Table of contents

A 28-term molecular expansion is employed to calculate total and partial cross sections for charge exchange and excitation in Be⁺+H collisions in the range of impact energies 0.1-25 keV amu^-1, which are of predictive value.

A key process in the muon catalysed fusion cycle is a low energy collision of t mu with a D₂ molecule which leads, at appropriate incident energies, to the formation of a resonant complex containing dd mu or dt mu . In this letter, an outline is given of a formal derivation in which elements of Feshbach's treatment of resonances and Chan and Fraser's (1973) coupled scattering approach to positronium formation in e⁺H scattering in the region below the first target excitation threshold are used to obtain an expression for the p-wave cross section for resonant dt mu formation. The result is similar to the Breit-Wigner formula. Full details of the calculation will be published elsewhere.

The relativistic plane wave Born approximation for (e,2e) processes is reconsidered. It is pointed out that, in contrast to the non-relativistic case, the triple differential cross section does not factorize into the bound-state momentum profile and a bound-state structure-independent scattering function. Due to the different description of the spin degrees of freedom, significant discrepancies exist between this theory and the factorization approach of Bell (1989), though both theories have the same non-relativistic limit. Implications of this feature and of the nonorthogonality of the scattering wavefunctions to the bound state for the use of semirelativistic wavefunctions in the description of relativistic (e,2e) processes are addressed.

The ratios of the cross sections for the formation of H⁺ and H₂⁺ from H₂ by the impact of fast fully stripped ions (C⁶⁺, O⁸⁺, S¹⁶⁺) have been measured at various incident projectile energies in order to understand the projectile charge and energy dependence on two-electron processes. The measured values have been corrected for the small contribution from dissociative ionization through single electron process. A comparison of the existing low charge projectile data with the present measurements tends to show a universal scaling behaviour for projectile charge and velocity.

Accurate calculations of the energies of several states in Be and Be^- are reported. Calculations with the valence shell correlation are compared with calculations including core-valence and core-core correlation. It is found that excitation energies and electron affinities can be calculated with an accuracy of a few meV, if core-core as well as core-valence contributions are included. The use of a core polarization operator approach to include the core-valence correlation is also tested, and it is shown that this operator is able to reproduce a large fraction of the core correlation contributions for the energy differences examined. The calculated electron affinities of the 2s2p ³P⁰ and 2p2 ³P^e states are 285+or-5 meV and 286+or-5 meV as compared to the recent experimental values of 261+or-10 meV and 265+or-10 meV.

For pt. II see ibid., vol. 26, p.1403 (1993). Configuration interaction gf values for transitions between the ²P⁰ and ²S, ²D states of some low-lying configurations are reported for the indium isoelectronic sequence up to Ba VIII. As in previously investigated aluminium and gallium sequences, we employ a self-consistent-field method to generate one-electron orbitals. The method includes relativistic effects albeit in an approximate way and the configuration interaction scheme accounts for correlation effects. Comparison with other theoretical or experimental data is difficult due to a severe shortage of such data; nevertheless in most cases fair agreement is found for those that are available.

An extended group function model has been applied to determine the interatomic potential for the X¹ Sigma state of ArLi⁺. By adopting a (14s, 10p, 7d, 4f, 1g/7s, 5p, 3d, 2f, 1g) contracted Gaussian type basis set, the following potential minimum parameters are obtained: R_e=4.50 au and D_e=10.569 mHartree. On the basis of an error analysis it is concluded that the calculated binding energy is in error by no more than 0.3%. The accuracy of the potential is superior to previously determined potentials.

Transitions between highly excited quadratic Zeeman levels of atomic hydrogen are considered. If the lower state is l-mixed, simple semiclassical rules are shown to exist determining transitions with the largest probabilities. These rules link the turning angles of oscillation of the Runge-Lenz vector of the atomic electron and (in the case of the irregular upper states) the direction of the periodic orbits of the electron at the moment when it passes through the nucleus. The Fourier transform of intensities of the Rydberg-Rydberg transitions in the magnetic field is investigated.

Photoelectron spectroscopy using monochromatized synchrotron radiation has been made to study the 2s²2p⁴nl correlation satellites of the Ne atom in the range of photoelectron kinetic energy E_K from 30 eV to 160 eV (hv=8 approximately 220 eV). For five main satellites including 2p⁴(³P)np²P(n=3, 4) and 2p⁴(¹D)3s²D states, the (satellite)/(2p-mainline) intensity ratio has been determined as a function of E_K. From a combination of these relative intensities with the reported 2p cross section, partial cross sections for the satellite excitation have also been obtained. The intensity ratios for the satellites are not constant over the photon energy region studied, indicating that the dynamic electron correlation cannot be neglected.

Photoionization cross sections for the 3p subshell electrons, correlation shake-up and conjugate shake-up satellite transitions in a relaxed-orbital Hartree-Fock framework within the dipole approximation are calculated. The potassium atom in the ground and first excited state 3p⁶4p²P is considered. The single-configuration approach using pure LS and intermediate coupling of electron momenta is applied. The strong enhancement of the relative intensity of the shake-up correlation satellites in 3p inner-shell photoionization of the 3p⁶4p²P excited state was obtained with respect to similar processes for the K atom in the ground state.

High resolution threshold photoelectron spectroscopy, in conjunction with synchrotron radiation has been employed to investigate single and double photoionization of argon in the 32-51 eV photon energy range. We have been able to distinguish between satellite and resonance states observed in our spectrum and thereby correct previous misassignments. Furthermore, we have developed a method to extract partial cross sections for satellite states from threshold to 150 meV. The excitation functions of these electron-correlated processes generally show a strong threshold peak, which we attribute to dynamic effects within the excitation complex, as well as other peaks due to the nearby neutral resonances. In addition, the apparent asymmetry of the Ar²⁺¹D threshold cusp is discussed in terms of a possible pressure effect.

The post-collision interaction (PCI) is investigated in Auger processes induced by electron impact ionization. (An Auger decay of an inner vacancy results in the creation of four charged particles in the final state of such a process.) General formulae allowing for PCI are obtained for energy and angular distributions of the Auger electrons. The expressions obtained beyond the eikonal approximation describe the Auger spectra measured in coincidence and non-coincidence experiments. Results of our approach are applied to the case of the Ar L₃-M₂₃M₂₃ Auger spectrum following electron impact ionization. The calculations show the essential PCI distortion of the lineshape and the angular distribution in coincidence measurements; in non-coincidence experiments only the lineshape is disturbed, not the angular distribution. Our results do not show the low-energy structure of the Auger line measured in coincidence with the scattered electrons.

The far-field spatial distributions of high-order harmonics in an ionized medium have been investigated using a computer model that includes both the single atom effects and the collective effects of the medium. The results show that depletion of the non-linear medium due to ionization and radial phase variations across the gas jet cause broadening and complicated structures in the far-field spatial distributions. These phase variations are due to three factors: focusing, ionization, i.e. depletion of the non-linear medium and electron dispersion, and intensity-dependent phase variations of the atomic dipoles.

The above-threshold photoelectron energy spectra of the hydrogen atom in an intense field are calculated by solving the Schrodinger equation in momentum space, which facilitates the extraction of the rapidly varying part of the wavefunction. It is found that the peak switching and the suppression of low-energy peaks in the above-threshold ionization is clearly manifested in this theory. We found that the apparent threshold shifts of the ATI peaks are strongly dependent on the final continuum state. The frequency dependence of the energy spectrum is also investigated.

We investigate cases of Coulombic systems near the break-up threshold for which the Wannier model holds, but not Wannier theory. Making use of the classical trajectory method, we derive threshold laws for a model system of fractional charge (Z= 1/4 au) nucleus and electrons, and a real (though perhaps impractical) system of two beryllium nuclei and an antiproton. For the first system we find the threshold law of the form exp(- lambda / square root E), where E is the total energy, and for the second one a number of characteristic features above the classical threshold have been obtained. Finally we investigate numerically a realistic case of an electron and two beryllium nuclei and discuss some general features of the ionization probability above the classical threshold.

A combined perturbation and close-coupling method is proposed to calculate cross sections for weak inelastic channels in ion-atom collisions. In this method, the time-dependent wavefunction is expanded in terms of basis functions of the dominant channels only, from which the perturbation method is applied to calculate cross sections for the weak channels. The method is applied to calculate probabilities for excitation and charge transfer to the n=2 states in p+H(1s) collisions in the intermediate energy region to check the validity of this approach. Cross sections for excitation to n=2, 3 and 4 levels are obtained and compared to experiment and other theoretical results.

A completely L² method to evaluate continuum states in multichannel scattering problems such as electron-atom collisions, photoionization, etc., is presented. The continuum wavefunction is built from a basis of discretized uncoupled channels. The coupling between channels is introduced, using formal scattering theory, in a pure algebraic way by simply solving a system of linear equations. The advantage of the present approach is that it can be used irrespective of the interaction strength between the uncoupled states. The accuracy of the method is illustrated with some results for both total and partial cross sections in H^- photoionization. Resonance parameters for the ¹P⁰ Feshbach resonances converging to the N=3 and N=4 threshold are also provided.

We report new measurements for the differential excitation of the vibrational modes V_{1, 3} and V_{2, 4} of CH₄ for incident electron energies of 3 to 15 eV and for scattering angles of 30 degrees to 140.

We report the implementation of covariance mapping mass spectroscopy to pulsed-electron-beam ionization time-of-flight mass spectroscopy. The technique has been applied to CF₄ and evidence is presented for the previously unreported doubly coincident ionization channels: CF₂⁺+F₂⁺, CF⁺+F₂⁺, CF₂⁺F⁺, F₂⁺+F⁺, C⁺+F₂⁺+F₂⁺F⁺, C²⁺+F⁺, and F⁺+F⁺ at electron impact energies of 100, 200, 300, 400 and 500 eV. In addition, indications for an unstable CF₄⁺: parent ion are presented. Furthermore, sufficient statistics were accumulated to allow an attempt to determine the momentum distribution for the ions in several previously reported double ion channels (CF₃⁺F⁺, CF₂⁺F+⁺, CF⁺+F⁺, and C⁺+F⁺) using a Monte Carlo method. Evidence for translational kinetic energies in excess of 10 eV was observed for some of the lighter ions. The magnitude of the resultant momentum from unobserved neutral fragments and the angle between the two detected ion momenta were calculated as well. The experiment was aided by the development of a fast data collection system. Contour maps of the major covariance mapping peaks were obtained with a time resolution of16 ns at an experimental repetition rate of 50 kHz over a time-of-flight range of 8 mu s.

We investigate the possibility of increasing the achievable index of refraction and the slope of the dispersion while simultaneously minimizing absorption in a driven two-level system. The analysis is performed non-linearly in both the probing laser field and Doppler and collision broadening have been incorporated for the sake of a realistic treatment. We compare the achievable index of refraction without absorption quantitatively with known results of several favorable three- and four-level systems, find our system less sensitive to Doppler broadening in a collinear set-up of both fields and apply our considerations to the particular example of caesium.