Table of contents

The analytical expressions for electron momentum distributions in the configurational Slater-type orbital approximation have been obtained for all elements of the periodic system. Cases of single-zeta functions and double-zeta functions were considered.

An analysis of 'collective excitations' in the wavefunction picture and conditions for their occurrence are presented.

The experimental differential cross sections for the incoherent scattering of 662 keV gamma rays from the K-shell electrons of bismuth (Z=83), reported earlier, are compared with the relativistic values computed by Whittingham (1971) and by Talukdar et al. (1971) for lead (Z=82) using the same energy. Good agreement is found between the experimental values on the one hand and the exact relativistic differential cross sections of Whittingham and the values of the relativistic incoherent scattering function (S_K) of Talukdar et al. on the other.

A dispersion relation for forward scattering, as formulated by Gerjuoy and Krall (1960) is studied numerically for electron and positron scattering by helium. The real part of the scattering amplitude for elastic scattering at impact energy E and scattering angle zero is evaluated from this relation and compared with its experimental or theoretical value. For electrons, it is shown that the dispersion relation of Gerjuoy and Krall has to be modified. The results suggest that an extra term should be included, which is related to the nonanalyticity of the exchange part of the scattering amplitude for negative energies.

Cross sections for excitation of metastable hydrogen by simultaneous collision with electrons and photons are calculated with the Bethe-Born approximation. The resulting second-order matrix element is calculated exactly with an analytic Coulomb Green's function. The cross sections are shown to be of direct physical interest with present-day lasers.

It is shown that the existing discrepancy between theory and experiment at low impact energies in H(2p) formation by H+He collisions may be removed by considering the molecular aspects of the problem.

The Wannier theory for double escape near threshold (1965) makes detailed predictions for the energy and angular distributions of the outgoing electrons. Not only the dependence on the excess energy above threshold but even the numerical coefficient in front that governs these distributions can be derived as a function of the charge Z on the residual core. These are of interest with the advent of recent accurate experiments and computer calculations. Such coefficients are presented and it is pointed out that there is a dramatic disappearance of some correlations for Z>or=3. Recent computer calculations for Z=2 can, perhaps, be extended to verify this result. Similar non-monotonic dependences on Z are to be expected for triple and other multiple escape.

The average trajectory effect is estimated for a van der Waals potential and it is shown that it is equivalent to the use of a purely attractive Lennard-Jones potential with a rectilinear trajectory.

Broad red and yellow bands have been observed in the emission spectrum of an electrical discharge containing caesium vapour mixed with rare gases at low pressures. By varying the discharge conditions, it is shown that these bands are correlated with well defined excited levels of the caesium atom. An afterglow study further reveals that at least a part of the red emission is from excimer states of the Cs-noble gas molecule.

The application of a convex mica crystal spectrometer to soft X-ray spectroscopy of laser-produced plasmas is discussed. Photographic spectra were obtained with resolving power 1500 and show isoelectronic scaling of oxygen-, nitrogen- and neon-like ion spectra of copper, zinc and gallium in the spectral region 8-13 AA. Photoelectric continuum measurements were made in the same spectral region as an electron temperature diagnostic for low Z targets. Line classification and plasma diagnostic possibilities are considered.

The electron-photon delayed coincidence technique has been extended to the measurement of the lifetimes of excited states of metals which require an oven to produce an atomic beam of sufficient intensity. Cadmium has been studied and the determination of the lifetimes of three neutral states is reported. These states, together with their measured lifetimes, are as follows: 6¹S (29.0+or-0.5) ns, 7¹S (60.8+or-2.6) ns and 5¹D (22.28+or-0.33) ns.

Transition probabilities between the upper 5p⁴6p configuration and the lower 5p⁴6s of singly ionized xenon are calculated in intermediate coupling. An effective correction for configuration interaction is introduced and its effect is discussed. Lifetimes of the upper states show a better agreement with experimental data than those obtained in LS coupling.

The magnetic resonance situation is considered where the static magnetic field and the oscillating magnetic field make an arbitrary angle with respect to each other and are applied to a system with a spin of one-half. Analytic expressions correct to fourth order are obtained for the eigenvalues of the interacting system in the neighbourhood of the crossing points of the unperturbed system and from these the positions of the resonances are deduced. Formulae are also given for the corresponding power-broadening contributions to the lifetime. The relationship between the power series for Shirley's parameter q (1965) and the series for q² is discussed. The model adopted is quantum mechanical and a continued fraction method is used.

General expressions for the angular distribution and polarization of photoelectrons in the region of autoionizing states have been obtained. It is shown that when an autoionizing state has two or more channels of decay a resonance structure can occur in both the angular distribution and the polarization. For the autoionizing states nsnp⁶mp of Ne and Ar, possible shapes for the energy dependence of the angular distribution have been discussed. This dependence is sensitive to the relation between the matrix elements which characterize excitation and decay of the autoionizing state through the different channels.

Many slow electrons in the field of a residual ion are shown to have Wannier-like stationary configurations. Solutions of the Schrodinger equation in this limited region of space are obtained and applied to the investigation of the cross section for multiple ionization near threshold. Numerical results are presented for double escape in several spin and angular momentum states and for triple escape in the ²S and ⁴S states.

The electron impact excitation of a large number of helium states has been studied in the energy domain where the autoionizing states lie. Apart from the well known negative-ion resonances, several structures are observed which can be explained in terms of a post-collision interaction (PCI) between the inelastically scattered electron and the electron ejected by an autoionizing state. It is found and made plausible that the PCI mechanism preferentially leads to the excitation of S states. From the experimental results it is possible to extract the dependence of the energy exchange between the scattered and the ejected electron on the excess energy of the scattered electron immediately after the excitation of the autoionizing state. From the magnitudes of the PCI structures in the excitation of states with different orbital momenta, conclusions can be drawn about the orbital momentum exchange in the PCI process. A brief discussion follows of the different theoretical approaches that have previously been proposed.

The theory of autoionization by electron impact is developed in a coherent fashion incorporating exchange, and making clear the links between existing theories. Parametrizations are established for analysing experimental results and expressions given for the theoretical evaluation of these parameters.

Experimental results are presented for autoionization from doubly excited states of He as a function of the angle of the ejected electron. These states are obtained by electron impact at three different incident beam energies. The data are analyzed using appropriate parametrizations and absolute values are obtained by comparison with known cross sections for simple excitation.

Experimental normalized absolute differential cross sections (DCS) for the excitation 1¹S to 3¹P in helium are reported at incident electron energies of 80 and 100 eV, and at scattering angles between 7 degrees and 135 degrees . The measurements are combined with results of recent electron-photon coincidence studies, and absolute DCS for the excitation of the magnetic sublevels 3¹P₀ and 3¹P_+or-1 are obtained. These experimental sublevel cross sections, and their sum, are compared with results of recent calculations in the multichannel eikonal and distorted-wave polarized-orbital theories.

This investigation presents estimates for relative intensities and transition energies of one- and two-electron transitions in highly ionized Fe and Ni. Two-electron transitions of the electric dipole and electric quadrupole type are treated. Experimental evidence for two-electron transitions in highly ionized Fe and Ni is discussed.

Using model potential curves V*, Gamma and V₊, quantum-mechanical calculations are carried out for the scattering of He(2³S) and Ar in the energy range 20-180 meV. Certain simplifying approximations in the theory are made which are discussed in detail. The potential curves are obtained by fitting calculated results to available experimental data. The model potentials derived in this manner reproduce differential and total elastic cross sections. Penning electron energy distributions, and Penning and associative ionization cross sections. The mechanism of quasi-associative ionization can account for as much as 8% of the measured HeAr⁺ ions under typical experimental conditions. It is found that most features of the scattering are adequately explained by a classical theory. Recent experimental results, however, indicate that there may be structure in the energy dependence of the associative ionization cross section. A simple quantum-mechanical explanation is suggested.

Semiclassical and exact quantum methods have been used to calculate the positions and residues of Regge poles for two interatomic potentials. A Lennard-Jones (6,4) potential with parameters approximating H⁺-Ar collisions and a Lennard-Jones (12,6) potential with parameters approximating the elastic scattering of K by HBr have been used. There is good agreement between the semiclassical and quantum calculations both for the pole positions and modulus and phase of the residues. Some properties of Regge trajectories for the Lennard-Jones (12,6) potential are also investigated.

Describes a flight-time difference method for the measurement of the velocity distributions of fragment pairs resulting from collisional dissociation of molecular ions in the keV range. Both dissociation fragments are detected in coincidence with respect to their flight-time difference, scattering angle and charge. Using this method velocity distributions were measured of the H-H, H-H⁺ and H⁺-H⁺ fragment pairs arising from collisional dissociation of 10 keV H₂⁺ ions, the centres-of-mass of which were scattered in collisions with H₂ molecules by less than 0.1 degrees . H-H fragments can be ascribed to dissociations in the lowest ³ Sigma _u⁺ state of H₂. The H-H⁺ fragments essentially come from dissociations in the 2p sigma _u state of H₂⁺. In addition some arise from H₂⁺ ions lying prior to collision in highly excited vibrational and rotational states just below the dissociation limit of the 1s sigma _g ground state. H⁺-H⁺ fragment pairs did not appear in dissociations of 10 keV H₂⁺ ions incident on H₂.

The ionization has been observed for most of the nine polar and three nonpolar organic molecules studied, and kinetic parameters have been evaluated for acetone, acetaldehyde-H₄ and -D₄ methyl bromide and ethylene-D₄ at excitation energies E in the range 0.01-0.08 eV below the ionization potential I. Also, the ratio k₂/k₂' of the rate constant for collisional ionization for collisions of the excited molecules with ground-state molecules to that for collisions with additive gas molecules has been evaluated for the binary mixtures of CH₃COCH₃+He, Ne, Ar or N₂, CH₃CHO+He, CD₃CDO+He, CH₃Br+He or Ar and C₂D₄+Ar. The k₂/k₂' values range from 20 to more than about 3000, with values not less than 800 for most of the mixtures with polar molecules and I-E<or=0.05 eV. The data for I-E approximately=0.02 eV are consistent with large values of k₂ for the polar molecules and with the unimolecular decay rate constant being determined by the predissociation rate. Also, the mechanism of collisional ionization is discussed briefly.

The problem of rotational excitation of a rigid rotor is formulated in a body-fixed frame of reference. Full account is taken of the simple form of the close-coupling equations to reduce the amount of computing time. In particular, the excitation of carbon monoxide (¹ Sigma ground electronic state) by helium (¹S ground electronic state) is considered, in the temperature conditions of the interstellar medium. Also, the validity of the centrifugal decoupling approximation is examined. It is found that good agreement is obtained in the partial cross sections for low J, the total angular momentum, and low rotor levels; for high J, the centrifugal decoupling cross sections converge too quickly.