Table of contents

General expressions are presented and investigated for the degree of polarisation of electrons produced in the resonant two-photon ionisation of unpolarised atoms from the ground S state. The exciting and ionising electromagnetic fields are assumed to have arbitrary directions of propagation and arbitrary degrees of polarisation. It is shown that completely polarised electrons can be obtained when the matrix element of the P to S transition from the resonant state to the continuum is larger than the matrix element of the P to D transition. Such cases are realised in sodium and potassium atoms.

The result of electronic structure calculations at the POL-CI level are presented for the lowest few doublet and quartet states of N₂⁺. Dipole transition moments and radiative lifetimes of a number of states of spectroscopic interest are also given. These results are used to suggest interpretations of a number of recent experiments.

The vibrational excitation cross section of the 001 asymmetric stretch mode of CO₂ by electron impact at 10 eV is calculated by quantum mechanical scattering theory. Although the calculations contain no parameters adjusted to experiment, the results are in good agreement with the rotationally unresolved experimental results. The calculated cross sections are dominated by the final rotational state j'=1 for scattering angles 0-58 degrees and by j'=3 for 59-180 degrees .

On introducing natural units (the reduced mass of the ion pair for mass, e²/k theta for length and k theta for energy, theta being the temperature) the physical similitude of the recombination process becomes apparent. It is hence shown that the dependence of the recombination coefficient alpha on theta and on the ambient gas number density n(Z) must be such that a plot of ( theta / theta _s)^3/2 alpha against ( theta _s/ theta )^tn(Z) is rigorously theta independent where t=3/2 for a purely polarisation interaction and t=1 for a simple hard sphere interaction and where theta _s is some standard temperature. This enables results at one temperature to be deduced from results at another for the same species of ions in the same gas.

Partial-wave (PW) increments to the second-order pair correlation energies for K-, KL- and L-shell symmetry-adapted pairs are analysed in order to lay down some rules governing their transferability properties. The changes of the PW increments for members of isoelectronic series (Z dependence) and for sequences of systems with fixed nuclear charge and varying numbers of electrons (N dependence) are discussed. It is found that when considering the transferability properties a distinction has to be made between the contributions defined by the PW that have to be orthogonalised to the occupied Hartree-Fock orbitals (OPW), and the remaining ones (FPW). When analysing the N dependence it is found that the FPW increments increase very slowly and monotonically for increasing N. For dynamical pairs the FPW increments can be considered to be transferable in most practical applications. As a result, when evaluating pair energies for larger systems, one can use the FPW increments obtained for the most simple ion containing the pair considered; this leads to a considerable reduction of computational effort.

Some results obtained with the quantal wavefunctions are compared, as n to infinity , with those obtained from the Bohr-Sommerfeld picture. In particular, it is shown that the probability to find the electron in the classically forbidden domain may not be negligible even when all the quantum numbers go to infinity, a fact which leads one to analyse the conditions for using the correspondence principle.

The transition rates of the forbidden lines KL_I, KM_I, KN_I and L_IM_I, L_IN_I, L_IO_I have been calculated for the elements Z=60 to 92, using Dirac's unscreened hydrogenic wavefunctions. A fair agreement is found between the authors' values and the results of some earlier workers for the KL_I transition. These calculations have enabled the authors to find the characteristics of the oscillator strengths for these forbidden lines.

Oscillator strengths for the electric dipole transitions within the ground complex of the boron isoelectronic sequence have been calculated up to Np⁸⁸⁺ from the relativistic parametric potential method. The electrostatic interaction between the states of the complex is explicitly introduced. The Z dependence of the f values for low and high stages of ionisation can be understood within the frame of the relativistic Z expansion method. For intermediate Z values, anticrossings appear between levels belonging to the same configuration; these anticrossings can give rise to striking irregularities in the trends of the f values. Finally the behaviour of the oscillator strengths throughout the sequence can be explained.

The lifetimes of the 5p(⁵/₂)₂, 6p(⁵/₂)₃ and 6p'(¹/₂)₁ levels of Kr I have been measured by the delayed coincidence method. Relative transition probabilities for eight 5s-5p lines and for twenty-seven 5s-6p lines have been found and for six 5s-5p transitions, absolute values are given. Comparison is made with experimental and theoretical values.

Selective laser excitation of Cd atoms from the collisionally excited metastable 5s5p ³P₀ and ³P₂ states was used to investigate the 5sns ³S₁ states (n=6-8) and 5snd ³D_3,2,1 states (n=5-8). The lifetimes of these states were measured with the method of delayed coincidences.

The authors report the results of measurements of transverse polarisation normal to the reaction plane of K-shell photoelectrons produced in a gold target by unpolarised 662 keV gamma rays. Energy selection of the photoelectrons was performed using a double focusing 90 degrees sector magnetic spectrometer specially constructed for the purpose. The electrons were counted in 300 mm² silicon surface barrier detectors of 1 mm depletion depth and the polarisation was deduced from the left-right asymmetry observed at backward angles in Mott scattering from a gold target. Measurements were performed at six values of the mean angle theta between electron and photon momenta with the results: theta =0 degrees , C₀₂=0.009+or-0.017; theta =6 degrees , C₀₂=0.173+or-0.018; theta =10 degrees , C₀₂=0.160+or-0.014; theta =20 degrees , C₀₂=0.133+or-0.025; theta =30 degrees , C₀₂=0.162+or-0.023; theta =50 degrees , C₀₂=0.198+or-0.064.

Nine emission bands of the main A ¹ Pi -X¹ Sigma ⁺ system of ¹¹BF have been reanalysed. The new data as well as Onaka's (1957) have been reduced to molecular constants using a single least-squares fit for each set of band lines. Revised constants are given.

The inner valence shell photoionisation of N₂ has been studied using monochromatised synchrotron radiation in the region of 44 to 65 eV. The experimental results are compared with recent theoretical calculations predicting shape resonance features and a breakdown of the independent-particle (MO) picture of ionisation.

The authors report self-broadening and self-shifting measurements on the J=0 to 1, J=1 to 2, K=0 and J=1 to 2, K=1 transitions of CH₃Br and CH₃I as a function of temperature. The experimental results are compared with calculations based on the Anderson-Tsao-Curnutte-Frost theory (1976) and on the modified Murphy-Boggs theory (1967) as formulated by Cattani. In all cases the modified Murphy-Boggs theory provides the best agreement with experiment as far as the width is concerned. With regard to the shift, however, both theories seem to be equally valid. They also comment on some passages of their previous paper (see ibid., vol.12, no.10, p.1687, 1979).

The measurement of the absolute Be K X-ray production cross sections for the projectiles ¹₁H⁺, ²₁D⁺, ⁴₂He⁺, ⁹₄Be⁺, ¹¹₅B⁺ impinging with 1-120 keV amu^-1 on 'thin target' Be foils reveals the change over from Coulomb ionisation-which satisfactorily describes the ionisation process with H⁺ and D⁺-to the ionisation by electron promotion (MO theory), for which Be⁺ to Be and B⁺ to Be are good representatives. For the first time, the 'K-K level-matching' effect for a Be target is demonstrated, indicating that besides evidence given by comparison of the calculated (in PWBABCPR) and measured X-ray production cross section for He⁺ to Be, MO theory may also be relevant for He collisions.

A term g₁ which corrects for the lack of orthogonality between the atomic wavefunctions of the initial and final states occurring in the OBK scattering amplitude g_OBK for electron capture from a hydrogenic ion by an incident nucleus, is evaluated using relativistic kinematics and Dirac atomic functions. A non-relativistic treatment using the same approach provides a one-dimensional integral for g₁ which yields good accordance with the non-relativistic calculations of McCarroll (1961) for electron capture from ground-state H atoms by protons having 0.5 and 1 MeV impact energies. Simple analytical expressions for g₁ and the capture cross sections are derived at relativistic as well as high but non-relativistic impact energies E. It is found that for small atomic numbers g₁ becomes negligible in comparison with g_OBK before E is sufficiently high for relativistic effects to be significant, but that for large atomic numbers g₁ remains important at high energies. Moreover, in the ultra-high energy limit g₁ produces an E^-1 (1n E)² decay of the capture cross sections which is more gradual than the E^-1 decay of the relativistic OBK cross section.

Results of a differential time-of-flight energy-loss study of H^- scattering by D (and D^- scattering by H) are presented. Differential cross sections for charge exchange, detachment and excitation were measured in the 100-1000 eV laboratory energy range and 0-1 keV deg reduced angular range. Contrary to the low-energy (less than 1 eV) predictions of Mizuno and Chen (1969) no oscillatory structure was observed in these cross sections. The charge exchange and elastic DCS are found to be strongly absorbed by detachment. Integration of the low-energy DCS and comparison with the total charge exchange cross section of Hummer and co-workers (1960) yielded an absolute calibration. A total detachment cross section for energies below 500 eV was thus obtained. This cross section is not found to decrease with energy. Experimental evidence for detachment from the ² Sigma _gH^-₂ state into the 'parent' ³ Sigma _u H₂ state is presented.

Absolute cross sections for the electron capture process H⁺+X to H(2s)+X⁺ have been determined for protons in He, Ne, N₂ and O₂ at 16 keV and for Ar and H₂ within the range 12-28 keV. A nitric oxide filled photoionisation chamber has been used to record the Lyman alpha radiation emitted by electric field quenching of fast metastable H(2s) atoms formed in single collisions. Unlike previous measurements by Andreev (1966) full account has been taken of the polarisation of the quench radiation. The present results, which involve smaller uncertainties, are found to agree on average to within 4% of the cross sections at 16 keV reported by Andreev.

Previous coupled-channels calculations of inner-shell charge transfer and vacancy production for p+Ar collisions are extended to bare projectiles with nuclear charges Z_p=2 and 6, in the collision energy range from 1 to 9 MeV amu^-1. An improved expression for the charge transfer amplitude is derived in a 'distorted-phase' formulation of the impact parameter method. The authors denote by sigma _C,VK the inclusive cross section, which has been measured by coincidence experiments, for producing all final states in which at least one target electron has been transferred to the projectile and at least one vacancy has been produced in the target K shell. In the single-particle model sigma _C,VK reduces to sigma ^SP_C,VK, which contains only the exclusive single-electron process of direct transfer from the target K shell. The contributions of multi-electron processes to their calculated sigma ^IPM_C,VK increase strongly with Z_p. For Z_p=6, sigma ^IPM_C,VK differs by up to a factor of three from sigma ^SP_C,VK. The charge transfer cross sections sigma ^IPM_CT and sigma ^IPM_C,VK show a Z_p dependence less than the Z⁵_p of the OBK approximation. The calculated sigma ^IPM_VK and sigma ^SP_I,VK for Z_p=6 are compared with experimental K vacancy production cross sections for C⁶⁺ and C⁴⁺ projectiles, respectively, and good agreement is found.

The authors measured positron total scattering cross sections for Ne, Ar, Kr and Xe between 1 and 6 eV employing a time-of-flight spectrometer which has a straight beam-line geometry. A comparison with other results shows rather good agreement for neon but serious deviations for the other gases; the authors' cross section values lie distinctly above the others.

A molecular theory of harmonic generation in free molecules is developed using the principles of quantum electrodynamics. A Cartesian tensor formalism is employed and emphasis is placed on the rotational average which is required to account for the random molecular orientations in the pump beam. It is demonstrated that even harmonics are forbidden to all orders of the multipolar interaction Hamiltonian and in the appendix a new relation involving the rotational average of an arbitrary rank Cartesian tensor is presented.