Table of contents

We have performed, using an ab initio basis-set method, calculations of the absolute cross section for double photoionization of helium for energies from 2 to 80 eV above threshold, and we have extended the results down to 0 eV through spline interpolation. Our results are consistent with the Wannier threshold law, and agree well with the near-threshold measurements. At higher energies (20 eV or so above threshold) significant discrepancies with experiment are evident. We find a simple relation between the integrated cross section for double photoionization and the singly differential cross section at the midpoint of the energy distribution.

Quasi-resonant charge-exchange reaction C²⁺+B⁺ to C⁺+B²⁺ by coincident detection of the product ions C⁺ and B²⁺ for CM-energies between 1.3 and 160 keV. The reverse reaction was also investigated in the energy range between 2.5 and 80 keV: B²⁺+C⁺ to B⁺+C²⁺. The theoretical treatment is based on the correlation diagrams for the system (CB)³⁺ and shows that distinct channels are responsible for the direct and inverse reactions. The results of calculations are in good agreement with me experimental data.

The triple differential cross section for e-Li has been calculated in a distorted-wave approximation at incident energies in the range 0.5 to 20 eV above the ionization threshold in the coplanar theta _ab=180 degrees and perpendicular geometries and equal energy sharing situation. The results are compared with those for hydrogen. The variations in the two cases are found to be qualitatively very different. It is found that, in the case of lithium, the D and F partial waves in the final state are quite significant except when the excess energy is less than about 0.5 eV.

Semiempirical formulae for multiple-ionization (MI) cross sections sigma _n of atoms and ions by electron impact are suggested for ejection three or more electrons n>or=3. The formulae have been deduced on the basis of available experimental data on sigma _n and the assumption of the Born-Bethe dependence of sigma _n on the incident electron energy E. The formulae suggested can be used for estimation of MI cross sections sigma _n, n>or=3, from threshold up to high electron energies E approximately=10⁵ eV for an arbitrary atomic or ionic target.

For pt. III see ibid., vol. 26, p. 1605 (1993). Matrix Hartree-Fock calculations for a polyatomic molecule, the ground state of the carbon dioxide molecule, are carried out using basis sets which afford an accuracy approaching the sub- mu Hartree level for the energy of the carbon monoxide molecule. Using a 28s14p14d14f atom centred and a 24s10p11d bond-centred Gaussian basis set, an upper bound for the Hartree-Fock energy of the CO₂ ground state at the experimental geometry is computed as -187.725 408 Hartree. Based on our experience with diatomic molecules, we believe this value to be accurate to 5-6 mu Hartree. The results of the present calculations are used to access the accuracy of many of the basis sets currently used in molecular self-consistent field studies including those designated STO-3G, 4-31G, 6-31G, 6-31G(3d), 6-311G, 6-311+G(3df), D95 and D95V+(3d) as well as the basis sets cc-pVDZ, aug-cc-pVDZ, cc-pVTZ and aug-cc-pVTZ, which have been proposed for electron correlation studies.

The stabilities of eleven electronic states of BN³⁺ were investigated using ab initio multireference-CI methods. Three states, 1⁴ Sigma ^-, 1² Delta and 2² Sigma ⁺, are found to be metastable, an unexpectedly large number for a diatomic trication from the first row. However, small dissociation barriers, from 0.10 to 0.21 eV, result in short tunnelling lifetimes in the order of nanoseconds for 1⁴ Sigma ^- and of picoseconds for 1² Delta and 2² Sigma ⁺. Hence, metastable BN³⁺ may elude mass-spectrometric detection. Generation of BN³⁺ by ionizing BN (66 to 77 eV) or BN⁺ (54-65 eV) should lead directly to atomic products. Ionization of BN²⁺ (31-41 eV) could reveal some vibrational structure due to metastable states of BN³⁺. Most of the unbound states of BN³⁺, stabilized with respect to the repulsive Coulomb potential due to B²⁺N⁺, as indicated by the calculated kinetic energy release and other data.

A theoretical investigation of static electric-field effects on the 2p²³P^e state and the doubly-excited ³P^o states below the H(N=2) threshold is carried out using the method of complex-coordinate rotation. Products of Slater orbitals are used to represent the two-electron wavefunctions, with l_max=8 employed for individual electrons. Block matrices with up to L_max=4 (G-states) are used to investigate the convergence behaviours for the resonance parameters (resonance energy and width). When the external electric field is turned on, the Stark effect is observed for the M=+or-1 components of the two nearly-degenerate 2p²³P^e and 2s3p ³P^o (2) states. Results for electric-field effects on the lowest 2s2p ³P^o(1) state are also given, as well as effects on the 2s3s ³S^e(1) and 2s4s ³S^e(2) states.

K alpha to L alpha intensity ratios of 11 lanthanide elements have been measured following photoionization at 59.5 keV by using a Si(Li) detector. The measured values are in good agreement with theoretical results. The theoretical results were calculated from sigma _Li expressions for n_KLi not=0 and n_KLi=0.

We give a theory of magneto-optic rotation and pseudorotation spectra for forbidden transitions. The case considered is that of a weak magnetic field at an arbitrary angle to the direction of light of low intensity passing through a dilate atomic vapour. The theory is applied to the 1279 nm M1 transition in atomic lead and the 1283 nm M1/E2 transition in atomic thallium. Experimental Voigt and Faraday effect data for lead measured in a sensitive optical polarimeter provide confirmation of the predicted profiles.

The static electric dipole polarizabilities and hyperpolarizabilities for different states of the ground configuration of the atoms He through Kr have been calculated at the restricted Hartree-Fock level using a finite perturbation method. The required field-dependent energy and dipole moment data have been computed with a recently developed numerical MCSCP code for atoms in a static electric field.

We use the complex-basis-function technique to calculate one- and two-photon photoionization rates in intense light sources for atomic hydrogen, the hydrogen negative ion and the hydrogen molecular ion. For H₂⁺ we also calculate three-photon ionization rates. Rather than calculating the ionization rates in the perturbation approximation, we take account of strong field effects by calculating complex quasienergies using a mixture of real and complex-basis functions. This allows us to extend the calculations from the perturbative to the non-perturbative regime as the laser intensity is increased. The method works well and gives reliable estimates of these rates.

The M_2,3-M_4,5N Coster-Kronig and the M_2,3-NN Auger electron spectra from krypton have been measured using synchrotron radiation and electron beam excitation. By combining information from the different excitation methods, a large number of transitions have been identified. Intensity distributions between the line groups have also been studied both experimentally and theoretically. Coster-Kronig decay widths of the 3p hole states have been determined. The effects due to the initial-state configuration interaction have been revealed by comparing the experimental results with multiconfiguration Dirac-Fock calculations.

The formalism of inclusive probabilities allows the computation of many-electron cross sections from single-particle amplitudes in the framework of the independent particle model. We use this formalism to calculate projectile Lyman X-ray emission cross sections in collisions between Kr³⁴⁺ ions and various dressed targets. We show that target electrons play a major role in the collision dynamics and that Lyman X-ray emission cannot be interpreted as arising from pure single excitation of projectile electrons. This brings new insight into the saturation of excitation cross sections and the validity of the one-electron model for the case of many-electron targets. Our results compare very well with recent experimental data.

Differential cross sections for electron elastic scattering in mercury have been measured over the incident electron energy range 9 to 25 eV and the angular scattering range 10 to 120 degrees . The cross sections have been normalized to the absolute value of Holtkamp et al. (1987) at 25 eV and 60 degrees . The integral elastic cross sections and total excitation cross sections in the above energy range have also been obtained.

The 29-state R-matrix calculations in which physical target states of helium with n=1, 2, 3, 4 and 5 are included in the total scattering wavefunctions have been reported for the 1¹S-3³P differential cross sections at scattering angle 90 degrees by Fon et al (1993). The same R-matrix calculations are now used to obtain integral and differential cross sections for 1¹S-3^3,1P and 1¹S-3^3,1D excitations. The calculations are performed at 400 energies ranging from the n=3 thresholds to 24.1 eV for electron scattering angles 20, 60, 90 and 120 degrees. The results are compared with the only experimental data reported by Allan (1992). There is a good qualitative agreement.

Differential and integral cross sections for the elastic scattering of electrons by unoriented oxygen molecules have been calculated and measured for selected impact energies in the range from 5 to 20 eV. The present theoretical results were obtained using the R-matrix method. The calculations have been carried out using a model with nine electronic target states and configuration-interaction representations of the target wavefunctions. Ten scattering symmetries with a fixed internuclear separation of R=2.3 a₀ and the vibrational averaged resonance symmetries ² Pi _u and ⁴ Sigma _u^- have been used for calculating the cross sections. The current experimental data were derived using standard crossed-beam techniques. Both the calculation and measurements were conducted to provide further insight into the worrying discrepancies between the differential cross sections of various experimental groups for low-energy elastic electron scattering from O₂, as were recently highlighted in the literature.

Differential and total cross sections for the excitation of the 3d¹⁰4p ²P state in copper by electrons have been measured at incident energies of 20, 40, 60, 80 and 100 eV. The differential cross sections cover the angular range from 2.5 degrees to 130 degrees . Absolute values of the cross sections have been assigned using a generalized oscillator strength technique and the validity of this technique is explored in some detail. The measured cross sections are compared with those predicted by several theories where it is found that the close-coupling calculation of Msezane and Henry (1985-6) accurately predicts the value of the total cross sections for incident energies greater than 20 eV. Agreement between theory and experiment for the differential cross sections is poor. The present measurements of the differential cross sections resolves the ambiguity in the absolute values of the cross sections measured by Trajmar and coworkers (1977).

Integral cross sections are presented for vibrationally inelastic electron-methane scattering. They were calculated by a method which is an extension of the single-centre expansion approach previously applied by us to elastic electron-molecule scattering. The scattering wavefunction is first calculated within the adiabatic nuclei approximation, and is then substituted once into the Lippmann-Schwinger equation in order to correct for non-adiabaticity. This yields inelastic cross sections in good overall agreement with experiment. Threshold peaks are found in the cross sections for excitation of the infrared active modes. The results show that our method is a promising approach with which to investigate the inelastic scattering of electrons from polyatomic molecules.

Electron impact spectroscopy has been used for the first time to obtain energy-loss spectra for chlorine dioxide, OClO, over an energy range 2.5 to 12.5 eV. The differential oscillator strength (DOS) obtained from the energy-loss spectrum is compared with the DOS obtained from optical measurements. Oscillator strengths for several transitions have been calculated from a summation of the DOS and comparisons are also made with previous optical data.

Dynamic multipole polarizabilities of 1, 2 ¹S and 2 ³S states of helium are calculated through a variation-perturbation approach. They are used to calculate the dispersion coefficients, C₆, C₈, and C₁₀, using 32-point and 64-point Gauss-Legendre integrations. The dispersion coefficients govern the interaction energy for the long-range electronic interaction between two helium atoms. The present numerical results are compared with other calculations.