Table of contents

Laser excitation in the far red wing of the second principal series doublet of Cs mixed with Xe revealed a diffuse structure associated with the ²P_3/2 component. The structure is thought to originate from a reflection type of spectrum between the weakly bound E ² Sigma _1/2 excited state and the X ² Sigma _1/2 repulsive ground state of CsXe.

The author investigates the Autler-Townes effect (also known as AC, optical or dynamical Stark splitting) on the Balmer- alpha transition of hydrogen in 0.5-5 Torr of helium. In the double optical resonance scheme. H(3p) is excited by absorption of two 243 nm photons and a 656 nm photon via the 2s resonant intermediate level. Calculated results show that in a Doppler-free or a 'Doppler-reduced' beam configuration, the required intensity of the 656 nm laser (0.2 cm^-1 bandwidth) for observation of the Autler-Townes doublet has to be greater than 2*10⁴ and 2*10⁵ W cm^-2 for 243 nm laser bandwidths of 0.1 and 0.5 cm^-1, respectively.

The molecular constants recently reported for the A0⁺state of InBr by Singh et al (1987) are found to be inaccurate by a comparison with recent values published by other authors, and through attempts to reproduce the wavenumbers of the lines given in their figures.

The semiclassical impact parameter model with rectilinear trajectories has been applied to collisions between ground-state hydrogen atoms for impact energies in the range 1.0<or=E<or=100 keV. The total wavefunction of the system was expanded in H(n=1) and H(n=2) travelling atomic orbitals on each centre. The calculated cross sections for the formation of H(2s) and H(2p) states are compared with the existing theoretical and experimental data.

The L X-ray spectra of Au and Th have been measured in collisions with C, F, Mg, Si, S and Cu ions in the energy range 0.5-2.5 MeV amu^-1. From the shifts of the yield ratios and energies of the L/sub // , L_alpha , L_eta and L_y1 lines relative to those for 2.5 MeV proton bombardment, the average numbers of M_4.5 (and N₄) vacancies present during the L-vacancy decay have been determined. The corresponding outer-shell ionisation probabilities show remarkable agreement with the geometrical model of Sulik et al. (1985) except for the Cu projectile.

The theoretical model for collisional l mixing in atomic intermediate Rydberg states proposed by de Prunele and Pascale (1979) is applied to the calculation of J-mixing cross sections of nP Rydberg states with higher quantum defect delta _l values of K (n=8-24) and Rb(_n=10-24) perturbed by noble-gas atoms and N₂. The interesting cases of J mixing in K(nP_1/2)-K(4S) (n=10-24) and Rb(nP_1/2)-Rb (5S) (n=12-24) collisions are also considered.

The vertical double ionisation energy of methane is determined to be 35.6+or-0.25 eV by means of an experimental technique in which the kinetic energy spectrum is measured of product negative ions resulting from double electron capture collisions of OH⁺, O⁺ and Cl⁺ ions with neutral CH₄ at 6 keV impact energy.

The authors present a comparison of theoretical cross sections for electron capture to Rydberg states (ECR) with the corresponding experimental quantities for electron capture to the continuum (ECC).

Observations of resonant electron transfer and L-shell excitation (RTE) using coincidence techniques are reported for ₄₁Nb³¹⁺ and ₅₇La⁴⁰⁺ ions colliding with H₂. The measured RTE cross sections are about ten times larger than those previously found for RTE involving the K shell and constitute as much as half of the total L alpha beta X-ray production. Overall agreement with theory is reasonable but discrepancies similar to those previously reported for K-shell RTE exist. The results demonstrate the importance of electron correlation in the production of L vacancies for projectile energies where RTE is possible.

A quasi-relativistic Hamiltonian for an atom and a molecule is constructed. The Foldy-Wouthuysen transformation is applied to the Hamiltonian. Consequently, extra terms from interactions between the electronic motion and the nuclear magnetic field contributing to the Darwin term and the spin-orbit coupling are derived explicitly. Moreover, the coupling between nuclear motion and the spin of the electron is obtained.

Local (including self-correlation corrections) and non-local correlation energy functionals are used to calculate the correlation energy of atoms and diatomic molecules. The reliability of five schemes is tested for both correlation and dissociation energies. Recently developed spin-polarised correlation energy functionals incorporating gradient corrections are found to be generally superior for the properties examined. The theory of atoms in molecules is used to aid in the discussion of the trends in correlation and dissociation energies.

Ionisation potentials of multiply charged ions have been calculated with the x alpha method using self-consistently determined parameters alpha . Self-consistent ab initio exchange parameters alpha have been determined for several ions. The ionisation energies have been determined by the transition state method of Slater and in a few cases by the method of Williams et al. (1975). The calculated ionisation energies are compared with Hartree-Fock and experimental data and found to be in agreement with them.

Relativistic calculations of the M1 transition between the ²P_1/2 to ²P_3/2 levels of the ground-state boron isoelectronic sequence were performed using the multiconfigurational Dirac-Fock extended average level (MCDF-EAL) method. The three configurations 2s²2p¹, 2s¹2p² and 2s⁰2p³ were used. Breit and QED effects were treated as first-order perturbations. Comparison of the authors' results with earlier calculations shows an average agreement to within 2-3%. Excellent comparison was obtained throughout the range of Z. Experimental values were unavailable for comparison.

The resonance structures corresponding to autoionising states interacting with many continua are parametrised using the multichannel quantum defect theory. All the involved continua are taken into account in an effective way, and collisional parameters which describe the position and width of the resonances are then introduced. The interactions between open and closed channels are parametrised in an original way in terms of vectors whose relative orientation allows stabilisation effects to be described. The introduction of a unitary transformation which rotates the 'degenerate' closed channels greatly simplifies the analysis. This theory is well adapted to the case of core-excited Rydberg states of barium, as is briefly discussed.

The theory of the field isotope shift (FS) in atomic spectra is discussed critically and the results are presented in a form which is more straightforward than the expressions traditionally used to interpret experimental data. First-order perturbation theory is applied within the single-particle framework to the case of an s or p_1/2 electron outside closed shells. Parameters to take account of many-body effects are subsequently included. The nuclear and electronic dependences of the FS are separately discussed. In a generalisation of the work of Seltzer (1986), it is shown that the former can be represented as a series of even charge moments: the coefficients of the first three terms for an s electron are given. The difference between the series for an s and a p_1/2 electron is shown to be generally insignificant. The electronic dependence enters through the (relativistic) probability density N of the electron at the origin; for the case of an s electron. They relate N to the magnetic hyperfine splitting factor alpha _s. A table which allows N to be found from a measurement of alpha _s is given. Second-order field shifts are of the order of 10^-3-10^-4 of the first-order shifts for a two-neutron change.

An explicit form of a classical mapping which governs the dynamics of a three-dimensional hydrogenic system driven by an impulsive electric field is presented. The mapping, in a unified way, accounts for both bound and unbound motion between the impulses. A one-dimensional model is obtained with a special choice of initial conditions. The two models are compared by studying the time evolution of the ionisation probabilities and their dependence on strength and frequency of an alternative impulsive field. It is shown that the effects of the recapture of the electron are important. Comparisons with previous calculations are made by studying ionisation of atoms initially in a specific Stark state. Relationships to systems driven by a monochromatic field are briefly discussed.

Different methods are used to obtain both the Auger widths and the relative Auger line intensities of some 2s2p⁶nln'l' Na terms. If the relative line intensities are expected to be correctly estimated in the distorted-wave method, the widths are overestimated owing to the use of the same 3s and 3p orbitals in the initial and final states.

The L XRF cross sections sigma ^x_Li (i=l. alpha . beta , gamma ), have been measured for all rare-earth elements with Z=60-71, except for Z=61 and 63, at excitation energies of 17.80 and 25.80 keV. Measurements have been performed using an X-ray tube with a secondary exciter system as the excitation source. Experimental results have been compared with theoretically calculated values and other available experimental results of L XRF cross sections. The experimental results are higher than the theoretical estimates. This necessitates the remeasurement of physical parameters such as fluorescence yield and X-ray emission rates.

Simple analytic expressions for the classical rotational rainbow and the quantal rotational rainbow maximum curves are obtained. A flexible and reasonably realistic Lennard-Jones-type model potential for atom-molecule interactions is assumed. Significant effects of the long-range attractive component in the potential have been identified, which may be useful in semi-empirical inversion procedures. Numerical illustrations for a potential modelled on the He-Ha₂ system are presented.

The measured time of flight spectra and angular distributions for proton collisions with CO₂ and HF targets at various energies (10-30 eV) reveal marked differences in the behaviour of these systems with regard to the dominance of either vibrational or rotational inelasticity during the encounters. Previous theoretical calculations of the full potential energy surfaces are employed to generate IOS integral cross sections for rotational energy transfer and a comparison with the corresponding experimental results shows that there is a good agreement between computed and measured quantities.

Collisional excitation to Rydberg states of hydrogen by multiply charged ions is examined in the frame of the symmetric eikonal approximation and the classical trajectory Monte Carlo method. The latter method is also employed to obtain single differential ionisation cross sections. The dependence on the projectile charge Z_p is analysed for different impact velocities. Departures from the Z²_p law predicted by the first Born approximation and saturation effects are discussed.

The authors presents an ab initio calculation of the double ionisation of helium by protons, antiprotons and alpha particles using the forced impulse method (FIM). This method builds on the fast numerical computer codes developed to solve the single-electron problem. They begin to address fully correlated systems. The FIM divides the collision time into sequential short segments such that an impulse approximation is forced to be valid. The system is allowed to collapse back into a fully correlated eigenstate at the end of each segment but the electrons propagate independently during each segment. In this high-energy situation they used just two segments. They solved coupled channel equations for transitions between 423 correlated pseudostates obtained by diagonalising the two-electron Hamiltonian in a truncated single-electron basis limited in the first calculation to 36 s and p orbitals.

The authors measured the excitation cross sections of He I 3^1,3S, 2¹P, 3^1,3P, 4^1,3P, 3^1,3D, 4^1,3D, 5^1,3D and He II 2²P and 3²P levels at ion energies from 50 to 2*10⁴ (lab) eV. Two types of structures have been recognised on the energy dependence of the cross sections, the origins of which are ascribed to excitation from the incoming channel by radial and rotational couplings through potential degeneracies at small internuclear distances. Profiles of the excitation functions depend systematically on angular momentum and spin multiplicity; the outgoing channels of the Sigma state strongly populate the triplet levels of the He atom, following the statistical weights of the spin multiplicity and those of the Pi state mainly populate the singlet levels. Rotational coupling is important for P-level excitation, as is radial coupling for D-level excitation. The characteristics of the polarisation of emitted light are consistent with our assignments for the excitation processes. The two-level model calculations give excitation functions which agree qualitatively with the observed ones.

Rate coefficients for Hornbeck-Molnar ionisation in a K-Rb mixture at thermal energy are measured using CW laser excitation of the atoms. Special attention is paid to the formation of the KRb⁺ molecular ion in Rb(nl)-K(4S) and K(nl)-Rb(5S) collisions. The absolute rate constant values are deduced from the analysis of collisional currents by mass spectrometry techniques. They are compared with the theoretical values calculated using a model recently developed by Janev and Mihajlov (1980).

The authors measure rate coefficients for Penning ionisation occurring in a vapour mixture of rubidium and potassium at thermal energy. The experimental technique uses the mass spectrometry analysis of collisional currents resulting from continuous laser excitation of the atoms. They report the first experimental evidence concerning the non-occurrence of an exchange-type mechanism. Although a second exchange-type reaction cannot be discarded by their experimental results, it seems that Penning ionisation, involving low-lying excited alkali atoms, most probably proceeds via the direct mechanism.

An approximation suitable for studying differential electron loss collisions with 'heavy' targets (e.g. Ar) is proposed. It consists of an impulse approximation to singly inelastic (SI) collisions, i.e. those in which the target is not excited, together with a first Born treatment of doubly inelastic (DI) processes, where the target is excited or ionised. Collisions of H(2s), H(1s), He⁺(1s) and O⁴⁺ with a ground-state Ar target are treated. It is assumed that O⁴⁺ behaves as if it contained only two 2s electrons, each of which is independently ejected. Comparison is also made between electron loss in light and heavy targets, ground-state He and Ar being taken as examples. In comparison with experiment it is concluded that the approximation works very well for small angles of ejection (<or approximately=15' in the cases studied) but shows some systematic discrepancies at larger angles, particularly towards 180'. It is suggested that these discrepancies may be mainly a result of the failure of the first Born approximation for DI collisions at large angles, a higher-order approximation being needed here.

In the energy range 5-100 keV the collisional destruction of H(3s), H(3d) and H(2p) atoms in He, Ar, H₂, N₂ and O₂ has been investigated. In the case of 3s and 3d states the measured cross sections which are subject to errors in the range 20-30% are in general agreement with the cross sections predicted by a simple model in which the orbital electron and the proton nuclear core are assumed to act independently on the target. Collisional destruction cross sections for n=2 and 3 were found to be independent of the l value within the experimental error, typically 20-30%.

Electron detachment in Li^--He, H₂ collisions in the 1-20 keV amu^-1 energy range is studied in an 'independent scattering' model, previously introduced by Vu Ngoc Tuan et al (1985) to study electron detachment in H^--molecule collisions. In this model, the outer electron and the neutral core of the Li^- ion are assumed to collide independently with the target. The knowledge of the electron-target and Li-target scattering properties thus allows the determination of the Li^- destruction cross section. The results are compared with available experimental data. In particular, the problem of the origin of the proportionality of the H^- and Li^- cross sections observed by Andersen et al (1984) is examined. This scaling law cannot be obtained if the detachment of the outer electron is the only process taken into account. However, the authors' model, considering both the outer electron and the neutral-core contributions to the negative-ion destruction, reproduces the scaling law. This aspect is further illustrated by looking at the electronic wavepackets formed by travelling H^- and Li^- ions.

A relativistic matrix continuum distorted-wave model is introduced. A first-order scattering amplitude is calculated for K-shell-K-shell charge exchange. Differential cross sections are given for the cases without and with spin flip using different theoretical approximations. Comparison with experimental data and other theoretical relativistic approximations for total cross sections are presented.

An intersecting beam technique has been used to obtain cross sections sigma _c for charge transfer in collisions of protons with C⁺ and N⁺ ions within the CM energy range 46-141 keV. The present values of sigma _c have been used to derive cross sections for ionisation of C⁺ and N⁺ by protons by reference to the authors' previously measured total cross sections for C²⁺ and N²⁺ formation. The results are compared with a number of theoretical predictions and other relevant data.

One-electron capture processes of highly stripped ions of Ne^q+(q=8, 9), O^q+(q=6-8), N^q+(q=5-7) and C^q+(q=4-6) in slow collisions with H atoms and H₂ molecules have been investigated. The final-state distributions of product ions are analysed using translational energy spectroscopy at a collision velocity of about 600 eV amu^-1, which is much slower than 1 au. The electron is selectively captured into a single n shell in all the systems except for Ne⁹⁺-H₂, O⁷⁺-H₂ and N⁷⁺-H₂ systems, where two n shells are populated with comparable fractions. The observed selectivity is explained by using the simple Landau-Zener mode.

Out of the scattering plane angular correlation measurements have been made of the L_alpha radiation arising from the cascade of the 3²D_j states of atomic hydrogen to 2²P_j detected in coincidence with electrons with n=3 energy loss. The experimental results, together with previous measurements in the authors' laboratory, constitute a complete determination of all the second-order state multipoles characterising the excitation. Data are presented at incident energies of 54.4 and 100 eV and at electron scattering angles theta _e of 20 and 25 degrees.

The authors present collision strengths calculated using the R-matrix method for transitions between the five lowest LS states for sodium isoelectronic sequence ions, Al III, Si IV and S VI. The importance of resonances, particularly for forbidden transitions, is investigated. Collision rates have been calculated assuming a Maxwellian electron energy distribution and their use in the interpretation of astrophysical plasmas is briefly discussed.

The energy levels of low-lying resonance states of ions are important in studies of ionisation via excitation to resonance states. The authors use the one-electron orbitals of Clementi and Roetti (1974) to calculate the energies of 189 low-lying resonance states of Ni II, III, IV, V and VI. The 46 ground-state energies of these ions are also calculated.

The normalised total cross section for the excitation of the ¹ Pi _g metastable state in N₂ is measured using a time of flight technique for incident electron energies from threshold to 150 eV. Comparison is made with theory and experimental work both for cross section and lifetime measurements.

Utilising a crossed electron-beam-molecular-beam collision geometry and the relative flow technique normalised values of total and partial ionisation cross sections for H₂O, CO, CO₂ and CH₄ have been measured. The present total ionisation cross sections agree well with those reported previously by Rapp and co-workers (1965). However, data on the partial ionisation cross sections widely disagree with some previous results.