Table of contents

It is pointed out that the quasiparticle picture, which has been found to be valid for both outer valence and core electrons and is commonly used in atomic, molecular and solid-state physics, may completely break down in the inner valence region. Many-body calculations of inner valence ionisation potentials and their relative intensities are reported for several atomic and molecular systems. The calculations demonstrate that the absence of quasiparticles is a common phenomenon.

Ab initio calculations on HCl dimers lead to quantitative results for the angles of H-bonded HCl molecules in solids.

A discussion of the semiclassical two-channel close-'curve-crossing' S matrix is given, with special reference to Stueckelberg phases as calculated within the non-adiabatic parabolic model. It is shown that the phase Gamma ₁, normally associated with elastic adiabatic evolution through the 'curve crossing', is considerably in error when calculated within the Landau-Zener approximation, but shows favourable agreement with earlier numerical evaluations from coupled equations, provided the full Zwaan-Stueckelberg phase-integral interpretation is effected.

Radiative electron capture (REC) by swift heavy ions in encounters with target atoms has been studied for a variety of collision systems. It is shown that the intensity profile in the peak region of REC distributions can be accounted for quantitatively on the basis of proper momentum distributions of target electrons.

The forward elastic exchange amplitude for electron-hydrogen scattering is singular at k²=-1. The dominant contribution to the difference between this amplitude and its first Born approximation is given by the second Born term in the neighbourhood of this singularity. It is shown that this second Born term has a third-order pole at k²=-1.

Experimental determinations of the elastic scattering cross sections for 2-18 eV positrons in helium have been made using a time-of-flight system specifically designed to achieve statistically accurate low-energy measurements. The cross sections are in close agreement with those of Kauppila et al. (1976) but differ at positron energies below 6 eV from the earlier measurements of Canter et al. (1972) and from the recent theoretical results of Campeanu et al. (1977).

Collision strengths for 1s-2s and 1s-2p excitation of hydrogenic ions of nuclear charge Z are calculated in distorted-wave (DW) and close-coupling (CC) approximations. The main source of error in the DW calculations is the neglect of strong coupling between the 2s and 2p states in calculating the wavefunctions. For near-threshold 1s-2p excitation, the DW calculations are correct to better than 10% only for Z>or=10.

Excitation functions for lithium were obtained by measuring the intensity of ejected electrons at 75 degrees to an incident electron beam. The maximum incident electron energy was 550 eV. Excitation functions were obtained for the 1s(2s2p ³P)²P, (1s2s²)²S, (1s2s2p)⁴P and 1s(2s2p ¹P)²P states. The excitation function for 1s(2s2p ¹P)²P suggests that an optically forbidden level lies close to the optical level and an electron configuration is suggested for this additional level. The very recent experimental data for caesium presented by Pejcev et al. (1977) are reconsidered in the light of the present work.

The generalised oscillator strengths f_if^l(Q) for atomic ns-electron transitions (n=1-4) to continuum states with energy E=0 and orbital momentum l from 0 to 6 have been calculated in the Born approximation with Coulomb wavefunctions. The importance of the functions f_if^l(Q) in forming the characteristic dependence of the total generalised oscillator strengths f_if^l(Q), for transitions of those electrons to all free states with E=0, the momentum transfer h(cross)Q has been considered. A qualitative relationship has been established between the structure (minima and maxima) found in f^l(Q), f_if(Q) and structural features of the radial wavefunctions of the initial and final states.

The effect of residual many-electron interactions on the parity non-conservation in heavy atoms due to the nuclear spin-independent part of the weak neutral current interaction is calculated. The special feature of the method is the inclusion of the parity non-conserving term in the zeroth-order single-particle Hamiltonian. The basic single-particle states are eigenstates of angular momentum but not parity. Standard many-body perturbation methods are used to derive diagrammatic expressions for the resulting parity non-conserving E₁ transition matrix elements. Some simplifying theorems are proved and the method for evaluating the zeroth- and first-order diagrams is outlined.

The halfwidths of seven Si III and four Si IV lines have been measured in a pulsed arc. The electron density of 5.8*10¹⁶ cm^-3 was determined by laser interferometry, while the electron temperature of 25600 K was measured from a Boltzmann plot of the relative intensities of the Cl III lines. The experimental Si III and Si IV Stark-profile halfwidths were compared with calculated values obtained from Baranger's semiclassical (1962) and Griem's semiempirical (1968) approaches. The agreement between experiment and semiempirical (1968) approaches. The agreement between experiment and semiempirical results is within 15%.

For pt.II see ibid., vol.9, p.1221 (1976). Discusses the steady-state response of a three-level atom in two-photon absorption. The applied laser fields can have arbitrary intensities (within the rotating-wave approximation (RWA)). A general expression is derived for the population excited on the uppermost energy level. Two special cases which allow a trivial Doppler averaging are treated in detail. For an off-resonant intermediate state the two-photon spectrum is a simple Lorentzian showing a power shift and modest cross power broadening. The same features are retained when several off-resonance intermediate levels are assumed. For very strong laser fields the Doppler shifts can be neglected in the first approximation. The corresponding spectra are analysed.

On the basis of a stochastic model of Burshtein et al. (1967) which assumes the phases, frequencies and/or amplitudes of the radiation field to constitute a randomly time-varying jump-like Markoff process, a generalisation of Mollow's theory (1975) of the dynamic Stark effect is formulated leading to predictions for the resonant scattering spectrum for a two-level atom quite similar to those published in the work of Agarwal (1976), of Eberly (1976) and of Kimble et al. (1977) who use a different approach. Apparently, the general form of the resonant scattering spectrum in a modulated radiation field depends less strongly on the particular stochastic model of the laser light than might be expected.

The cross sections for the production of the N₂⁺(B ² Sigma _u⁺ to X ² Sigma _g⁺), CO⁺(B ² Sigma ⁺ to X ² Sigma ⁺) and N₂O⁺(A ² Sigma ⁺ to X ² Pi ) fluorescence in the 3000-6000 AA region are measured with synchrotron radiation from their fluorescence thresholds to 175 AA. The technique used to correct the second-order effect is described. The present results are compared with previous data. Near 540 AA the cross section for the production of the N₂⁺ fluorescence shows structure at the positions of the Rydberg states converging to the N₂⁺(C ² Sigma _u⁺) state. The cross section for the production of the CO⁺ fluorescence shows a broad band with its peak at 400 AA. The cross sections for the production of the N₂O⁺ fluorescence shows structure at the positions of the Rydberg series VI-IX.

Proton impact double ionisation of He has been investigated in the double-encounter model of Gryzinski (1965). The binary-encounter expressions developed by Vriens along with an accurate quantum-mechanical velocity distribution for the bound electrons in both encounters have been used to obtain double-ionisation cross sections. These results have been compared with the experimental observations and earlier calculations by Gryzinski's method. The variation of sigma ₊/ sigma ₊₊ (single-ionisation cross section/double-ionisation cross section) with incident proton energy is also found to be in qualitative agreement with the experimentally observed variation.

The semiclassical theory is developed for a simultaneous collisional and radiative process,, and applied to the laser-induced ionisation in a collision of two excited alkali atoms (in their lowest n²P states) in a laser field. Perturbation theory is used to evaluate the probability for this process in the case of straight-line paths at large impact parameter. The collision perturbation is taken to be the dipole-dipole interaction between the two atoms, which together with the dipole interaction with the radiation leads to the occurrence of s and d intermediate atomic states and p and f final states for the ejected electron.

An impulse approximation to the charge-transfer amplitude in an ion-atom collision is derived. When a straight-line trajectory is used for the nuclear motion the impact-parameter method provides the same charge-transfer cross section as the plane-wave treatment of nuclear motion. The impact-parameter formulation allows a simply physical interpretation to be given to various peaking approximations that have been used to simplify the impulse approximation. The full impulse approximation provides the same high-velocity limit as the second Born approximation but an additional peaking approximation only provides the correct limit when Z₂/Z₁ to 0.

The work of O'Malley et al. (1961) and of Levy et al. (1963) is extended to obtain higher-order terms in the energy dependence of elastic-scattering phase shifts that arise from long-range inverse power forces. The expressions, in terms of the coefficients of the long-range inverse power potentials, give useful predictions of higher partial-wave phaseshifts. A further application would be in work on the determination of phaseshifts from scattering data.

Electrons are scattered elastically by caesium atoms in a crossed-beam experiment. The dependence of the cross section on collision energy and scattering angle has been measured; the collision energies studied are in the range from 0.8 to 20 eV. Resonant structures are observed in the elastic cross section at an energy of 1.49 eV and around the 6²P_1/2 threshold at 1.39 eV. It is believed that the 1.49 eV resonance is due to an autoionising F state of Cs^- which is consistent with theoretical predictions. The angular distributions of the elastic cross section agree satisfactorily with theory for collision energies below 2 eV. At higher energies larger deviations from theory are observed.

A multichannel theory of free-free transitions of an electron in the presence of a general atomic system is developed. Expressions are presented for total and differential cross sections, and full account is taken of both fully polarised and non-polarised photons. The evaluation of the association reduced matrix elements is discussed within the framework of the R-matrix theory of electron-atom scattering.

The eikonal approximation has been used to calculate differential cross sections for the 1s to 2s exchange scattering of 50, 100 and 200 eV electrons by atomic hydrogen. Symmetrised differential and total cross sections have also been calculated, and comparison of calculations is made with other theoretical results.

The spectra in the energy region 18 to 75 eV has been measured for electron impact excitation with electron energies of 0.75, 2.0, 4.0 and 16.0 keV and with a resolution of 80 or 30 meV (at 25 eV). The identification of almost all lines was possible by using the results of the photoabsorption spectrum and the ejected-electron spectra for other than electron impact excitation and by comparing with theoretical values. The intensity distribution of the Coster-Kronig transitions 2s2p⁶3s ¹S, ³S to 2s²2p⁵²P has been calculated.

An impact-parameter method using hyperbolic paths and perturbation theory has been used to calculate rotational-excitation cross sections for polar ion-electron collisions. Good agreement with corresponding Coulomb-Born calculations is obtained even close to threshold. The focusing effect of the Coulomb field is shown to be important for close collisions. Cross sections including an empirical estimate of short-range effects have been performed for HD⁺, CH⁺ and H₃O⁺ at electron energies up to a few electron volts.