Table of contents

Without electron repulsion there were accidental degeneracies between some of the excited states of two- (or more) electron atoms among themselves and also with ionization thresholds. The effects of these degeneracies on resonances of highly ionized atoms decaying by the Auger effect of autoionization, are investigated by treating the repulsion between the electrons as a perturbation.

Propagation of strong circularly-polarized 'video' pulses in a two-level non-resonant medium without using the approximations of slowly varying envelope and rotating wave has been studied. New solutions to the Maxwell-Bloch equations called 'bisolitons' are obtained. Bisolitons are running localized 'video' pulses of the electric field vector rotating in transverse plane.

A new toroidal analyser has been used to study the double photoionization of helium, for energies down to 4 eV above threshold. The two electrons are detected with equal energies, and coincidences between them for various angles of emission can be obtained simultaneously. The plane of detection contains the photon beam, in contrast with recent experiments performed in the plane perpendicular to the photon beam. The angular correlation function is extracted directly from the present measurements, which are shown to be insensitive to the Stokes parameters of the photon beam. The width of that function is not observed to vary significantly from 4 eV to 17.6 eV above threshold.

The linear polarization Stokes parameter P₁ for electron impact excitation of the 3¹D state of helium is investigated following a recent publication by Wedding et al (1993) in which values reported at small scattering angles at 40 eV incident electron energy strongly disagree with earlier measurements of Beijers et al (1986-7) and Donnelly and Crowe (1988). The earlier data are confirmed by new measurements. Only a multichannel eikonal calculation gives even qualitative agreement with the experimental data. It is shown that the data of Wedding et al are consistent with a normalization error. Their method of simultaneous determination of three Stokes parameters is shown to lack sensitivity to substantial changes in the Stokes parameters and is inappropriate for data with their statistical quality.

The author has confirmed the existence of a resonance shape around 35.6 eV, previously found by Higgins and Burke (1991) in the S-wave positronium formation cross sections that has been calculated also within the coupled-static approximation but with the use of the Harris-Nesbet variational method. Still within the spirit of the coupled-static approximation, the author added the polarization effects to the process of including the pseudo-state 2p in the coupling scheme, and found a whole range of structures formed at this intermediate energy region of the positronium formation cross sections. These structures were also seen in the elastic cross sections. The author discussed the formation of these structures.

A resonant interaction between a laser beam, a standing light wave and a relativistic electron beam can be built up in the rest frame of the electrons. Use can be made of this result to work out a new scheme for an 'optical-wiggler' free-electron laser. The conditions for which external electromagnetic radiation with frequency lower than that of the laser pump field can be amplified via an electron scattering process of the Kapitsa-Dirac type are then determined. The linear gain of this device is estimated and is found to be of interest in the infrared domain.

High-precision matrix Hartree-Fock calculations are reported for a series of five isoelectronic diatomic systems in their ground states. A universal even-tempered basis set of Gaussian-type functions is developed for the nitrogen molecule at its experimentally derived equilibrium nuclear separation and then used to parameterize the single-particle state functions in the matrix Hartree-Fock description of the neutral molecules CO and BF, and in the ions NO⁺ and CN^- at their respective experimentally determined equilibrium geometries. The calculated energies are compared with the results of previously reported fully numerical calculations which were performed by using finite difference and/or finite element techniques and which are taken to define Hartree-Fock limit. Energies obtained using the universal even-tempered finite basis set expansion are well within what is usually regarded as 'chemical accuracy' of approximately 1 mHartree and are only in error by approximately 2.3, approximately 1.5, approximately 90, approximately 40 and approximately 140 mu Hartree for N₂, CO, BF, NO⁺ and Cn^-, respectively.

M shell X-ray production cross sections in thick targets of Pb, Th and U by K X-rays of Rb, Nb and Mo respectively have been measured. As the incident K X-ray energies are above the L₃ edge but below L₂ edge energies of the respective target elements under reference, the M X-rays are produced not only due to direct interaction of incident photons with M shell electrons but also due to the decay of L₃ subshell vacancies to M shell. The measured values have been compared with those calculated using known values of M and L₃ subshell photoionization cross sections and fluorescence yields etc, wherever possible. The percentage contribution due to shift of L₃ subshell vacancies to M shell to the total M shell X-ray emission were evaluated and it is seen that the shift contributes nearly 50% of the total M X-rays produced in the case while it is nearly 75% in the case of Th and U at the incident energies under reference.

Threshold and near-threshold photoelectron spectra were taken around the Ar K edge and up to 150 eV above threshold in order to study the behaviour of the (1s3p)4p shake-up satellite in the energy region where the transition between the sudden limit and threshold takes place. The threshold spectrum is compared to photoelectron and absorption data exhibiting features of both. The satellite intensity at zero kinetic energy is 3.8% of the 1s main line. The whole data set indicates a smooth increase to the higher kinetic energy values of about 9% around 50 eV above threshold. This behaviour can be well described qualitatively by the adiabatic transition model of Thomas, although the sudden limit value of this model seems to be reached too slowly and is, as a consequence of this, too high. Nevertheless, this strongly supports the idea that the strongest K-shell satellites are predominantly 'shake-up' in character, exhibiting a smooth transition to the sudden-limit intensity in the near-threshold region. Furthermore, other related phenomena, such as post-collision interaction and enhanced intensity of zero-kinetic-energy electrons above threshold, are observed and briefly discussed.

The photoionization of a one-dimensional Dirac delta-function potential is examined analytically for the high-frequency range in which the individual photon energy exceeds the binding energy. Closed-form expressions are presented for the population amplitudes, the spatial wavefunction, and the probability flux with no approximations other than the rotating-wave approximations, the instantaneous turn-on approximation, and the neglect of continuum-continuum transitions. The expressions are examined in various levels of approximation and compared with what would be expected for exponential decay.

Expressions of the vibration-rotational, Z_kl^v, and additional rotational, Z_kl^r, coefficients in the vibration-rotational energies of a diatomic molecule to take quantitatively into account the separate vibrational adiabatic and non-adiabatic vibrational and rotational terms in the Hamiltonian for nuclear motion have been generated by method of symbolic computation. All the radial functions have as argument z=2(R-R_e)/(R+R_e). An algorithm to evaluate these coefficients from available spectral data was tested on simulated data and applied to HCl. The relative contributions of adiabatic and non-adiabatic effects to selected term coefficients and the need for experimental measurements of the Zeeman effect on rotational and vibration-rotational transitions are discussed.

The leading singularity as the momentum transfer tends towards zero of the elastic, off-shell, two-particle, Coulomb scattering amplitude is investigated and shown to coincide with that of second Born approximation, independently of whether the intermediate-state Coulomb scattering is represented by the exact or only by the asymptotic Coulomb Green function. This indicates that for investigations of this type Coulomb rescattering in the intermediate state can be neglected. Proofs of this result are developed in both momentum as well as coordinate space.

Charge-state distributions for ions in beam-foil experiments are discussed in detail and analysed using the maximum entropy method (MEM). The symmetric distributions can be well described by one Gaussian in the whole charge-state interval. It is found that asymmetric distributions, exhibiting the shell effect can be treated using two disjoint Gaussians, in two separate intervals. The relative weights of these Gaussians are interpreted by using Fermi Dirac statistics. Physically this corresponds to varying occupancy of some of the shells in the ions. The fact that no more than two moments, in one interval or two, is sufficient is of significant importance in the analysis of charge-state distributions in general. An interpolation procedure to obtain the charge-state distribution for an energy is developed and tested. The procedure, which is made possible by the Gaussian character, uses MEM and experimental information. It is only possible to predict distributions in energy regions where experimentally measured values for other energies exist. The method is tested on asymmetric distributions of copper ions in the energy range 0.902-1.505 MeV u^-1 and symmetric distributions for calcium, titanium, yttrium, rhodium, holmium and bismuth ions for energies from 0.191 to 3.184 MeV u^-1.

An analytical formula for the electronic stopping power S was derived for swift ( nu >or approximately=Z₁ nu ₀) frozen-charge-state projectiles (atomic number Z₁) with two electrons in metastable 1s2s singlet and triplet states using a first-order perturbation method. The spatial electron distribution around a projectile was determined by the variational method. In order to demonstrate the magnitude of S, the authors also calculate the effective stopping-power charge. Compared with the ground state (1s²), S in the 1s2s configuration is found to be enhanced especially at the lowest velocity ( nu =Z₁ nu ₀) investigated. In addition, it is found that (i) there is no appreciable difference between 1s2s singlet and triplet state, and (ii) S for the 1s2s singlet can be scaled in the form S*Z₁^-2.20.

Excitation cross sections of Kr³⁴⁺ ions colliding with various target atoms, (Z_t between 6 and 40) at nu =35 au have been investigated both experimentally and theoretically. The experimental cross sections were obtained from those for production of projectile He-like and H-like Lyman X-rays. The combined experimental and theoretical analysis of H-like emission allowed for a determination of the competitive capture-ionization process, whose contribution to the Lyman production cross sections is important for near-symmetric collision systems and has to be subtracted. A saturation effect of the experimental excitation cross sections with Z_t is analysed from the theoretical point of view through atomic and molecular coupled-state calculations.

Secondary ions produced from condensed NH₃ and CH₄ molecules under MeV amu^-1 heavy ion impact have been observed. Intensities of multiply charged atomic ions are found to depend upon the chemical structures of the parent molecules, while those of cluster ions seem to be influenced through the chemical reactivity of the parent molecules or collision products.

The atomic-orbital close-coupling method (AO-CC) has been employed in calculations of electron capture and target excitation in collisions of fully stripped ions A^q+ (q=2-6) with Li(ns,p) atoms (n=2,3) at low and intermediate energies. Total and state selective single electron capture (SEC) cross sections have been determined. Data for SEC into nl sub-levels of the projectile have been used to calculate the related line emission cross sections, which are compared with available experimental and theoretical data. In addition cross sections for various target excitation processes, in particular the Li(2s-2p) resonance transition are presented. The scaling properties of the total SEC and target excitation cross sections are investigated, and their implication for lithium beam edge plasma spectroscopy is being discussed. For completion, scaling relations for ionization are also given.

Results of numerical calculations of the spectrum and angular asymmetry parameters of two-photon bremsstrahlung for a 8.82 keV electron moving in a static atomic field are presented for different targets: He, Ne, Ar, Kr and Xe. The data obtained are compared to previous theoretical and experimental results.

A pulsed cross beam technique previously developed in this laboratory has been used to study the electron impact ionization of copper. Previous measurements have been very limited in scope and exhibit large discrepancies. Relative cross sections sigma _n for the formation of 1 to 5 times ionized copper have been measured with high accuracy within the range 7.8-2100 eV. Individual cross sections have been obtained by normalization to absolute values of sigma ₂ obtained by Freund et al (1990) at energies below 200 eV using a fast crossed beam technique. Weak structures in sigma ₁ can be attributed to Auger decay processes following the creation of 3s subshell and L shell vacancies but there is a lack of other pronounced structures in sigma _n for n>1 where many close-lying subshell vacancies are involved. At 2100 eV cross sections sigma ₅ are less than three orders of magnitude smaller than sigma ₁.

The authors present results for the change in polarization of transversely polarized electrons scattered by randomly oriented oxygen molecules over the energy range 0.25 eV to 15 eV. They use the nine-state R-matrix calculations of Noble and Burke (1992) to obtain values for transitions between the four triplet states X³ Sigma _g^-, C³ Delta _u, A³ Sigma _u⁺ and B³ Sigma _u^-. They confirm that the change in polarization is small for elastic scattering from the ground state, X³ Sigma _g^-, over the whole of the energy range, through structure can be seen due to the ² Pi _g, ² Pi _u and ⁴ Sigma _u^- resonance states. Larger changes are observed for electronic excitation with considerable variation in the 8 eV region due to the ² Pi _u resonance.

The energy scheme of ethyl iodide has been studied by excitation of the valence and Rydberg states lying in the near UV and UV (3-10 eV) by electron impact and the angularly resolved electron energy loss spectroscopy. The lowest energy valence triplet states have been identified in the 3-5.5 eV region with a maximum of transition probability around 4.3 eV and at 4.78 eV respectively. Additional valence singlet and/or triplet states have been suggested to be located between 5 and 6 eV. For the Rydberg series, several electronic states contribute to the lowest energy terms of the Rydberg bands. The differential cross sections for the B, C and D Rydberg bands have been measured and confirm that each state within the electronic multiplet in each band has a differential cross section different from that of the other. In addition excitation of the A₁ states is very likely responsible of a minimum in the D/C and D/B differential cross section ratios. In addition the first terms of two new nd series converging to each spin-orbit component of the ionic ground state have been reported. Vibrational levels of the E(1) states which are the most intense in pseudo-optical conditions have been partly resolved for the first member of the ns, np and nd series.

For atoms of three-state ladder or V configuration, a large population may be 'locked' into a single high-lying excited state via atomic lower-state coherence even when it exceeds the total population in the two lower states. The conditions are analysed and the two systems are compared.