Table of contents

The X-ray absorption spectra of the atomic rare earth cerium, samarium, gadolinium, erbium and ytterbium were measured for photon energies close to the 2p threshold. Correcting for the instrumental broadening, the widths of the white lines were determined. Except for cerium, these experimental widths fall below the theoretical total L level widths. Combining the available experimental data for the elements between Xe (Z=54) and Hg (Z=80) and new data on Xe it is demonstrated that theory and experiment agree for atomic numbers Z>or=70 but differ for the low-Z elements.

An R-matrix LS-coupling calculation of the photoionization of neutral iron is presented, in which the 17 terms with configurations 3d⁷, 3d⁵(⁶S)4s², 3d⁶(⁵D)4s and 3d⁶(⁵D)4p are included in the Fe⁺ core. the photoionization cross section for excited states of Fe show large broad resonances, known as PEC resonances, which arise due to photoexcitation of the core. The mechanism is illustrated for the 3d⁶(⁵D)4snd ⁷S^e series in Fe, where the photon is absorbed by a 4s4p transition in the Fe⁺ core with the nd electron acting as a spectator, forming autoionization resonances below the 3d⁶(⁵D)4p thresholds.

The spectra of electrons emitted in collisions between H⁺ and He²⁺ projectiles and He, Ne and Ar targets at energies of 50 and 100 keV amu^-1 have been studied. The data are in qualitative agreement with results of Irby el al. (1988), but are in disagreement with measurements of Bernardi et al. (1989, 1990). It is shown that the observed electron spectra have a dependence on both target-ion and projectile effective charge that can be understood qualitatively in terms of 'saddle-point' ionization. Several issues relevant to saddle-point ionization are discussed.

The consistency of the authors' previous results, for He ionization by H⁺ and ³He²⁺ impact, with theoretical calculations is briefly commented upon. The results do not support the hypothesis of an independent saddle-point mechanism at the projectile energies studied.

The yield spectra of Xe²⁺ and Xe³⁺ ions near the 4d ionization limits have been measured by a threshold electron-photoion coincidence method. These spectra consist of some prominent peaks corresponding to the 4d^-1np Rydberg states and two dominant peaks corresponding to 4d threshold ionization. It is proposed that threshold-electron emission from the inner-shell resonance states is due to not only shake-off (e.g. Xe 4d^-16p to Xe²⁺ 5p^-2) but also two-step autoionization (e.g. Xe 4d^-16p to Xe⁺ 5s^-15p^-17p to Xe²⁺ 5s^-15p^-1). The two-step autoionization process plays an important role in decays from the lower n Rydberg states. Two dominant peaks somewhat above the 4d ionization limits show the effect of post-collision interaction, namely, shifts of peak positions and asymmetric profiles. The shifts are almost the same in the Xe²⁺ and Xe³⁺ spectra.

The spectrum of lead was observed in the 1300-700 AA wavelength region in which the 5d⁸6s-5d⁸6p transitions of Pb VI are located. In the 5d⁸6s configuration 15 out of 16 energy level values could be determined. Only the highest J=1/2 level is missing. In the 5d⁸6p configuration 43 of 45 levels were established. In both cases the levels based on the 5d⁸(¹S₀) parent are missing. 170 lines were classified in this spectrum. The level structure was calculated using the conventional set, containing Slater-Condon parameters.

Three different calculational methods (AUTOLSJ, MZ, MCDF) were used to obtain energy levels and autoionization rates for doubly excited states 2l3l' of two-electron systems (10<or=Z<or=42). When all the transpositions of the levels are done according to the Wigner-von Neuman theorem (noncrossing of the levels with the same total angular momentum J and parity of the system along the Z isoelectronic sequence), it becomes possible to compare energy levels and autoionization rates obtained by the three methods. The Z dependences of the quantities are investigated for each calculational method for 10<or=Z<or=42. The final comparison reveals rather good agreement between the results obtained by the three methods.

Near-threshold photoionization of krypton and xenon has been studied, in the region of the 4s and 5s satellite states respectively. Photoelectron spectra for various constant values of photoelectron energy, from 0-1 eV, have been measured under high resolution revealing a rich structure due to satellite states. Partial ionization cross sections for many of the satellite states, up to 3 eV above their respective thresholds, have been obtained which demonstrate that the mechanism for satellite excitation is a two-step process via doubly excited neutral states or resonances.

For xenon, the angular distribution of photoinduced N_4,5-O_2,3O_2,3 Auger electrons and of the spectator autoionization lines following 4d_5/2 to 6p_3/2 and 4d_5/2 to 7p_3/2 excitation have been measured and analysed on the basis of a two-step formulation for the photoprocess and the subsequent radiationless transition. A close relationship between the angular distribution of Auger electrons and spectator autoionization electrons is established theoretically and found to be in general agreement with the experimental observation. It yields deeper insight into the radiationless decay mechanisms above and below the 4d photoionization threshold.

Three mutually related problems concerning the interaction of an electron with an alkaline-earth atom are examined. (i) The scattering problem: the phaseshifts for p-electron scattering by Ca, Sr and Ba are calculated. (ii) The negative ion problem: the wavefunction and the binding energy are obtained for the outer electron in Ca^- 4s²4p, Sr^- 5s²5p and Ba^- 6s²6p ions. (iii) The photodetachment problem: the photodetachment cross sections are calculated for the outer electron in each of these ions. The most intriguing feature of these problems is the strong correlational potential, attracting the electron to the atom due to the large dipole polarizabilities of Ca, Sr and Ba. The calculations are performed by means of the Dyson equation of the many-body theory.

Heavy-particle collisions involving strong electronic coupling can be conveniently described by using a complex (optical) potential in the entrance channel. Uniform JWKB stationary-phase techniques are used to evaluate T-matrix elements for transitions where an electron is ejected. The semi-analytic expressions for the resulting electron energy spectra are no more difficult to implement than corresponding ones for totally real potentials. Numerical results are reported for Penning and associative ionization from subthermal He*(2³S)+He*(2³S) collisions. These are in excellent agreement with fully quantal, complex-potential computations. The stationary-phase expressions for T-matrix elements and differential cross sections are employed to elucidate the rapid and slow rainbow interference oscillations in the spectra, including the significant effects of turning points and the imaginary width of the entrance-channel potential.

Positive ions produced from C₂H₂ by electron impact were collided with Cs, K and Na vapours in the keV energy range. Negative ions formed by two electron transfers were mass-analysed. C^-, CH^-, C₂^- and C₂H^- ions formed by dissociative and/or nondissociative processes were observed. The dependences of each ion on target density were measured. Formations of nondissociative C₂^- ions from C₂⁺ ions and C₂H^- ions from C₂H⁺ ions were determined to be double electron transfers in one collision from the density dependence for the Cs target. The cross sections of nondissociative C^- and CH^- ion formations were much less than those of C₂^- and C₂H^- ion formations. The order of cross sections for nondissociative C₂^- and C₂H^- ions in the condition of a Cs target at the collision energy of 3.0 keV was estimated to be 10^-18 cm². Nondissociative peaks of C₂^- and C₂H^- ions which were detected with a Cs target were not, however, observed with either the K or Na targets. The double electron transfer cross sections are discussed in relation to the endothermicity.

The integral and differential cross sections for the electronic excitation of the 2l and 3l states of hydrogen obtained by the multichannel eikonal theory in a 10-channel basis are reported. The lambda , R and I parameters for the 2p state of hydrogen at 54.40 and 350 eV are also provided. Good agreement with the experimental data of Mahan et al. (1976) for the integral cross sections sigma _{1s to 3l} is achieved by the updated and extended multichannel eikonal theory. Two general conclusions follow from the results reported in the paper: (a) the first Born results underestimates experiment in s to d transitions and (b) indirect coupling mechanisms for the excitation of the 3d state of hydrogen are important and cannot be neglected. These conclusions indicate that the pseudostate basis used in a number of optical potential model calculations needs further refinement in the intermediate energy region.

Calculations of the integral and differential cross sections for the electronic excitation of the 2¹L and 3¹L states of helium are reported. The multichannel eikonal theory employing a 10-channel basis set is used. The lambda and chi parameters for the 2¹P and 3¹P states at 80 eV are also reported. A number of misprints concerning the original multichannel eikonal theory results of Flannery and McCann (1975) have occurred in the literature. These misprints are corrected in the paper. A detailed description of both the theoretical and experimental difficulties involved in the determination of the integral cross sections sigma ₁¹_{S to 3}¹_S and sigma ₁¹_{S to 3}¹_D is provided. In addition, a critique of the assumptions made in previous experimental determinations of the differential cross sections (both composite and magnetic sublevel) for the 2¹L states of helium is given. A principal conclusion of the paper is that the first Born approximation underestimates experimental data in the intermediate energy region for ¹S¹D transitions, in contrast to the trend seen in ¹S¹P transitions. In the paper the importance of indirect mechanisms in the excitation of the 3¹S and 3¹D is shown, thereby confirming the early results of Chung and Lin (1968, 1969) and the tentative conclusions of Massey (1969, 1974).

The authors have formulated a polarized-orbital treatment of electron-atom scattering which includes dynamic distortion effects. Detailed formulae are given in terms of their previous formulation of the polarized-orbital method. Results for electron-helium scattering are presented and compared with various experimental measurements as well as with the very accurate theoretical calculation of Nesbet (1979). Definite improvements are noted over their previous polarized-orbital calculations. They have also tested the reliability of their procedure for incorporating dynamic distortion effects by performing an atomic structure calculation of the ionization energies of several low-lying valence states of lithium.

Electron-impact cross sections for excitation from ground 3s ²S and excited 2p⁵3s3p ⁴S states to the lowest core-excited quartet states of Na I arising from the configurations 2p⁵3s3p, 2p⁵3s3d and 2p⁵3s4s are calculated and contrasted using a 12-state R-matrix method for energies from near threshold to 6 Ryd. In the threshold region resonance structure characterizes the cross sections with those from the 2p⁵3s3p ⁴S state dominating the corresponding ones from the ground state by at least two orders of magnitude. Significantly, the cross sections from the inner-shell 2p⁵3s3p ⁴S state are on the average comparable with or even greater than those from the ground state to the 3d ²D, 4s ²S, 4p ²P degrees or 4d ²D previously calculated. The results may have interesting implications for the Na I XUV laser.

Calculations are presented for electron-H₂ collisions for energies up to 25 eV. Excitation from the ground, X ¹ Sigma _g⁺, to the lowest six excited electronic states, b ³ Sigma _u⁺, a ³ Sigma _g⁺, c ³ Pi _u, B ¹ Sigma _u⁺, E, F ¹ Sigma _g⁺ and C ¹ Pi _u, is explicitly considered for all total symmetries up to and including ² Phi _g. The target states are represented using a full configuration interaction treatment within a basis of Slater-type orbitals optimized to give accurate vertical excitation energies. Results are presented for eigenphase sums, in which the resonant and other features are analysed, as well as total electronic excitation cross sections. Extensive comparison is made with the available experimental data. In contrast to many previous ab initio calculations the results are in good agreement with the observed electronic excitation cross sections for all the processes considered.

The production of metastable O(⁵S) atoms by electron impact dissociative excitation of O₂ has been observed using the technique of translational spectroscopy. The kinetic energy spectra show two distinct structures with maxima near 0.13 and 2.5 eV. These distributions indicate that two distinct excitation processes are involved in the production of the O(⁵S) species, one from the dissociation of O₂* bound states in their repulsive region, one coming from the dissociation of completely repulsive O₂* potentials. Excitation functions for both these excitation processes have been measured and compared with earlier data.