Table of contents

The difference between the frequency of the 2S-4P_1/2 transition and a quarter of the 1S-2S transition frequency in atomic hydrogen has been measured by high-resolution laser spectroscopy. The result is 4696.3 (2.0) MHz, from which the value 8168 (8) MHz is derived for the 1S Lamb shift. This is the first determination of this quantity from continuous wave spectroscopy without frequency standards.

The decay parameters alpha ₂ for krypton M_4,5-N₁N_2,3 Auger transitions induced by photon impact are determined experimentally and compared to electron impact data and theoretical predictions. Good agreement is found with respect to theoretical data obtained in the LS coupling scheme and treating many-electron effects.

The orientation parameter and the dipole moment of the n=2 states of He⁺ resulting from electron capture in He²⁺+H collisions are examined using the classical trajectory Monte Carlo (CTMC) method and the close-coupling expansion using two-centre atomic orbitals at 10, 25 and 50 keV amu^-1. It is shown that the orientation parameters calculated from the two theories are in good agreement but large discrepancies exist for the dipole moments. Together with previous similar comparisons for p-H collisions, the authors conclude that the CTMC method is not reliable in predicting the coherence parameters of excited states formed in atomic collisions.

Total and partial cross sections are calculated for single- and double-electron capture in C⁴⁺-He collisions from 0.75 keV to 11 MeV projectile energy in a two-centre close-coupling expansion of the wavefunction in atomic orbitals. The calculated cross sections are in good agreement with experiments. An extended Bohr-Lindhard model for double-electron capture suggests that the dominant dynamics at intermediate energies can be interpreted as a simultaneous release of both electrons combined with independent particle probabilities for each electron to adjust to the projectile.

The total charge-transfer cross sections in Sr⁺+Mg and Sr⁺+Ca collisions are investigated experimentally using the crossed-beam technique in the energy range 250-1300 eV. The theoretical two-state calculations are performed by numerical solution of the Schrodinger equation. A considerable underestimation of the experimental data is found and discussed. It is concluded that the simple theoretical models used relatively successfully for alkali-ion-alkali-atom systems, are not adequate for alkaline-earth-ion-alkaline-earth-atom systems.

The scattering of unpolarized electrons from hydrogen atoms is treated using the hyperspherical coordinate and the multichannel quantum defect approach with particular emphasis on the formation of quasi-stable resonant e^--H complexes. Resonant positions and widths, formed below the H(n=2), H(n=3), and H(n=4) thresholds, are calculated for symmetries ^1.3S^e, ³P⁰, and ¹D^e. Resonant cross sections are also given for the singlet and triplet L=0 partial waves. Good agreement is found with other more computationally intensive calculations.

Estimates of the ground state energies of several closed-shell atoms and ions with atomic numbers 1<or=Z<or=102 are presented. A relativistic formulation has been employed: energies are obtained as expectation values of the appropriate combination of the Dirac, Coulomb and Breit Hamiltonians with respect to the corresponding Slater determinants of four-component spherical spinor orbitals. Atomic nuclei have been modelled as uniform spherical charge distributions. An expansion method using a kinetically-balanced basis constructed from Gaussian type functions ( approximately rⁿe^{- alpha ir2}, where n is an integer and the alpha _i form a geometric progression) has been used to formulate the variational problem for the optimal orbital radial functions in each case. The resulting sets of coupled algebraic eigenvalue problems were solved approximately using the self-consistent-field iterative method. The authors' results are compared with those of other workers and with finite-difference calculations carried out specifically for this purpose. They estimate the accuracy of their work to be within a few parts in 10⁸ in most cases.

Dynamic dipole ( alpha ), quadrupole (C), dipole-quadrupole (B) polarizabilities and hyperpolarizabilities ( gamma ) of H^-, for frequencies below the ionization threshold, have been calculated using accurate, explicitly electron-correlated wavefunctions. The static values are alpha =2.06165*10² au, C=2.5916*10³ au, B=-4.843*10⁵ au and gamma =8.03*10⁷ au.

For autoionizing levels converging to the N=2 threshold of helium-like ions with Z=2-26, the 1/Z expansions of CI energies are compared with the predictions of the S-matrix perturbation theory. The combined data for the expansion coefficients can improve the validity of calculations. Autoionization decay amplitudes and widths are obtained with both the unscreened and screened hydrogen-like orbitals for the free electron, as well as in the first order in continuum Coulomb coupling. Terms of order 1/Z can be evaluated to improve the perturbational data for autoionization amplitudes, involving both screening corrections and the first-order continuum coupling.

The authors report calculations of energies and wavefunctions for doubly-excited states using a direct diagonalization method in a B-spline basis. They report results for bound ¹S^e and ¹P⁰ states in He and quasi-bound 3lnl' states in N⁵⁺ and N³⁺. The latter case has a 1s² core and they show that the B-spline technique can easily be extended to quasi-bound states outside a closed core. They show that the B-spline approach in this case can compete with the most accurate techniques available. Also the influence of the core on the composition of the 3l3l' states is studied.

Doubly excited states of two-electron atoms and ions are calculated using the configuration interaction method and the resulting wavefunctions are analysed in the body frame of the atom. The authors have computed the percentage of the projection (T) of the total orbital angular momentum L along the interelectronic axis for the 3l3l' and 3l4l' doubly excited states. It is shown that along each rotor series, the projection is dominated by a single T component for small L states and that these states are well described by the molecular model. For the high-L states along the rotor series, admixture of different T components is large and these states display properties characteristic of a shell model. The relation between the purity of T projection and the T doubling is also discussed.

Doppler-free laser spectroscopy has been used to measure the broadening of the 657.3 nm calcium intercombination line by the noble gases helium, neon and argon. A multiparameter least-squares fit was used to deduce the pressure broadening parameters from the raw data. To clarify the shape of the Doppler-free line, core deviations from a simple Voigt profile were taken into account. For this purpose the Zeeman splitting of the 657.3 nm calcium line due to small magnetic fields was determined. The results for the broadening parameters are compared with those of Smith (1967) who applied classical spectroscopy to the same broadening phenomena.

Stabilization techniques are used to investigate the existence of 'shape' resonances of H^- above N=2. For each of the ¹P⁰ and ³P⁰ symmetries, the authors only detect a single resonance state: the energy of the singlet one, -0.1244 Hartree, agrees with previous calculations and measurements; and the triplet resonance, of energy -0.1234 Hartree, has not yet been detected experimentally. Both states had been predicted using the hyperspherical coordinate method. On the other hand, further predictions of this method relating to series of (K=-1, T=0) resonances and on the 'size' of the 'shape' resonances are seen to be probably invalid. The 'shape' resonances have a complex configurational structure and a 'size' of about 80 a₀.

Using synchrotron-radiation-based angle-resolved photoelectron spectrometry the authors measured the relative partial cross sections and asymmetry parameters for all 4p^-1 ionic states of atomic bromine between the 4p^-11S₀^e and 4s^-13P₂⁰ thresholds. The cross sections are placed on an absolute scale using previous data obtained with an electron spectroscopy modulation method. Attention is focused on autoionization phenomena due to the 4s²4p⁵ to 4s4p⁵np one-electron and 4s²4p⁵ to 4s²4p³nln'l' two-electron excitations. The 4s to np Rydberg series strongly resembles the equivalent series in krypton. The autoionization profiles of the doubly-excited states are found to depend strongly on the continuum channel. Outside the resonances, nonrelativistic Hartree-Fock calculations are in reasonable agreement with the present experimental results. Discrepancies are attributed to correlation and/or relativistic effects.

In an (e,2e) simulation of photoelectron spectroscopy the authors have measured the parity unfavoured spectrum of the cadmium 4d⁹5s²5p J=1 autoionizing region. This result is combined with their earlier measurement of the parity favoured spectrum to yield the spectral behaviour of the photoelectron asymmetry parameter. The results are in fair agreement with the true photoelectron data of Jimenez-Mier et al. (1989).

The charge state distribution of photoions from K and Ca targets has been measured in the L-shell ionization region (270-400 eV for K, 330-500 eV for Ca) using a time-of-flight technique coupled with synchrotron radiation as a light source. In both the K and the Ca targets, doubly and triply charged ions are the prominent products, and outstanding doublet peaks due to the 2p-3d resonance excitation are observed in the yield spectra. The doubly charged K²⁺ ion predominates over the K³⁺ ion throughout the present energy range. On the other hand, the Ca²⁺ ion is the most intense species below the 2p ionization threshold and the Ca³⁺ ions dominate over Ca²⁺ ions above the threshold. These features of the photoion can be understood mainly by successive Auger decays from the 2p-shell vacancy states. The yield of higher charged ions indicates some contribution of shake processes.

The shake-up Auger satellite spectrum associated with the N 1s Auger spectrum of N₂ is recorded using monochromatized X-ray excitation. As many as 11 structures are observed in the spectrum. Assignments in terms of participator and spectator processes are discussed based on data from normal Auger and shake-up spectra and from a simple one-particle model. The interpretation is assisted also by a new N 1s photoelectron shake-up spectrum showing several hitherto unobserved details. A new X-ray excited normal Auger electron spectrum of N₂ is also presented.

Equations are derived which describe the effect of an electric field gradient on the intensity of allowed rotational transitions. The effect, although small, should be measurable and provides another method for the determination of the sign and magnitude of the molecular electric quadrupole moment.

Two-photon absorption in methanol solutions of Rhodamine 6G is investigated by photoacoustics using the second harmonic of a pulsed Nd:YAG laser. Competition between one-photon and two-photon processes is observed, depending critically on the sample concentration and input light flux.

The author presents a general unified model of electron transport in a dense gas of attractive or repulsive scatterers. It attempts to graft the multiple scattering theory of waves, and wave motion in a random medium, plus a newly derived second Ioffe-Regel criterion onto classical Boltzmann theory, with help from drift velocity experiments. The model describes diffusion as well as drift, electric fields strong enough to cause nonthermal energy distributions and high density (nonlinear) effects. Some good agreement is shown with experiment for the mobilities and drift velocities, and a tentative agreement for the diffusion coefficient where experiments are scarce.

The intramultiplet relaxation processes of Cd(5³P₂) by rare-gas atoms were studied by employing a pump-and-probe technique. Cd atoms in the 5³P₂ state can be produced by exciting Cd-rare-gas collision complexes with the output of a XeCl laser at 308 nm. The cross sections for the 5³P₂ to 5³P₁ and 5³P₂ to 5³P₀ processes induced by collisions with ⁴He, ³He and Ar were determined to be: sigma ₂₁(⁴He)=8.6, sigma ₂₀(⁴He)=0.8, sigma ₂₁(³He)=14.7, sigma ₂₀(³He)=1.2, sigma ₂₁(Ar)=15.1 and sigma ₂₀(Ar)=4.3 in units of 10^-20 cm². Close-coupling calculations were carried out to evaluate the cross sections by assuming various interaction potentials. Information on the potentials could be obtained by comparing the calculated cross sections with those obtained experimentally.

The deviations of the experimental L-subshell ionization probabilities relative to the semiclassical approximation are discussed in terms of couplings between the target atomic states during the collision by means of a relativistic one-centre coupled channel formalism with hydrogenic wavefunctions. Numerical results are compared with experimental data for systems He+Pt, Ne+Pt, Ne+Yb at 0.9 MeV amu^-1 and S+Au at 1 MeV amu^-1. A simple inclusion of the screening effect presented in this work yields a major improvement in the description of the L-shell ionization probabilities. The intra-L-shell couplings modify slightly the ionization probabilities for light projectiles but they have a marked effect for heavier projectiles. The couplings between the L and M shells were also included, but they were found to have only a small influence as compared with intra-L-shell couplings.

The modified two-orthogonal state expansion method published previously is applied to calculate electron capture cross sections for two-electron systems: H⁺, He²⁺+Be²⁺, B³⁺, C⁴⁺, N⁵⁺ and O⁶⁺, H⁺+He and He²⁺+Li(1s²). The active electron is described by the independent-electron model. The calculated results agree well with the existing experimental data for H⁺+He and He²⁺+Li collisions in the ranges 5-130 keV and below 500 keV amu^-1 respectively. The author's results for H⁺, He²⁺+C⁴⁺ systems are in good agreement with the experimental values for H⁺, He²⁺+CH₄ collisions in the intermediate energy range. A comparison with other theoretical results is presented. Scaling laws for these cross sections are examined.

A previous paper of Avakov et al. (1990) proposed an approach to the calculation of electron transfer reactions in ion-atomic collisions based on the Faddeev three-body equations written in the Alt-Grassberger-Sandhas form. This approach is used to describe the electron capture in proton collisions with alkali atoms. The results of calculation of the total and partial cross sections for charge exchange in proton collisions with Li, Na, K and Rb atoms are presented. The calculated total cross sections are in good agreement with experiment for light target atoms. In going over to heavier targets, the theoretical total cross sections, while agreeing in form, tend to be larger than the experimental ones. The calculated partial cross sections for electron capture into the 2s state of the H atom are also in agreement with experiment. Some other partial cross sections were also calculated.

The authors present a theoretical study of the autoionizing double capture in low energy collisions between highly charged N⁷⁺, O⁸⁺ or Ne⁸⁺ ions and helium. Close coupling calculations have been performed to evaluate the branching ratios of the various processes occurring in such collisions, the corresponding cross sections and the angular momentum distributions of the states (3l,3l^') formed by double electron capture. The results are found to be in good agreement with the experimental results; the general features of the autoionizing double capture are discussed.

Using the same method by which Dinterman and Delos (1977) have generalized the formula of Rosen and Zener for the near-resonant charge transfer transition probability in the noncrossing case, the authors have generalized the well known Demkov formula, removing most of its defects. They have compared the generalized Demkov formula to the direct numerical solution to the coupled Schrodinger equations for two actual ion-atom systems: Li⁺+Na and K⁺+Rb and have shown that the generalized formula is much better than the original Demkov formula and is in good agreement with the numerical results in a large region of velocities.

A detailed study of a modified Born approximation (MBA1) calculational method, proposed earlier by the authors, has been made in connection with ionization of hydrogen atoms by electrons for the highly asymmetric kinematic conditions of Ehrhardt (1986). Another modification of it which results from the post form for the interaction has also been studied in greater detail. Results of this latter calculation (MBA2) are, generally, better than those of their previous calculation and of the first Born approximation, particularly for angles belonging to regions which include recoil peaks for larger momentum transfer cases. The MBA2 method may also be expected to be useful in the study of other cases of ionization of hydrogen atoms and hydrogenic ions by charged particles under Ehrhardt-type kinematic conditions and for studies of various integrated cross sections.

The authors present absolute triple differential cross sections (TDCS) measurements for ionization of helium leaving the ion in both n=1 and n=2 final states, obtained under asymmetric geometry at an incident energy approximately 5.5 keV and ejected electron energies of 5, 10 and 75 eV. The kinematics are chosen to correspond either to a constant ejection energy, or to a constant energy transfer to the target. Angular distributions are measured at both constant ejection angle ( theta _a mode) and at constant scattering angle ( theta _b mode). In the theta _a mode experiments, the momentum transfer dependence of the n=2 triple differential generalized oscillator strength is investigated for the first time. In both modes, the n=2 angular distributions show several new features which are not present for the n=1 ones, and which tend to vanish as the ejected energy is increased. They are attributed to final state interactions between the ejected electron and the excited ion. Comparison with first-order theoretical models shows the inadequacy of a Coulomb wave representation of the ejected electron, while in the R-matrix formalism it is found that a five-state multichannel calculation qualitatively describes the shape (but not the amplitude) of the TDCS measured in the theta _b mode. Comparison is also made with the photoionization in the dipolar limit where the momentum transfer approaches zero. When integrated over the ejection direction, the double differential generalized oscillator strength ratio for ionization to the n=1 and n=2 states is found to agree with an earlier first Born close coupling prediction.

The rate coefficient ( nu sigma ) for radiative recombination between free electrons and highly charged Si⁶⁺ and Si¹¹⁺ ions has been measured in the energy region from 0 to 1 eV. The experimental results were obtained with a merged-beams technique. Knowledge about the relative velocity distribution allowed a direct comparison with theory. It was found that for Li-like Si¹¹⁺, the effective charge to be used in a calculation developed for bare ions is the total ion charge, Z_eff approximately=11. This is in agreement with earlier findings for H-like ions and other Li-like ions. For O-like Si⁶⁺, the effective charge is found to be Z_eff approximately=9. For Si⁶⁺, the measured rate coefficient is more than a factor of two larger than calculations based on the total ion charge (Z_eff=6). This shows evidence for significant incomplete screening in the case of Si⁶⁺.

Ab initio calculations at the SCF and CEPA levels have been performed for the interaction potential between O(³P) and H₂(X ¹ Sigma _g⁺) at separations in the van der Waals region. Potential curves for ³ Sigma ^-, ³ Pi (collinear approach), ³A₂, ³B₁, ³B₂ (orthogonal approach) and ³A' (45' approach) are presented and used in quantum mechanical close coupling calculations of the cross sections for the de-excitation of fine structure transitions in O(³P_J) by ortho- and para-H₂. The rate coefficients for these and the reverse transitions are evaluated for a wide range of temperatures.

A systematic study of the equal-time Hamiltonian two-body differential equations is presented, for all combinations of spinless and Dirac particles. Components with 'mixed' indices are eliminated in a new way. A new radial variable r' is introduced which reduces the equations to simpler 'quasipotential equations' which in some cases have exact solutions. A nonzero total momentum and a constant magnetic field are also discussed.