Table of contents

We propose a novel approach for Hamiltonians allowing adiabatic separation of variables. Our method is based on the assumption of smoothness of the motion associated with the adiabatic variable instead of its slowness, which is assumed traditionally. Convergence in terms of the number of coupled channels in our method corresponds to the standard adiabatic expansion. However, neither laborious calculations of non-adiabatic couplings nor a priori information on locations of avoided crossings are required. The method is illustrated by calculating bound-state energies for several three-body Coulomb systems for states with zero total angular momentum.

Using the optical Ramsey method we have observed the configuration interaction in Ba drive atoms back and forth between the doubly excited 5d7d state and the degenerate 6snd Rydberg states. Our observations can be understood qualitatively as autoionization of the initially excited 5d7d state into the Rydberg states, followed by capture of the Rydberg electron by the ion a Kepler time later. After a time delay, the whole process repeats. A quantum defect theory description provides a quantitative picture and clarifies the connection to resonance scattering.

The regularity for the relative intensities of the strongest lines in lanthanide spectra, emitted in calibrated arcs, is explained.

The photoabsorption spectrum of atomic phosphorus has been recorded in the 1400 - 600 Å region. The Beutler - Fano profiles due to autoionizing transitions have been observed and fitted to a Rydberg series. Calculations made with the Cowan code show that interaction with the (also written as ) configuration is negligible, unlike the situation for 3s photoexcitation in Ar I.

The presence of two metastable states in the negative beryllium ion, and , which are connected by an optical transition, offers a unique possibility for gaining new information about this ion. Utilizing resonant, two-photon detachment, combined with resonant-ionization detection of the resulting neutral atoms, we have obtained results yielding the lifetimes of the () and () levels, the fine-structure splittings of the levels, and an improved transition wavelength.

We present highly accurate relative measurements of 2 excitation of helium by electron impact at 30, 40 and 50 eV. These, when normalized to the convergent close-coupling (CCC) theory at one angle, yield excellent agreement at the remaining angles. This resolves some of the outstanding discrepancies between the CCC theory and earlier measurements.

The method of continued fractions (MCF) of Horácek and Sasakawa is applied for the first time to the study of low-energy electron scattering by linear molecules at the two-state close-coupling level. In particular, we have calculated the excitation cross sections for the transition X in for incident energies from near threshold to 40 eV. In contrast to the early two-state studies of Chung and Lin, in this work no orthogonality constraint between the bound and continuum orbitals is imposed and the one-electron exchange terms are considered explicitly. The calculated cross sections are in very good agreement with the available experimental data and with some recent theoretical results.

The bound state spectra in the symmetrical Coulomb four-body systems with unit charges are considered. The total energy and other properties for such systems depend only on the mass ratio . It is shown that there is an additional charge - mass-permutation invariance in these systems. By applying this invariance we can determine the total energies for the systems with if the energy dependence is known for , and vice versa. To produce quite accurate interpolation formulas for the total energies we have made calculations for a number of exotic systems, including and eight others. For all these systems the variational results are the best values to date. This is the first study based on numerical results of bound state spectra for Coulomb four-body systems with unit charges.

Spectra of Ba autoionizing Rydberg states (n = 60 - 80) in a varying external electric field, keeping the classical scaled energy constant, have been measured. Series converging to the - as well as to the -limit have been studied for three values of the scaled energy: , -2.35 and -1.76, deep in the n-mixing regime around the classical field ionization limit . The experiments are interpreted using semiclassical closed-orbit theory by taking the Fourier transform of the experimental spectra. High resolution is obtained using CW laser excitation and large frequency scans. Effects of the large non-hydrogenic core of Ba are investigated by comparing the experimental Fourier spectra with hydrogenic closed-orbit theory for - 3 incorporating the metastable ground-state wavefunction and assuming only 5d - nf excitation.

We studied the 1s2s nl and quartet states of the lithium atom by a model-potential method with B-spline basis functions in the CI scheme. The transition wavelengths and probabilities, and lifetimes for electric dipole decay are calculated and compared with experimental and other theoretical results. A large number of transitions, which can possibly be observed, are presented. Our calculated transition wavelengths and lifetimes are in good agreement with the available experimental results. Including relativistic, radiative, and mass-polarization corrections for 1s2p and 1s2s cores, we calculated for - transitions. Some of these agreed with experimental results within 1 Å. The lifetime of [1s2p3s] is calculated to be 9.89 ns, which is the best agreement with the experimental result, ns, among the present theoretical results. The lifetime of [1s2s3s] is also improved.

Brightness profiles of x-ray and VUV lines from eight molybdenum charge states between and have been measured in Alcator C-Mod plasmas. These spatial profiles agree very well with those predicted by a model which includes ionization, recombination, excitation and transport. Comparison with the profiles of many different charge states provides severe constraints upon the rates used in the model. The charge state density profiles are calculated using measured impurity transport coefficients, measured electron density and temperature profiles and newly calculated ionization and recombination rate coefficients. These new rate coefficients include direct collisional ionization, excitation - autoionization, dielectronic and radiative recombination. Excitation - autoionization is shown to be an important process, since the excellent agreement between the measurements and predictions is obtained only with its inclusion. Fits to newly calculated excitation rate coefficients for the transitions are also presented.

The configuration-interaction effects on the excitation - autoionization cross sections for the Na-like Cu ion from (l = 0,1,2) transitions were investigated in some detail. The excitation cross sections were calculated using the distorted-wave Born exchange approximation (DWBE) and configuration-interaction, bound-state wavefunctions. The autoionizing branching ratios were calculated without and with configuration interaction. The calculated results indicated that, the configuration interaction can increase the excitation - autoionization cross sections for the transition by about a factor of two, and reduce the autoionization cross sections for by about 35% but only has a slight effect on the autoionization cross sections for the transition. In addition, the autoionization cross sections make a substantial contribution to the total electron-impact ionization cross sections, it is about a factor of four of the direct ionization cross sections.

The R-matrix Floquet approach has been extended to treat multiphoton processes involving two incommensurate laser frequencies. The extensions to the R-matrix Floquet theory are discussed and the approach is illustrated with an example of light-induced continuum structure in He, linking the 1 ground state with the 1s2s state. The results obtained for this example are compared with model calculations and good agreement is found, if the q parameter is suitably chosen. A further example is provided by linking the ground state of He and the 1s2s state with the autoionizing state by two-photon processes. The differences due to the autoionizing state are illustrated by showing the changes in the behaviour of the and 1s2s states. The first example demonstrates that the code is able to describe light-induced continuum structure, while the second example demonstrates that the code is capable of describing multi-electron excitations in two laser fields with different frequencies. It is also demonstrated that for more extensive calculations, it is necessary to develop new approaches which limit the amount of memory required.

We have undertaken extensive configuration interaction calculations to determine the positions of 3p-hole states of of the form ; ; . Comparison is made with the 3p photoabsorption spectrum measured by Costello et al (1991 Phys. Rev. A 43 1441) with a dual-laser - plasma technique. It has thereby been possible to identify most of the lines in that spectrum within the energy range 47 - 65 eV.

Coupled-states calculations have been made to interpret the L-subshell ionization cross section data measured by Padhi et al for collisions of B ions with Au and Bi in the energy range 0.48 - 0.88 MeV . The calculated cross sections are in a reasonable agreement with the measured values. It has been shown that simple static Stark mixing of the target 2s and 2p atomic wavefunctions is inadequate for the description of the ionization process, and the dynamical treatment of the subshell couplings based on the solution of the coupled equations of the problem is necessary for the satisfactory description of the process.

We have investigated a systematic procedure for representing, as a separable expansion, the exchange interaction in electron - molecule scattering. Illustrative calculations of scattering quantities (eigenphase sums, total cross sections and rotationally-resolved differential cross sections) have been performed for electron scattering from molecular hydrogen in the fixed-nuclear-orientation approximation. The exchange basis can be constructed from the same Cartesian Gaussian functions used to generate the near-Hartree - Fock static interaction, supplemented by an even-tempered series of Gaussian functions located on the molecular centre of mass. Particular emphasis is placed on examining the convergence properties of this series.

In this work the multiply-differential cross section for electron-impact ionization is considered. A coplanar, equal-energy, fixed-relative angle kinematics is chosen and the particular case where the scattered and ionized electrons emerge perpendicular to each other is emphasized. It is shown how new structures arise in the spin non-resolved cross sections due to an interplay between single-binary collisions and the ratio of singlet to triplet scattering. We deduce that the angular behaviour of these structures in the spin non-resolved cross sections gives a direct hint as to the spin states occupied by the escaping electrons.

The general theory of electron - photon coincidence experiments for electron impact ionization plus simultaneous excitation is developed. Orientation and alignment parameters are defined and related to the angular distribution and polarization of the light emitted by the residual ion. As a first guide to experimentalists in this field, we present results for ionization of helium atoms, as calculated within a combined DWBA+R-matrix method.

Recent electron-impact double ionization (e, 3 - 1e) experiments on helium are investigated theoretically. An ab initio calculation has been performed on the basis of the first Born approximation using different correlated wavefunctions for the initial bound-state and a correlated double-continuum wavefunction which satisfies the exact asymptotic boundary conditions for the final state with two slow electrons. The results of this calculation are compared with those of other theories.

A variant of the Born approximation is used to calculate doubly, singly differential and total ionization cross sections for the (e, 2e) interaction between an electron and the rare-gas atoms neon, argon and krypton. The calculation is done for the ionization of the valence electron. This work is a continuation of a previous study of the alkali atoms, and the purpose, as it was there, is to elucidate basic features. Comparison to experiment is made where possible.

The distorted-wave approximation combined with the Born - Oppenheimer approximation is applied to calculate rovibrationally resolved state-to-state differential and final-state-specific integral cross sections associated with the transitions of , as well as the anisotropy parameters for the excited state. The comparison of our final-state-specific cross sections with the available recent laser-induced fluorescence experimental data in the 12 - 22 eV range is very encouraging.

We investigate - molecule scattering using both the adiabatic nuclear rotation (ANR) and the rotational laboratory-frame close-coupling (rotational LFCC) models. Results are reported for rotationally elastic , rotational excitation and the total cross sections (summed over the rotational cross sections) in the energy range 0.0408 - 9.74 eV. The present total cross sections are in good agreement with the measured values below the positronium formation threshold. The rotational excitation cross section is found to be sensitive to the short-range potential.

We have measured the extreme ultraviolet (EUV) spectrum of neon produced by electron impact excitation. The measurements were obtained under optically thin conditions, and at a spectral resolution of 0.5 nm full width at half maximum (FWHM). The most prominent features of the EUV spectrum between 45 - 80 nm are the resonance lines of Ne I at 73.6 and 74.4 nm and a multiplet of Ne II at 46.14 nm (the average value for the line centre of the two closely spaced ion lines at 46.07 and 46.22 nm). Absolute emission cross sections of these lines at 300 eV were measured and compared to other previous measurements. The measured emission cross section values at 300 eV for the Ne I lines at 73.6 and 74.4 nm are found to be and respectively, and for the Ne II multiplet at 46.14 nm is found to be (sum of the 46.07 and 46.22 nm line cross sections) with an uncertainty of 41%. In addition, excitation functions were measured for the Ne I resonance lines (0 - 400 eV) and Ne II ion line (0 - 1 keV). The excitation functions for the Ne I resonance lines were corrected for polarization because these are strongly polarized.

Several recent papers have examined the theory of electron - atom scattering in a low-frequency laser field. Only one of these papers obtained rough agreement with the size of the experimental cross section through the identification of a novel physical process: collective field effects. The purpose of this paper is to demonstrate that an error in the formulation led to an overestimate of the interaction potential by five orders of magnitude. The experimental results cannot be explained by collective field effects.

The momentum transferred to the photo-electron was specified in terms of coefficients a₁(u), u = ω/ω₀, ω = photon frequencey, ω = threshold frequency. Those coefficients were calculated analytically for hydrogen 1s, 2s, 3s, 2p, 3p and 3d. Recent work by Dr G Peach, using numerical integrations, shows that the expression given for the 3d state was in error. A discrepancy for 3d is also noted by Massacrier (1996). An error occurred in the final step of the algebra for the 3d state. The corrected expression for 3d is

which gives agreement with the calculations of Peach and with equation (50) of Massacrier.

References

Massacrier G 1996 Astron. Astrophys. in press