ionization decays of resonantly excited single-electron states. The formulae for the transition probabilities were derived in the LS coupling scheme, and the orbital angular momentum and spin selection rules were obtained. In addition to the formulae, which are exact in this order, three approximate expressions, which correspond to illustrative model mechanisms of the transition, were derived as limiting cases of the exact ones. Numerical results were obtained for the decay of the resonantly excited Kr I 3d^-15p(¹P) state which demonstrated quite clearly the important role of the interelectron interaction in double-ionization processes. On the other hand, the results obtained show that low-energy electrons can appear in the photoelectron spectrum below the ionisation threshold of the 3d shell.

The interaction of a pair of coupling two-level atoms with a radiation field is studied. The effects of a dispersion force (dipole-dipole interaction) on the emission spectrum are analysed. It is found that the dipole-dipole interaction induces an asymmetry of the emission spectrum, which is most obvious in a vacuum field.

Analytical expressions for photoelectron spectra for Rydberg autoionization induced by a resonance electromagnetic field in the case when the atom is completely ionized within the electromagnetic pulse have been derived. Depending on the ratio between the problem parameters, various numbers of autoionizing states can take an essential part in the photoelectron spectrum formation: (i) a single resonance autoionizing state; (ii) many autoionizing states including infinite numbers as well; (iii) none.

The formation of H^- in the collision of excited (3s) atomic hydrogen with helium and neon is investigated. The ratio ( sigma _3s/ sigma _2s) of the cross sections for electron capture on H(3s) and H(2s) is measured through an analysis of the kinetic energy spectrum of H^-, which, in the case of helium, also allowed the measurement of the ratio ( sigma _2s/ sigma _1s). A resonant cavity is used, in the adiabatic rapid passage configuration, to increase the laser power used to excite the 3s state from the metastable state. An excitation yield as high as 90% is obtained. The measurements cover the range 300-3000 eV and confirm the important growth of the cross section ratio at the low energy side.

This paper describes the first experimental measurements of sigma ₂₃^P, the cross section for removal of the final electron from lithium by charge exchange and impact ionization with protons. The implications that this cross section has with regard to lithium ion beam heating and/or current drive in tokamaks are discussed. Since the expected cross section is quite small, non-standard techniques were employed to make this measurement. Singly ionized lithium ions, formed in a magnetically insulated racetrack diode, were passed through an energy selector and then directed along the axis of a theta pinch produced hydrogen plasma. The selector restricted the energy range of the lithium beam to the neighbourhood of 400 keV. The theta pinch coil was 71 cm long and had an inner diameter of 10.5 cm. It was energized by a 25 kj main capacitor bank operating at 150 kHz. After exiting the theta pinch plasma the ion charge states were analysed with a Thomson spectrometer.

The emission of electrons from light targets bombarded by hydrogenic projectiles is approximated by the elastic scattering of electrons by these ions. The electron exchange transition amplitude is considered in a first-order distorted-wave approximation, and contributions attached to the non-orthogonality of continuum and bound electron states are fully incorporated into the calculation. A close agreement with measured cross sections is obtained.

An intuitive model of the binary encounter peak in the energy and angular distribution of ionization electrons employs the elastic scattering amplitude for electron-projectile scattering. When the projectile carries some electrons, electron exchange contributes to the elastic scattering. The authors show that the elastic scattering model, including exchange, derives from a distorted-wave strong-potential Born approximation for ionization to the projectile continuum. This theory incorporates both the binary encounter peak and the continuum capture cusp.

The crossed electron-ion beams technique was used to measure absolute cross sections for single ionization of Ar²⁺, Kr²⁺ and Xe²⁺ ions at electron energies ranging from threshold to 2000 eV. The total error in the cross sections at a 90% confidence level is estimated to be +or-4.8, +or-4.9 and +or-7% or less for Ar²⁺, Kr²⁺ and Xe²⁺ respectively, at energies more than a few electron volts above threshold. In contrast to some previous measurements, the metastable contents of the ion beams were small even in the case of Xe²⁺. All measured cross section curves show significant contributions from excitation-autoionization and possibly direct ionization of inner-shell electrons. There is evidence for resonance-excitation-double-autoionization in the case of Xe²⁺. Comparisons are made with other available experimental and theoretical data.

The authors have made a preliminary study of several allowed M1 transitions within the 4f⁶6s² ground state configuration of samarium with the aim of observing enhanced parity non-conserving (PNC) optical rotation. No evidence for R=Im(E1)^PNC/(M1) greater than 10^-6 has been found. They conclude that, although opposite parity states are comparatively close (ca 200 cm^-1) in the cases studied, the strength of the admixed E1 transitions is low thereby reducing any potential enhancement. This also complicates any theoretical analysis since there is no single dominant E1 component in the admixture, and in any case calculations of oscillator strengths for samarium with its complicated level structure are notoriously difficult. Finally, the reactivity of metallic samarium poses considerable experimental difficulties in achieving sufficiently high vapour pressures.