Table of contents

Accurate non-relativistic energies of the only two bound states of have been computed variationally using basis sets of -correlated functions. The upper bounds to the non-relativistic energies of the and states are, respectively, -2.17807725 au and -0.72305872 au.

An explanation of anomalously long time scales in the transient formation of forbidden lines is proposed. The concept is based on a collisionally induced density dependence of the relaxation times of metastable level populations in transient plasma. Generalization leads to an incorporation of diffusion phenomena. We demonstrate this new concept for the simplest atomic system: the He-like isoelectronic sequence. A new interpretation of the observed long duration and anomalously high intensity of spin-forbidden emission in hot plasmas is given.

The angular correlation between photo- and Auger electrons in xenon has been measured for right and left circularly polarized light with 110 eV energy. Thanks to the high degree of circular polarization of the helical undulator radiation, the circular dichroism in the angular correlation has been clearly observed.

We report the development of the amplitude of the beat-wave accelerator in a relativistic electron oscillation plasma. The changes in the amplitudes of the incident lasers and the plasma wave occur at different three-wave phases and electron collision damping coefficients. The results present the new phase dispersion relation that depends on rates of amplitude changes of the three waves.

The variational approach to the solution of the hyperradial-adiabatic three-body Hamiltonian is analysed in a new way. The hyperspheroidal coordinates, introduced earlier by Matveenko and Abe, are used to construct the channel-independent pair-collision- and triple-collision-limit solutions of the HA problem. The breakdowns of the adiabaticity in the vicinity of the avoided-crossing points and in the regions of the two-body dissociation limits are critically overlooked. In order to treat the avoided-crossing regions we define the separable HA Hamiltonian which naturally follows from the exact one, and has proper behaviour at the singular points and is simpler than the one introduced recently by Tolstikhin et al. The dissociation-limit problem is clearly demonstrated to be the asymptotic tail of the unphysical hyperradial coupling: we discuss the possibility of the general solution of the problem for a rearrangement scattering process in the hyperradial-adiabatic approach.

The use of appropriately screened one-electron diatomic orbitals (OEDOs) in a variational configuration interaction (CI) of ground or excited diatomic states guarantees the nature of the desired state and speeds up the CI convergence. The method can predict, within the variational OEDOs, the possibility or impossibility of a two-centre state and can simultaneously compute both diabatic and adiabatic states. As an application, : is shown to be impossible, and four previously unreported excited states of are calculated.

The optical properties of Mg(I) have been computed using a configuration-interaction (CI) approach in which the multielectron basis includes single and double excitations from the M shell plus single excitations from the Ne-like core. The orbital set is of type and comprises STO, hydrogenic plus spatial oscillating STOCOS functions. The continuum states have been obtained by solving a discretized integral equation for the K matrix half-on-the-energy shell. For the , , and manifolds we have computed the discrete and autoionizing levels of the 3s nl, 3p nl, 4s nl and 3d nl series, the last two lying beyond the 3p ionization threshold.

The optical properties of Mg(I) have been computed using a CI approach in which the multi-electron basis includes single and double excitations from the M shell plus single excitations from the Ne-like core. The orbital set is of type and comprises STO, hydrogenic plus spatial oscillating STOCOS functions. The continuum states have been obtained by solving a discretized integral equation for the K-matrix half on the energy shell. Oscillator strengths and radiative lifetimes for the discrete states of the , and manifolds have been computed together with one- and two-photon cross sections of the ground and the states evaluated in the LOPT approximation. The maximum energies reached were 0.005 au and 0.035 au below the 4s ionization threshold for one- and two-photon processes, respectively. All the quantities have been determined in both the velocity and the length gauge.

The empirical correlation energy of atoms and singly charged positive and negative ions in their ground state shows a roughly linear dependence with respect to the number of electrons. However, the deviations from this linear behaviour follow a well defined trend. A formulation based on density functional theory allows us to understand these features. This formulation focuses on the number of pairings between antiparallel-spin electrons in the same main shell.

CASSCF/MRCI calculations have been performed for five states of ScCl , , , and for which values of the spectroscopic constants , , and are reported.

The influence of solid and chemical effects on photo-induced x-ray transitions from the valence states was investigated by means of high-resolution x-ray diffractometry. The study was devoted to metallic Mo and , and compounds. A significant enhancement of the emission rate of the 4d - 1s and 4d - 2s quadrupole-allowed transitions was found for the targets studied. The change of the transition probability is interpreted as being due to the mixing of the 4d states with p character orbitals. Further, the effect of exchange interaction between the 4p core level and the

The population of the S level in a beam ( - 17.5) keV) has been measured by means of laser-induced fluorescence. For optical pumping the transition ( nm) was chosen. The S projectiles were produced by quasi-resonant charge transfer collisions of in lithium vapour. Densities between and were investigated. By comparison with the beam neutralization it could be shown that the fraction of S particles in the beam can be manipulated by variation of the lithium density in the neutralization chamber. An explanation of this behaviour is proposed on the basis of collisionally-induced de-excitation of the S state. An effective cross section of the de-excitation could be derived with a simple model of the de-excitation processes.

We discuss the problem of the choice of the gauge in which to represent the electromagnetic field in the non-perturbative time-dependent study of the interaction of atoms with intense laser fields. Even though quantum mechanics is gauge invariant, the velocity gauge is more adopted then the length gauge for dynamical reasons. This property is even more severe when the problem is solved by expanding the wavefunction in spherical harmonics. The point is illustrated through the calculation of the above-threshold ionization photoelectron energy spectrum produced by atomic hydrogen under an intense laser pulse.

The level-crossing technique is a valuable tool for precise measurements of energy separations at electric-field anticrossings. To exemplify this possibility, we measured the energy separations of electric-field singlet - triplet anticrossings of the 1s5d and 1s5f configuration of He I. The level-crossing measurements on the 1s5d anticrossings are more accurate than previous radiofrequency experiments by about an order of magnitude. The experimental results agree with calculated values.

Near-threshold photoionization of argon in the () inner-shell region has been studied by means of threshold photoelectron - photoion coincidence spectroscopy. The observed line profiles are clearly influenced by the post-collision interaction effect and this influence is observed to be stronger for double than for single-Auger decay. The spectra have been interpreted using a classical theory which takes into account the possibility that a single photoelectron can interact with several Auger electrons.

The problem of low-power spin decoupling over a broad range of chemical shifts in liquid-state nuclear magnetic resonance (NMR) is addressed through the design of periodic amplitude-modulated irradiation schemes. A principal feature of these is the composition of each decoupling period which contains a single modulated pulse in place of a composite-pulse train of the kind used traditionally. To satisfy the theoretical criterion for decoupling, the pulse amplitude is shaped such that the propagator is made cyclic and broadband, meaning here that it equals the identity matrix over a frequency range which is broad compared to the root-mean-square (RMS) pulse amplitude. The method of design is based on the use of the Floquet formalism to provide insight into the influence of the modulation on the dynamics of the irradiated spin-. Modulation functions formed from simple Fourier series are derived in the first instance using perturbation theory to impose the required cyclicity on the propagator. Broader bandwidth solutions are then obtained by the addition of higher-order Fourier components. Finally, numerical refinement of a selected solution is shown to raise the decoupling quality to the standards acceptable in routine high-resolution NMR.

The excitation function of the dissociative photoionization excitation ( DIE) of has been measured with the detection technique of coincidence between photoion and a fluorescence photon emitted from an excited atom N* using extreme- UV synchrotron radiation ( SR) as an excitation source in the energy region of 35 - 65 eV. The axis of the detection system was set at two angles, parallel or perpendicular with respect to the major axis of the elliptically polarized SR, in the present measurement. The coincidence spectra obtained in this energy region show at least two qualitatively different structures originating from two different DIE processes. The major components of the DIE in the present energy region are ascribed to the dissociation of the state which is strongly coupled with correlation states in the symmetry. In addition, DIE from other precursor states in the symmetry or dissociative photo-double ionization excitation ( DDIE) was also observed.

The dependence of electron capture and ionization processes on the pre-collision electronic distribution is examined for the systems primarily in the energy range 1 - 15 keV. Populations of the capture and ionization channels as well as the coherence of the post-collision electronic distribution of the capture product are formulated using the classical trajectory Monte Carlo (CTMC) method. Total capture and ionization cross sections arising from initially aligned Na(3p) targets exhibit transient behaviour near the velocity-matching energy of 5.7 keV. This observation is described in terms of anisotropy parameters; anisotropy effects are also analysed for the capture states n = 2, 3 and 4. In addition to the population manifold, we present theoretical calculations of the alignment parameter and the z-component of the dipole moment , in order to relate the alignment of the post-collision charge cloud to that of the initial Na(3s, , ) target states. The z-component of the cross product of the angular momentum and the Runge - Lenz vector is used to describe the orientation of the post-collision product generated from the aligned Na(, ) pre-collision reactants.

Over the last two decades, there has been considerable interest in the electron - atom and positron - atom scattering problems. Most of this work has concentrated on elastic scattering and excitation of the lowest energy states. However, due to recent technological advances, it is now becoming possible to examine the excitation of higher lying states experimentally. The important question then concerns whether these higher states contain new information. Earlier, we have examined this issue for electron - hydrogen scattering. The purpose of this work is to examine the N dependence for positron - hydrogen scattering. The second-order distorted-wave method has been used to calculate scattering amplitudes for excitation of s- and p-states of hydrogen for N = 2, 3, 4, 5 and 6. The N dependence of the integrated cross sections, differential cross sections and polarization fractions will be discussed for energies up to 1 keV and the results will be compared with those for electron - hydrogen scattering.

Over the last two decades, there has been considerable interest in the electron - atom scattering problem. Most of this work has concentrated on elastic scattering and excitation of the lowest energy states. However, due to recent technological advances, it is now becoming possible to examine the excitation of higher lying states experimentally. The important question then concerns whether these higher states contain new information. We have recently examined the N dependence for electron - hydrogen scattering. The purpose of this work is to examine the N dependence for electron scattering from alkali atoms. The second-order distorted-wave method has been used to calculate scattering amplitudes for excitation of the higher lying levels of Li, Na and K atoms up to N = 7. The N dependence of the differential cross section and angular correlation parameter are discussed for energies from threshold to 250 eV.

Calculations using the distorted-wave first Born model are reported for the triple differential cross section for electron-impact ionization of atomic hydrogen at low energy. They are carried out in the coplanar asymmetric geometry for various deflection angles. Different kinematics are considered: unequal energy sharing between the outgoing electrons at 54.4 eV and equal energy sharing at 27.2, 17.6 and 15.6 eV. We attempt to improve the theoretical description of the initial state and propose four models, three of which include a polarization of the target orbital. The associated polarization potential is used in addition to the static and the static-exchange potentials to calculate the distorted wave. We account for electron-pair correlation in the final state by use of a Gamow correction factor.

Electron impact emission cross sections of Ar were measured for the , and transitions from the threshold to 1000 eV by a polarization-free method.

Two different methods were used in determining absolute emission cross sections for the first resonance lines. The first method is to measure the cascade cross sections to the 4s state by detecting visible to near-infrared radiation and the total cascade cross section was then combined with the level excitation cross section to the 4s state to give the emission cross section for the resonance line.

The second method is to measure the vacuum ultraviolet sensitivity of the apparatus and an absolute emission cross section for the resonance line was obtained by comparing its spectral intensity with that of the Lyman- radiation produced from by electron impact. The two results showed good agreement within the experimental errors.

The level excitation cross sections to the and states were obtained by subtracting the total cascade cross section to these levels from the corresponding emission cross sections.

We report on measurements of the parameter and absolute differential cross section for electron-impact excitation of the state in barium from the metastable, isotropically populated level at 20 eV impact energy. The determination of these scattering parameters was facilitated by the utilization of a novel laser excitation geometry and measurement scheme. Along with our previous measurements of electron-impact coherence parameters for this transition we now have available a complete set of the five parameters which characterize the excitation of a P state by the scattering of unpolarized electrons from an isotropic (metastable) D level. Comparison of measured values with recent calculations carried out in the unitarized distorted-wave approximation reveals good agreement.

A method is presented for constructing configuration-interaction (CI) Hamiltonians for electron - molecule collision calculations based on close-coupling expansions. This method utilizes the structure of the wavefunction used in scattering calculations to greatly reduce the computational cost compared to conventional procedures. In particular, CI target configurations are treated only at the prototype configuration state function stage of the calculation. Only then are these prototype configurations expanded over all scattering functions using an improved version of a previous algorithm by Morgan and Tennyson. This new algorithm has been implemented as part of the UK R-matrix codes. Test calculations show that it leads to a large reduction in both the number of integrals and matrix elements which need to be explicitly constructed and evaluated. Use of this new program should allow significantly more sophisticated target wavefunctions to be employed in future scattering calculations.

The doubly excited states of molecular hydrogen have been studied by means of electron-energy-loss spectroscopy in coincidence with the detection of H(2p) formation at 80 eV incident energy and scattering angle. This is the first observation of doubly excited states of in electron energy-loss spectra. It is shown that a forbidden doubly excited state makes a large contribution as well as allowed states. The measured spectrum has been analysed by theoretical fits based on the reflection approximation.

First measurements on the ionization cross sections for the production of and ( + ) ions from by electron impact are reported for an electron impact energy range of threshold energies to 1 keV and for at 100 eV. These measurements were carried out in a crossed-beams apparatus using the pulsed technique and the partial pressure method to determine the absolute cross sections. Total ionization cross sections at each electron impact energy were obtained by numerical summation of the respective partial ionization cross sections.

The partial doubly differential cross sections (PDDCS) for the ions , , , , (, ), , , and resulting from the dissociative ionization of the molecule by electron impact employing the ejected-electron and produced-ion coincidence technique have been measured. Measurements have been made for incident electron energies of 100 and 200 eV and for ejected electron energies of 30, 50, 65, 100 and 150 eV. The angular range for the detection of the ejected electron is between and . No trace of ions is found at any energy used. No previous data or theoretical predictions exist with which to make comparisons.

We report a measurement of parity non-conserving (PNC) optical rotation in the vicinity of the 1.279 magnetic dipole transition in atomic lead. We obtain a value for the conventional parameter, , with limits on the nuclear spin-dependent contribution set by the anapole constant . The experimental results, when combined with the relevant atomic calculations, lead to a value for the mass of the boson or, alternatively, place a limit on physics beyond the standard model through the isospin-conserving parameter, S.

It is shown that the non-additive part of the Casimir - Polder potential between three neutral isotropic ground-state atoms or molecules can be obtained in a simple way by generalizing a method that has recently been used for two-body forces. The electric field correlations of vacuum fluctuations in the presence of one atom are evaluated at the position of the other two atoms. This fluctuating field induces and correlates the dipole moments of these two atoms, such that their correlation function contains a term depending on the presence of the first atom. When the dipolar interaction between the induced dipoles is evaluated, this term yields the correct expression of the non-additive three-body Casimir - Polder potential. It is argued that this method prompts a new physical interpretation of three-body forces in terms of dressed vacuum field correlations.

In this recent article, the printers inadvertently printed figure 5 twice. The correct figure 3 is shown below.

Figure 3. Same as figure 1 but for the CI calculations. The calculated cross section at the resonance peak is 69.2 Ų.

The recent paper by M Pont and R Shakeshaft in J. Phys. B: At. Mol. Opt. Phys., 28 L571, and former discussions with M Pont have pointed out to us the importance of integrating the triple differential cross section (TDCS) of double photoionization towards the total cross section σ⁺⁺. Using our parametrized TDCS, the integrations over the solid angles of the ejected electrons and the final integration over the energy coordinate have been performed, assuming for the last step a flat distribution for the energies of the electrons and avoiding double counting. The resulting integrated cross section σ⁺⁺(E_exc = 20eV) appears to be too small by a factor of about two when compared to known experimental and theoretical σ⁺⁺ values, the same conclusion has been derived independently by M Pont, R Shakeshaft, F Maulbetsch and J S Briggs (to appear as a brief report in Phys. Rev. A). In the light of this result, our experimental TDCS values should be revised upwards by a factor of about two, even though we could not find in neither in the individual measurements nor the procedure to derive the absolute TDCS an obvious source for this discrepancy.