Table of contents

The singularity structure of the ion-atom forward scattering amplitude is analysed. The forward ion-atom dispersion relations are proposed.

The authors have studied the simultaneous excitation of both collision partners to the 2¹P state in He-He collisions at 1250 eV. This was done by measuring (in a coincidence experiment) the angular correlations between the two UV photons emitted by the two simultaneously excited atoms in their decay to the ground state. These measurements contain information on the cross sections for simultaneous 2¹P excitation of the two colliding atoms to the various substate combinations. A theoretical expression for the angular correlation function in terms of the various collision parameters is presented and is used to extract the available information from the experimental data. The results are not in agreement with conclusions which can be drawn from a simple reasoning based on the correlation diagram for the He-He system.

The electron loss and capture cross sections for helium ions at velocities nu =8-20*10⁸ cm s^-1 in helium, nitrogen, neon and argon have been measured and the electron capture cross sections for He⁺ and He²⁺ ions in various media have been theoretically calculated in the Oppenheimer-Brinkman-Kramers approximation. It has been found that the dependence of the electron capture cross section on the nuclear charge of the target atoms is oscillating. This phenomenon is caused by the analogous oscillations of the values of the equilibrium charge fraction and the average charge of the fast ions.

The first measurements of the angular correlation between a scattered electron which has ejected a bound electron from an inner atomic shell and one of the decay products of the ion formed are presented. Ionisation of the 2p_3/2 shell of argon has been studied by measuring the angular correlation between the scattered electron and the L₃-M₂₃M₂₃(¹S₀) Auger electron. The measured angular correlations exhibit considerable anisotropy.

The theory of the polarisation of the 6³P₁ from 6¹S₀ line of Hg after excitation by forward scattered electrons is reviewed. Certain problems encountered in previous studies are pointed out and it is shown that the Ochkur approximation fails to give adequate agreement with the experiment. It is also pointed out that in the case where hyperfine splitting is not resolved experimentally that a different means of averaging over the nuclear hyperfine contribution must be employed. A discussion is presented on why an Ochkur-type approximation might improve on the Born approximation near an excitation threshold.

The momentum profiles expected from analyses of (e,2e) coincidence experiments using intermediate energy electrons scattering from mercury are obtained within the relativistic plane-wave approximation. These momentum distributions have been calculated using both relativistic and non-relativistic Hartree-Fock wavefunctions revealing differences that are sufficiently distinct as to be experimentally observable.

A study is made of the effective operator Xi that reproduces (to second order in perturbation theory) the displacements of the energy levels of d^Ns² due to the interaction of this configuration with d^N+1s. The matrix elements of Xi are broken up into separate n-electron parts of all the states of d^N, and it is shown that Xi corresponds to quasi-spin ranks of 0 and 1 only. Contributions from the n=1 and n=2 parts of Xi to the classic electrostatic parameters F^k, alpha and beta are worked out, and it is shown that an orthogonalisation of the n=3 part to the electrostatic operators leads to corrections to the F^k but none to alpha and beta . This last result generalises to the f shell and also to all types of configuration interaction in which a single electron is excited. An orthogonalisation of the operators associated with alpha and beta is also carried out and a fit to the experimental levels of Cr IV 3d³ demonstrates that the parameters associated with the orthogonalised operators have smaller mean errors than the conventional ones.

Measurements are presented on the double-to-single photoionisation ratios for Zn, Cd and Hg in the photon energy range 27-37 eV. There is good agreement with earlier data which do not cover this region in such detail.

Calculations of the variation with photon energy of the partial cross sections to the ionic states of NO arising from 4 sigma , 5 sigma , 1 pi and 2 pi ionisation are described. The Tchebycheff imaging technique and the separated-channel static-exchange approximations are used. The results are compared with experimental data from (e,2e) spectroscopy and from synchrotron radiation.

The authors have measured double-differential cross sections d² sigma /dE_ed Omega _e for delta electrons emitted in energetic ion-atom collisions with energies between about 15 and approximately 100 keV. Various incident ions (p,O,S) with energies from 1.8 to 10 MeV amu^-1 and thin-foil targets (Cu,Ag,Au) were investigated. Comparison of the authors' results with theory shows that the semiclassical approximation (SCA) describes the data in the entire range whereas the binary-encounter approximation (BEA) often fails. Angular distributions, recoil- and combined-atom effects are discussed in some detail.

The strong coupling correspondence principle is formulated to provide an approximation to the T-matrix elements for vibrational excitation of polyatomic molecules in rotationally sudden collisions. The theory is valid in the forward diffraction zone and at wider angles. Using the long-range form of the interaction potential this theory is applied to diatoms and linear triatoms. It is shown that there are three ranges of scattering angle in each of which different subsidiary approximations are valid. Particular application is made to proton collisions with CO, NO and N₂; the author obtains approximate analytic expressions for the cross section in the forward direction and at larger scattering angles (greater than 0.2 degrees ). It is shown how various parts of the potential affect the cross section. Theory and experiment agree satisfactorily. The excitation of the infrared-active modes of CO₂ by protons is also considered. In particular, an analytic approximation to the ratio of the bend to the asymmetric stretch cross sections is obtained and is shown to agree well with experiment.

The atomic Coulomb excitation or ionisation by slow heavy particles is treated in the framework of the semiclassical approximation. A different approach is adopted which avoids the explicit calculation of the atomic form factors and greatly simplifies the computation of monopole transitions. This formalism is applied to the calculation of an analytical expression for the K-shell Coulomb deflection factor assuming a hyperbolic orbit for the projectile. This expression agrees well with previously reported numerical results.

Elastic differential, integral and momentum transfer cross sections are reported for the scattering of electrons by N₂ and CO in the energy range 40-800 eV. The two-potential approach of Hayashin and Kuchitsu (1976) is employed, in which the total interaction potential is represented as a sum of the short- and the long-range potentials. It is found that the interference cross section term arising due to the coherent sum of the scattering amplitudes from the short- and the long-range parts of the interaction potential, is stronger than the pure long-range potential cross section term, of course, in the small angular region (<or=40 degrees ). The author also studies the multiple scattering within the molecule and finds that in the multiple scattering series the single-double (SD) term dominates over the pure double-double (DD) term in the whole energy region considered. However, there is evidence (below 100 eV) that the DD term is compatible with the SD term in the middle angles and exhibits some peaking behaviour at about 90 degrees . To evaluate elastic scattering amplitudes for each potential, a partial-wave analysis was performed. Results are discussed and compared with the recent absolute measurements. All the author's results are in very good accord with the observed data.

The scheme of Wallace (1973) for obtaining higher-order eikonal approximations than the Glauber approximation, is used to calculate the cross sections for the excitation of the 2s and 2p states of atomic hydrogen by electron or positron impact, correct to the second order. As expected, inclusion of the second-order term introduces differences between electron and positron scattering. For electron scattering, an estimate of the eikonal exchange amplitude has also been included. Differential cross sections are presented and discussed in the angular range 0-30 degrees , where the Wallace formula may be expected to be valid.

Triple differential cross sections are obtained using different wavefunctions of the normal helium atom and with different expressions for the effective charges of the outgoing electrons. The results obtained are compared with experimental findings at intermediate and high energies. The exchange effect is considered at low energy. Double differential cross sections are calculated with a hydrogen-like wavefunction for helium and results are compared with experimental observations.

Differential cross sections and angular distributions of ions formed by dissociative electron attachment in HBr have been obtained, by means of a modified crossed-beam electron impact spectrometer, in the 5-10 eV energy range. The yields of H^- ions associated with both Br(²P₃2/) and Br(²P₁2/) atoms exhibit two peaks in the energy range 5-7 eV and 7-10 eV, respectively. The first peaks in the 5-7 eV energy range have an angular distribution characteristic of a d sigma wave indicating that H^- ions in this energy range are formed via a ² Sigma HBr^- resonant state. For the two higher peaks the angular distributions of H^- ions are characteristic of d pi waves in agreement with ² Pi HBr^- resonant states. The first peak of the H^- ions associated with Br(²P₁2/) atoms shows a non-diabatic transition from the ² Sigma ₁/2HBr^- resonant state to the H^-+Br(²P₁2/) dissociation limit which is due to a strong dynamical (d/dR) coupling between the two Omega =¹/₂ HBr^-1 resonant states. This coupling occurs at a large internuclear separation of the H^- and Br fragments as shown by the angular distribution of H^- ions. A d/dR coupling calculation does not account for the experimentally measured branching ratios.

Multipole polarisabilities and shielding factors for atoms and ions with a valence configuration (nd)¹⁰, n=3 and 4, have been calculated in the coupled Hartree-Fock approximation.

Effective electron-ion and electron-atom cross sections and energy changes in a slightly ionised gas when a laser field is present are derived. The numerical analysis is restricted to: (i) weak laser fields; (ii) mean electron kinetic energy greater than the laser photon energy; and (iii) an average electron-atom collision frequency much smaller than the frequency of the assisting field. The electron-ion cross section exhibits a sharp decreasing behaviour as a function of the laser intensity, and significant changes against the laser frequency. The electron-atom cross section shows a more smooth behaviour against the laser intensity and frequency. The energy changes are very rapidly decreasing functions of the laser frequency and are, generally, non-linear rising functions of the intensity.

The rate of disappearance of electrons has been measured in mixtures of 20% O₂ in N₂. The results show a three-body attachment rate coefficient of 2*10^-32 cm⁶ s^-1 for electron mean energies in the range from 0.6 to 1.8 eV. The results also show a two-body rate coefficient which decreases from 8*10^-13 to 5*10^-14 cm³ s^-1 over this same range.