Table of contents

Multielectron calculations for the 3p absorption cross section of free are reported for the first time. The calculations were performed within the `spin-polarized' random-phase approximation with exchange. Reasons are given why the giant resonance of is incomparably broader than that of Cr, whereas the rest of the Rydberg structure of is incomparably weaker compared to Cr. Good agreement with experimental data is obtained with regard to the resonance structure, and qualitative agreement with regard to the rest of the Rydberg structures. Surprisingly strong quantitative discrepancies between the theoretical and experimental data are found in the high-frequency shoulder of the giant resonance. Additional investigations of the absorption, both experimental and theoretical, could help to understand better what is `wrong' with the 3p absorption of .

Using stable, intense fs Ti:sapphire laser pulses, a distinct difference between the multiphoton ionization of and Xe, which have the same ionization potential, was observed. Considering electron rescattering and dissociative recombination, a new model for the dissociation of at low intensities is proposed which can explain this observation.

Using many-body perturbation theory, we have the calculation for resonance structure into the main lines and satellite lines in photoionization for neutral sodium. This resonance structure results from the interference between a main line or a satellite line and a resonant Auger process. The coupled equation method has been improved to calculate this interference by the summation of specific classes of diagrams of perturbation theory to infinite order. The theoretical results are in good agreement with experiment.

The Meyer orthonormal wavelet is applied for solving radial Schrödinger equations for atoms. The validity of the present method has been tested by calculating the energy eigenvalues, wavefunctions, radiative transition rates, and partial photo-ionization cross sections. The advantage of employing wavelets as a basis set for atomic and molecular systems over other basis functions is discussed.

An energy level survey of atomic gadolinium was performed in the 31 000 - region using simultaneous detection of optogalvanic and resonance photoionization signals; over 90 odd states, including 38 new ones, were revealed. The J-values of several new states were also determined by a method based on the J momentum selection rule.

An analytical approach is presented which aids the computation of the photoabsorption spectrum of atomic hydrogen in a weak external DC electric field. Separation constants in the parabolic frame and one of the normalization factors are evaluated by the Telnov algorithm. For matrix elements the series expansion after powers of parabolic coordinates is used. An enhanced precision arithmetic is applied to extract the second normalization factor from the power expansion. The results agree with those from the previous calculations and experiment.

In our experiment, measurements of pressure broadening of the helium emission line () from a gas discharge at liquid nitrogen temperature have been carried out using a scanning Fabry - Perot interferometer. The variation of the line width with number density was found to be nonlinear at low gas pressures which has been qualitatively explained previously, in relation to other spectral lines, in terms of non-thermal Doppler broadening which arises from momentum transfer associated with collisional excitation of short-lived states. In describing the results of these measurements we give a semiquantitative discussion of the excitation mechanisms and their effects on the pressure-broadened line profiles. These are compared with the somewhat different effects observed in the corresponding singlet line - .

Measurements of Stark profiles of the He II line at and of the He II line at from a well diagnosed plasma of a gas-liner pinch discharge are reported. The plasma parameters were determined independently by Thomson scattering, and we found - , - 7.5 eV when measurements of the He II line were carried out and - , - 5.4 eV for the He II measurements. We compare widths and shifts of the spectral lines with theoretical approximations and present new empirical scalings. While the width of the He II line shows significant discrepancies with lineshape theories, in the case of the He II line profile we find good agreement with some calculations. In particular, to investigate ion dynamic effects we present an accurate measurement of the dip in the centre of the spectral line.

The triply differential cross section (TDCS) for the photo-double ionization of neon to the final , , ionic states are calculated and compared with recent experimental results. The origin of maxima and minima (lobes) in the polar plots of the TDCS and their variation with detection geometry are analysed with the help of selection rules.

The influence of core scattering effects on the dynamics of a bifurcating electronic Rydberg wavepacket is investigated in the diamagnetic Kepler problem. Results are presented for a spherically symmetric nonhydrogenic core with particular emphasis on nongeneric bifurcations of the Edmonds - Garton - Tomkins orbit.

Photoelectron spectra of the ground state have been measured as a function of photon energy from 14.75 to 15.05 eV with monochromatic synchrotron radiation and a spectrometer incorporating a state of the art position-sensitive detector. The results cover the first 13 vibrational levels of the ionic state and show a strong influence of the decay of the neutral autoionizing states I, and . A comparison is made between the observed spectra and a model calculation which includes both direct and resonant mechanisms for the population of the ionic states. Based on Morse type potential curves the inter-nuclear spacing, for the I, and states were determined to be 1.380(3), 1.380(3) and 1.390(5) Å, respectively. Some limitations of the comparison are discussed.

In a one-channel QD approximation, an analytic expression is obtained for the electron Green function of a polar molecule exactly accounting for the dipole moment of the molecular core. This result is valid in the Born - Oppenheimer approximation.

Crossed-beam measurements of rotationally inelastic scattering of ions from are reported for collision energies in the range . From the energy-loss spectra of the scattered ions, state-specific differential cross sections in the CM system are derived for rotational transitions ranging from to . Absolute units are obtained by using elastic scattering as a reference system. The measurements show a strong rotational excitation of the target molecule, while elastic scattering is nearly absent. The rotational levels are found to be dominantly excited. The observations can be understood qualitatively from the properties of the potential energy surface. Integral cross sections for rotationally inelastic scattering are estimated and compared with those for reactive scattering and electron detachment.

When the autoionizing state formed by the impact of a slow positively charged ion decays, the line profile of the emitted electron is affected by the post-collision interaction with the Coulomb field of the outgoing projectile, displaying a strong enhancement in the forward direction. On the other hand, when the autoionization process is induced by a negatively charged projectile, we demonstrate that the electron spectrum displays a depletion in the forward direction and a sharp enhancement at a certain intermediate angle. We show that these structures can be interpreted as forward glory and rainbow effects, respectively. We discuss the similarities and discrepancies between a continuum-distorted wave (CDW) approximation and a classical description of these phenomena.

Single electron capture by impact of bare heavy ions on He targets is studied for intermediate and high collision energies. We focus our analysis on the use of distorted wave models within the impact parameter approximation. For proton impact, total cross sections for capture to selective final states are presented. Results corresponding to capture to any final state are also shown for several projectiles with charge . Theoretical total cross sections calculated in the present work using the continuum distorted wave - eikonal initial state and the continumm distorted wave - eikonal final state approximations are compared with continuum distorted wave results and available experimental data. A criterion to determine which, among the distorted wave approximations analysed in the present work, is the most appropriate to describe a given collision system is discussed.

We use perturbation theory to derive an expression for the effective potential in the scattering between a charged particle and a coulombic complex (an atom) in the presence of weak screening in terms of the screening length and the atomic structure sum rules. Contact-type terms are neglected.

Vibrationally elastic and inelastic cross sections (differential and integral ones) are calculated for electron scattering from hydrogen sulphide at the collision energies 3 - 30 eV. Vibrational excitation of all three fundamental modes is considered. The calculation is based on the rotationally sudden and a vibrationally close-coupling method using an ab initio electrostatic potential. The effects of electron exchange and target polarization are taken into account approximately. The resulting cross sections are compared with the experimental data available. The present differential cross sections (DCS) for the elastic scattering reproduce the experimental data well. For the inelastic scattering, the present DCS is too small at 3 eV, compared with the experimental data. This is probably due to a shape resonance, which the present calculation would not be sufficiently accurate to produce. In the higher energy region (i.e. above about 10 eV), the present vibrational cross section should be more reliable, but no experimental data are available so far.

Elastic scattering of electrons from a molecule is studied theoretically. Differential, as well as integral and momentum-transfer, cross sections are calculated at the energies 3 - 60 eV. The calculation is based on an ab initio electrostatic potential taken with the approximate effects of electron exchange and target polarization. The fixed-nuclei approximation is made and the coupled channel equations are solved for partial waves. The resulting cross sections are satisfactory in the energy range 10 - 60 eV, when compared with experimental data. The resonance structure at around 4 eV is not well reproduced by the present calculation. Detailed comparisons are also made with previous theoretical works.

Line intensity ratios of OV multiplet lines for the (J = 2,1,0) transitions are studied using a collisional radiative model and the results are compared with measurements from the reversed field pinch experiments Extrap T1 and T2 at KTH. The measured line intensity ratios deviate from the predictions of the model and the possible causes for the discrepancy are discussed with regard to errors in rate coefficients and non-quasi-steady state.

Measurements of the neon , and triple differential cross section (TDCS) for photoionization under equal energy sharing are presented. The experimental data are found to be well reproduced by a model in which the TDCS is parametrized in terms of the relative magnitudes and phases for the and continuum channels and an electron correlation factor. Within this model, the final-state continuum wavefunction is formulated with respect to the contributing LS components, which allows an analysis of the effects of their different symmetries upon angular correlation patterns.

In electron scattering from complex targets such as complex atoms or molecules, resonance features often occur at energies where the target has many excited electronic states. Standard methods of determining resonance positions and widths are unreliably close to electronic thresholds or fail when a resonance is too wide or found to overlap another resonance. By finding the time-delay matrix given by it is possible to fit the resonance positions and widths more precisely and with greater certainty. It is also possible to find the `branching ratios', the probabilities of decay of the resonance to the continuum of each target state. This helps in the accurate assignment of the resonance (particularly for core-excited shape resonances), reducing the extensive detective work that is often required. The usefulness of the time-delay matrix is demonstrated with two R-matrix electron - molecule scattering calculations: - and - , although the method is not restricted to any particular type of calculation or target species. Resonance positions and widths are found for - . For the case of - , resonances with symmetry are tracked as a function of bondlength. Their changing character is discussed and an avoided crossing at short internuclear separation is reported for the first time. The parentage of the 12 eV resonance has been classified as a contrary to previous classifications. Evidence of the notoriously shy series b resonances has been found.

The distorted-wave approximation is applied to study electron-impact excitation leading to the , , and states of CO in the 20 - 100 eV range. Our calculated DCS and ICS are compared with available theoretical and experimental data. In general our cross sections are in quite good agreement with previous two- and five-state Schwinger multichannel results. Also, good qualitative agreement with the experimental data is obtained. However, in general our theory overestimates the cross sections. In comparison with a previous DWA study, our work reveals the strong influence of the description of the target wavefunctions on the calculated cross sections.

It is proposed to produce a number of exotic systems in the charge-exchange reactions using overlapping storage rings as sources of initial particles. The high efficiency of this method is provided by using, in collisions, the appropriate ions and the absence of losses in the medium due to in-flight production. The yields of the positronium, protonium, antihydrogen and ions of the antiprotonic helium isoelectronic sequence are estimated and some possibilities for measurements on the beam of the produced particles are discussed.

We present calculations of the cross sections for muon transfer in collisions within the framework of the quantal close-coupling approximation. The total internal wavefunction of the system is expanded in terms of atomic orbitals on each atomic centre. The appropriate Jacobi coordinates are used in the direct and rearrangement channels. A pseudostate representation of the continuum is also employed. The t-matrix elements are obtained from the solution of coupled Lippmann - Schwinger equations using the momentum space formulation. For the isotopic exchange calculations the basis consists of a large number of s and p type atomic states symmetrically distributed between direct and rearrangement channels. The present results are in agreement with existing calculations in the case of elastic scattering for collision energies above about 0.001 eV. In the case of the rearrangement process the agreement is satisfactory for collision energies greater than 10 eV.

In table 4 of this recent article the K = 10, i = 10a, 10b entries should have a denominator of .

We would like to thank Dr Gary Burdick for pointing this error out.

In this recent article table 3 was printed twice. The correct version of table 2 is shown below.

Table 2. DCS and ICS (in 10^-19 cm²) for the excitation X¹Σ⁺ → ¹Π(2π → 8σ) in N₂O.

We regret that in the above paper the cross section data was incorrectly totalled. The corrected results are about 10% higher than those given in figures 1 - 4 and in table 4. The corrected version of table 4 is given overleaf. This in no way effects the resonance analysis reported in this article and the widths and positions of the resonant states of H^- are as reported in the original text.

We also note that in figure 3 the results for the 1s - 2s cross section attributed to Konovalov and McCarthy (1994) have been incorrectly reproduced and we do apologise to the authors for this error. Comparisons of these methods together with the CCC approach of Bray and Stelbovics (1992a, b) are found to be in a recent article by Bray et al (1996).

Table 4. Intergrated cross section in units of for 1s - 2s and 1s - 2p transitions.

We are grateful to Dr I Bray for bringing these matters to our attention.

References

Bray I, McCarthy I E and Stelbovics A 1996 J. Phys. B: At. Mol. Opt. Phys. 29 L245

Bray I and Stelbovics A 1992a Phys. Rev. Lett 69 53

------ 1992b Phys. Rev A 46 6995

Konovalov D A and McCarthy I E 1994 J. Phys. B: At. Mol. Opt. Phys. 27 L741