Table of contents

An analytical method, already applied to bound-bound and bound-free transitions, is extended to free-free transitions in the field of positive ions. The role of a cut-off factor, intended to cancel the diverging part of the irregular Coulomb functions, as well as the way to obtain photoionisation data for excited states from the extrapolation of free-free results, are carefully discussed.

Conditions for occurrence of spin polarisation of Auger electrons have been analysed. It is shown that if an inner-shell vacancy arising from particle impact is aligned then the Auger electrons can have spin polarisation transverse to the scattering plane. The expressions for the degree of polarisation as a function of the angle of detection of Auger electrons have been obtained. Using the LS coupling approximation it is shown that a significant spin polarisation may be expected for the Auger lines corresponding to the triplet terms of the residual ion if individual components of the fine-structure multiplet are separated out. After the summation over the multiplet the polarisation disappears.

The differential excitation of two very similar isotopic atoms with overlapping Doppler, but not natural, linewidths is studied theoretically. It is found that the addition of a nuclear radiofrequency pulse to a laser pulse sequence can, in principle, achieve complete differential excitation when one isotope has zero nuclear spin and the other has an integer-plus-half nuclear spin. This differential excitation is insensitive to Doppler shifts and applies equally to all velocity classes of atoms in the sample.

Finite-element integration of the Schrodinger equation for the time evolution of a wavepacket is used to solve the two-dimensional model of electron impact ionisation proposed by Temkin and Hahn (1974), and Peterkop and Rabik (1972), as a test bench for mathematical and numerical techniques. The quantitative features of Wannier's transition state theory (1953) are reproduced, not only at threshold, but at much higher energies. Ionisation probabilities and ejected electron spectra are presented from threshold up to initial energies nine times the threshold energy.

The paper is devoted to the development of an effective occupation number theory for application to the level schemes of nl^N configurations. The expression for the angular and radial coefficients was obtained through the N number of the occupation shells. Varying the values of the expression for the energy terms of the 3d^N and 4f^N configurations, one can get closer agreement with experiment and identity some specific features in the behaviour of correlation effects.

The X-ray spectra of highly ionised Ag, Cd, In and Sn produced in a low-inductance vacuum spark were observed. Spectral lines belonging to the 2-3 transitions of Ne-like isoelectronic sequence were identified and measured with high accuracy.

Individual L-subshell fluorescence yields and the average L-shell fluorescence yield have been measured for rhodium (Z=45). A graphite mosaic crystal spectrometer was employed to resolve the LX-ray spectrum from the decay of 17 d ¹⁰³Pd. The average fluorescence yields, omega _L and omega _KL, were measured in singles and coincidence experiments using Si(Li) and Ge(Li) detectors. Satellites from the decay of double-vacancy states in the L shell were detected and included in the analysis which yielded: omega ₁/ omega _L=0.411+or-0.013, omega ₂/ omega _L=0.927+or-0.021, omega ₃/ Omega _L=0.941+or-0.038, omega _L=0.038+or-0.04, omega _KL=0.036+or-0.06.

Charged-particle scattering by a static potential V(r) in the simultaneous presence of a magnetic and a laser field, both directed along the z axis, is studied within the Green's function formalism and the first Born approximation (FBA). For three types of scattering potential (screened Coulomb, pure Coulomb and exponential) the first-order matrix elements and the cross sections are calculated exactly and given in analytic closed form. The optical theorem is derived, with the result that the total scattering cross section is found to be expressed not through the imaginary part of the forward elastic scattering amplitude as is usually the case, but through the real part. The analytic structure of the cross sections in the presence of the two external fields is found to resemble the cross sections of collision processes involving particles with discrete internal structure. The analysis shows that the magnetic field assisted potential scattering is one of the physical processes where a beam of free electrons is found to amplify coherent radiation, especially in the low-frequency regions (infrared or less).

Starting from the general formulation of the plane-wave Born approximation (PWBA) an analytical expression for the low-energy total K-shell ionisation cross section is obtained. The reduced cross section is given as a power series in the adimensional parameters xi ² and theta which reproduces the values obtained by numerical integration within the precision inherent in the available tables. The universal part of the reduced cross section in the PWBA (that part of F( xi ², theta ) that does not depend explicitly on theta ) is expressed in terms of a few elementary analytical functions. The exact physical limits of integration are taken into account through the introduction of an effective parameter xi_eff.

An exact analytical expression for the energy differential K-shell ionisation cross section is obtained in the framework of the plane-wave Born approximation (PWBA). The excitation function I( eta , theta ) is expressed in terms of elementary analytical functions plus a rapidly convergent power -series expansion. With a few terms of the power series the values obtained by numerical integration are reproduced within the precision inherent in the available tables. Starting from the same general formulation an exact algebraic equation for the 1s-n excitation for atomic hydrogen is also obtained.

A detailed study of the collisionally induced alignment of the 2p electron is presented. The collision systems investigated are e^-, H⁺, He and Li⁺ with Mg for emission following 2p ionisation and e^- and H⁺ with Na and Mg⁺ with He for emission following 2 p excitation. Impact velocities, scaled with respect to the threshold velocity of the process being studied, ranged from approximately electron threshold ( nu _rel=1) to nu _rel=7 for electron impact and nu _rel=0.1-5 for the combined ion impact data. A comparison of the measured alignments for the various projectiles with each other and with theory is made for the two cases. Specific comparisons demonstrate how kinematics effect at lower velocities. In addition, pronounced maxima-minima structures resulting from nodes in the target wavefunctions are observed. Finally, the fires measurements of alignment in the region of electron threshold are presented and a prediction of the threshold alignment value is made in the case of 2p ionisation by electron impact.

K excitation and K-K transfer cross sections have been investigated in a systematic way for nearly symmetric collisions of 300 MeV Kr and 200 MeV Fe ions with various targets. Solid targets have been used and cross sections have been extracted by an analysis of the projectile and target K X-ray yield dependence on target thickness, taking into account the effects of non-immediate L equilibrium. The rotational model keeps some validity in this intermediate velocity range ( nu / nu _K<or approximately=0.5): a quantitative comparison of summed experimental K vacancy cross sextions and theoretical predictions is allowed by a separate estimate of the 2p pi MO vacancy number, taking into account the dynamical couplings of the 2p pi MO in the first part of the collision. K-K transfer cross sections are also extracted from the K X-ray yield dependence on target thickness and discussed in terms of various two-state models.

Using the resonant internal energy conversion mechanism for inelastic atom-Rydberg atom slow collisions, the energy spectra of ejected electrons and the rate coefficients for the ionisation process are calculated analytically. Reaction rates for H*(n)+H, Cs*(n)+Cs, Rb*(n)+Rb and K*(n)+Rb colliding systems are calculated and compared with the available experimental data.

Cross sections have been measured for the formation of Cs²⁺ by collisions between Cs⁺ ions with centre-of-mass energies between 19 and 79 keV. In this range, the results are much smaller than those published by Dunn et al. (1979). Details are therefore given of the development of experimental techniques and checks applied to the results. These measurements are relevant to proposed schemes for heavy-ion fusion.

The classical trajectory method is applied to the investigation of low-energy e+He to He²⁺+3e collisions. In contrast to the hydrogenic case numerical calculations appear unable to derive the ionisation threshold law by varying one or two initial parameters. Final electron energies appear as linear functions of the initial time parameter and it is conjectured that the energy distribution should be uniform within the total energy interval: 0.544<or=E<or=6.75 eV. Calculated angular correlations between the three escaping electrons conform to the full central symmetry at very small energies, but go over to axially symmetric configurations at the energy rises. The final angular momenta acquire large values, with dramatic enhancement at the edges of an ionisation interval, where momentum exchange becomes particularly strong. Finally, the applicability of the classical model to non-hydrogenoics is discussed, as well as some stochastic effects encountered.

The authors use a new expression to find fixed-rotor S-matrix elements from the first-order change in classical action and then employ frame transformation theory to obtain Arthurs and Dalgarno SMatrix elements (1960). They relate this result to the sudden approximation of Kramer and Berstein (1964) and apply it to scattering by the important electron-dipole potential, finding analytic expressions for S-matrix elements. These are shown to be in good agreement with close-coupling results for orbital angular momentum quantum number, L>or=5, for D=2.59 ea_O. For electron-dipole collisions, differential cross sections from the ground state have been calculated directly using this action for the angular region where the Born approximation is invalid. Interference effects are found to be responsible for the sharp minima obtained in the close-coupling calculations for the 0 to 0 and 0 to 1 cross sections. Employing the same action, integral cross sections from the ground state to any other level are obtained and empirical formulae presented for the cross section between any two levels and for the total cross section out of any level. Good agreement with a variety of close-coupling calculations has been obtained.

A new approach to measure values of the transverse diffusion coefficient from a drift tube experiment is presented. The analysis of the E/N dependence (E is the electric field strength and N is the neutral gas density) of the count rate of ions transversing a drift tube from a point source to a point exit aperture enables a determination of the variation of D_T/K (D_T is the transverse diffusion coefficient and K is the ionic mobility) with E/N for singly and doubly charged atomic Ar ions in Ar at 298K. Absolute values of D_T/K may be obtained by normalisation at low E/N to the Nernst-Townsend value. The two ion types measured to be 55 mV for Ar⁺ and 125 mV for Ar²⁺. This difference is believed to be related to the relative roles played by charge exchange in the drift motion of the two ion types. The analysis further discloses that the reaction rate coefficient for the loss of Ar²⁺ by collisions with H₂O is 3*10^-9 cm³ s^-1.