Table of contents

Partial cross sections for photoionisation of He 1s², leaving the ion in excited 2s and 2p levels, have been calculated. Resonance structure below the n=3 threshold due to doubly-excited autoionising states is included. The authors' calculation supports the result that, close to the n=2 threshold, the formation of ions in the 2p level dominates over the 2s level.

By using a statistical description of multiphoton processes in complex atoms, the author calculates the charge state distribution for various elements excited by UV light (193 nm). The results are in good agreement with experimental data.

From a general semiclassical expression for the spectral intensity distribution of electrons or photons emitted during a collision by the transient molecule, two analytical formulae are derived for special situations. These formulae are shown to be applicable to the analysis of spectral features reported in the literature.

The author obtains simple analytical expressions for the cross sections of the state-changing and ionisation collisions of Rydberg atoms with neutral atomic particles due to the Rydberg-electron-perturber interaction. The theoretical investigation is based on the impulse approximation and binary encounter approach for the form factor. The earlier results of Alekseev and Sobel'man (1965) and Omont (1977) turn out to be the limiting cases of the author's expressions. The calculated quenching cross sections for the Rydberg states agree well with experimental results and numerical calculations.

Four He^- resonances with all three electrons in the n=2 shell are accessible by electron impact from the He ground state but only two resonances have been identified unambiguously in electron-helium scattering experiments. Experimental evidence for the presence of a He^- resonance at 59.90 eV in the excitation cross section of the He^**(2s2p)³P autoionising state is presented and discussed.

Let N_c denote the maximum number of electrons that can be bound to an atom of nuclear charge z, in the Thomas-Fermi-von Weizsacker theory. It is proved that N_c cannot exceed z by more than one, and thus this theory is in agreement with experimental facts about real atoms. A similar result is proved for molecules, i.e. N_c cannot exceed the total nuclear charge by more than the number of atoms in the molecule.

The concept of operator orthogonality is examined and discussed in the context of mixed configurations of p and d electrons. Operators are constructed in the scalar form P^{( kappa k)}.D^{( kappa k)}, where P^{( kappa k)} denotes a collection of annihilation and creation operators for the p electrons whose resultant spin and orbital ranks are kappa and k, and where D^{( kappa k)} is similarly defined for d electrons. In addition to kappa and k, the irreducible representations of the symplectic groups Sp(6) and Sp(10), as well as those of the special orthogonal group SO(5), are introduced to distinguish the various three-electron operators t_i that can arise when configuration interaction is taken to second order in perturbation theory. Two methods are described for evaluating the matrix elements of the t_i. Tables of numerical results are given for pd, p²d, pd², and p³d. Quasispin ranks K_p and K_d are assigned to the operators so that the conjugate configurations (such as p⁴d, p²d⁹, and p⁴d⁹, the conjugates of p²d) can be treated.

Numerical comparisons of Breit-Pauli and MCDF-EAL estimates of fine-structure splittings are presented for the boron and fluorine isoelectronic sequences up to Z=26. With simple modifications a single basis set is found to be very satisfactory over the entire range. The authors stress the importance of anomalous magnetic moment corrections and examine higher-order relativistic effects neglected in the perturbative approach. A discrepancy between Breit-Pauli and MCDF-EAL estimates in the fluorine sequence is explained in terms of the inclusion of exchange core polarisation but not electron correlation. The authors present evidence that these effects cancel to a large degree, at least for this system. Direct comparison is made with experiment for F to Fe¹⁷⁺.

Expressions for the angle of the Faraday rotation in atomic hydrogen, at radio frequencies, that are valid for all temperatures and magnetic field strengths are developed taking into account the electron-proton spin interactions. Simplified expressions for the rotation are also given for the special cases when the Zeeman splitting is much greater than or much smaller than the hyperfine splitting and when the two splittings are equal. A numerical result is given for the latter case at the hyperfine resonance frequency, 1420 MHz.

The ⁴He-³He isotope shift of the 2³S₁-2³P transition is directly measured by Doppler-free absorption spectroscopy with a colour-centre laser operating in single mode around lambda =1.08 mu m. The splitting between the (2³S₁-2³P₀) line of ⁴He and the (2³S₁, F=³/₂-2³P₀) line of ³He is (811+or-3) MHz, leading to a shift of (33668+or-3) MHz for the transition 2³S₁-2³P between ⁴He and ³He. This value compares well with previous experimental data but is more accurate. The agreement with the calculated isotope shift is good.

New high-resolution spectra of Tl I are reported in the wavelength range 2100-2000 AA using the first order of a 3 m 6000 line/mm holographic grating. Synchrotron radiation from the 500 MeV accelerator in Bonn provided the background source of continuum. The observations comprise 6s²6p²P_1/2 to 6s²ns²S_1/2 (12<n<48) and 6s²nd²D_3/2 (12<n<70) spectra extending to much higher values of n than previous work. The authors have resolved the hyperfine structure of the ground state which is shown to have a significant effect on the determination of the ionisation potential.

The authors investigate collisional redistribution in a laser field with arbitrary lineshape. The phase diffusion model cannot be correct in general due to its inherent assumption of zero correlation times. The authors give general expressions for the state of the atom, the dipole correlation function and the steady-state fluorescence spectrum in the presence of collisions and phase fluctuations with finite correlation times. It turns out that collisional effects decouple from the free-atom stochastic problem to a good approximation and that modifications due to the laser bandwidth can be quite different from collisional effects, although they are additive on a microscopic level. They evaluate the fluorescence spectrum explicitly in the limit where the three lines are well separated and show that the spectrum is determined by simple field correlation functions. The arbitrary phase fluctuations do not affect the line strengths but only the lineshapes. Collisions, on the other hand, can transfer intensity between the lines, due to the very small collision time. For free atoms and strong driving fields, the fluorescence spectrum becomes a convolution of the normalised laser spectrum with the fluorescence spectrum at monochromatic irradiation, but this does not hold any more when collisions are present.

Electric-field-induced singlet-triplet anticrossing signals have been used to measure the 5¹D-5³D interval in helium and to study the differential Stark effect of the corresponding magnetic substates. The present result for the interval 5¹D-5³D_mean of 34066.0 (1.7) MHz agrees with previous, less accurate measurements though not quite with the global fit value by Farley et al. (1979) obtained by extrapolation of experimental results of n>or=7. The quadratic Stark constants are measured with an accuracy of the order of +or-1% and agree with the theoretical calculation based on hydrogenic wavefunctions.

The L-shell X-ray intensity ratios I(L_l)/I(L_alpha ), I(L_beta )/I(L_alpha ), I(L_gamma )/I(L_alpha ) for tungsten (W) and I(L_l)/I(L_alpha ), I(L_eta )/I(L_alpha ), I(L_beta )/I(L_alpha ), I(L_gamma )/I(L_alpha ) for mercury (Hg) have been measured at 17.8, 25.8 and 46.9 keV with a Si(Li) detector. A comparison is made of the experimental results with the calculated values obtained by using the theoretical X-ray emission rates, subshell ionisation cross sections, subshell fluorescence yields and Coster-Kronig transition probabilities. The experimental results are in reasonable agreement with the theoretical values.

Photoionisation studies provide a direct method for an accurate determination of vacancy decay parameters. The authors report on such measurements performed for the Au L shell. Thin Au foils were irradiated by photons with energies ranging from 11.5 to 15.2 keV. The mass absorption coefficient as well as the induced L X-radiation were measured. From the absorption coefficient the ionisation cross sections of the individual L subshells were derived. Comparison of the cross sections and the induced X-ray fluorescence gives Coster-Kronig factors, fluorescence yields and branching ratios for different X-ray lines. For the Coster-Kronig factors and fluorescence yields significant deviations from previously adopted values were found.

The problems of the determination of the rotation constant B_nu and the distortion constants D_nu , H_nu , . . . , for a given diatomic electronic potential U(r) and a given vibrational level nu , are considered. The recent 'eigenvalue function method' associating a function E(E) (of the parameter E) to the potential U(r) is extended to the case of the rotation and distortion constants. A set of eigenvalue functions B(B), D(D), . . . , associated with the potential U(r), is defined; they are independent of the rotational quantum number J and the vibration wavefunction psi _nu (r); they vary with the parameters B, D, . . . . The functions B(B), D(D), . . . are proved to be linear and to have, for every one, a unique solution: B_nu , D_nu , . . . . Examples of the numerical application are presented and discussed, they show an improvement by comparison with other methods.

The authors present new and powerful methods for the numerical evaluation of the relativistic ionisation form factor in coordinate space. Series expansions convergent for all values of the argument, as well as asymptotic expansions are given. Recursive methods are developed allowing a very efficient computation. The authors discuss limiting cases for high and low electron energies and the non-relativistic limit.

Continuum electron capture by H⁺ in He is studied for impact velocities 1-3 au by measuring (i) forward cone spectra with cone semiangle 0.38 degrees and (ii) cross sections sigma (E, theta ) at angles 1 degrees -4 degrees . Information has been extracted on the projectile-frame cross section sigma (E', theta ') near the limit E'=0 between capture and ionisation.

TDHF calculations using a two-centre basis expansion technique are carried out for two-electron systems in the energy range of 5 to 40 keV/nucleon. Cross sections are calculated for one-electron capture of p+He(1s²) and for one- and two-electron capture of He²⁺ and Li³⁺ from He(1s²) and compared with other theoretical and experimental results. The influence of the exchange potential is briefly traced by considering the collision systems He⁺(2s)+He⁺(2s) and He⁺(2s)+H(1s) for the spin-singlet and spin-triplet cases.

A relativistic generalisation of the third Born approximation is used to investigate the ground-state to ground-state capture of an electron from a hydrogenic ion A by a fully stripped positive ion B having atomic numbers Z_A and Z_B respectively. Formulae are obtained for the scattering amplitudes by expanding in powers of alpha Z_A and alpha Z_B and retaining the leading terms, alpha being the fine-structure constant. Closed analytical expressions for the relativistic electron capture cross sections for the cases without and with spin flip from the initial to the final states are derived making suitable approximations. In the limit of non-relativistic but high impact energies the authors obtain complete agreement with the electron capture cross section derived by Shakeshaft (1978) using the non-relativistic third Born approximation. At ultra-high relativistic impact energies E they find that the contributions to the total capture cross sections to leading order in alpha Z_A and alpha Z_B arising from the third Born correction terms fall off as E^-4 and E^-3 for the cases without and with spin flip respectively, which are both much more rapid than the E^-1 decay obtained with the second Born approximation.

A relativistic non-local two-channel theory is applied to the elastic scattering of 20-200 eV electrons from heavier noble-gas atoms. Total inelastic cross sections are obtained for Ar, Kr and Xe in semi-quantitative agreement with experiment. The differential cross section and spin polarisation results for Ar and Kr agree with experimental data at about the same level as results from less sophisticated (one-channel) theories. For Xe, however, the two-channel model gives a clearly improved agreement with the experimental data.

A theory is presented to formulate Kessler's idea that intramolecular double scattering should cause initially unpolarised electrons to obtain a spin-polarisation component in the scattering plane if the electrons are scattered elastically by optically active molecules (1982). The theory leads to an approximate yet feasible formula for the in-plane polarisation, and is applied to 1,3-dimethylallene. The in-plane component is an oscillating function of the scattering angle. The maximum degree of this component is about 0.5*10^-6-1*10^-8 at electron energies E=50-500 eV. These values are four to five orders of magnitude smaller than those of the perpendicular component. The difference from the in-plane polarisation in LEED is discussed within the framework of the present theory.

A classical Coulomb three-body problem is solved to investigate the post-collision interaction effects in the autoionisation process by electron impact. The resultant energy shifts differ greatly from those given by the Barker-Berry formula, especially at high energies. Reasonable agreement with experimental results is obtained in a wide energy range.

The electron impact ionisation cross section of Fe XXIV is calculated by two methods, both making allowance for excitation-autoionisation and for radiative decay. The breakdown of LS coupling for this ion does not seriously affect the results but the effects of radiative decay are to reduce the excitation-autoionisation contribution by a significant amount. The results obtained omitting radiative decay are up to 25% larger than those given by the semi-empirical formula of Burgess and Chidichimo (1983), which also omits this process.

Energy and angular dependences of the elastic scattering and of the excitation of the nu ₂, nu ₃ and nu ₅ vibrational modes have been measured in the angular range from 5 degrees to 108 degrees and for impact energies of the electrons from threshold to 3.6 eV. Absolute differential and integrated cross sections are presented. The study of the vibrations nu ₁ and nu ₄ is more limited because these Raman active modes are not excited strongly in this energy region. A previous theoretical treatment allows the determination of the amplitudes for direct excitation and for excitation via the ² Pi _g shape resonance and also the relative phases between these two amplitudes as a function of the impact energy. Branching ratios are presented for the decay of the resonance into the different vibrational channels of the molecule.