Table of contents

A recent identification in the absorption spectrum of atomic Ba (Ba I) of the 4d⁹4f ¹P level as lying above the ionization limit corresponding to ejection of a 4d electron is discussed. It is found that a Hartree-Fock calculation of this level places it below the ionization limit.

A recent proposal for the evaluation of the shape of the Fermi hole in atoms is investigated. It is pointed out that the arbitrariness inherent in the Hartree-Fock orbitals introduces an ambiguity in the definition, which can be overcome by using the unorthogonalized Hartree orbitals. This is still not quite satisfactory, the use of a proper Hartree-Fock as well as a proper Hartree wavefunction, each independently optimized, being essential for obtaining the shape of the Fermi hole. Calculations demonstrating these points are presented for the 2³S state of the helium isoelectronic sequence.

The one-electron three-dimensional exchange integrals which arise in the impact-parameter treatment of heavy-particle collisions, are reduced to one-dimensional integrals whose ranges are finite, and whose integrands contain only simple exponential functions.

Potential energy curves are reported for the diatomic hydrides BeH⁺, BH and CH⁺ in their ground states and, in each instance, an allowance was made for all the pair-correlation energies. Spectroscopic constants were obtained by means of a Dunham analysis and comparisons were made with experimental values.

An emission spectrum of SiSe is observed between 3900 and 4300 AA. The transition involves the ground state x¹ Sigma ⁺ and the ( sigma pi ) a ³ Pi state, most probably the a ³ Pi ₁ component. Silicon tetrachloride and traces of selenium vapour diluted in helium are sent into a Schuler discharge. Vibrational assignments are discussed: T_e(³ Pi ) approximately=25075 cm^-1.

The results of two experiments are reported. In the first, cross sections were measured for the production of protons by collisions between electrons and vibrationally de-excited H₃⁺ ions for interaction energies between 29 and 611 eV. This experiment employed electron and ion beams which intersected at right angles ( theta =90 degrees ). In the second experiment inclined beams ( theta =10 degrees ) were used to investigate lower interaction energies (19.2 to 29.5 eV) because it was found that the cross section increased rapidly with energy between 20 and 30 eV. The cross section reached a maximum value of 7.6*10^-16 cm² at 79 eV and the systematic errors were assessed to be less than +or-6%. There is excellent accord between the present results and earlier measurements (Peart and Dolder, abstr. A72242 of 1974) at near-threshold energies.

The relativistic contraction of inner-shell orbitals affects outer shells through the self-consistency requirement of the atomic field. The self-consistent field relativistic effects increase the binding energies of the 6s electrons of Au and Hg by about 30%, whereas those for the 4f and 5d electrons are decreased by about 15% compared to the non-relativistic results. The oscillator strengths for the resonance transitions (6S to 6P) of Au and Hg are reduced by about 30% as a consequence of the self-consistent field relativistic effects.

An entirely numerical approach to the Coulomb approximation for atomic dipole transition probabilities A_jj(j=n, l) is investigated. The wavefunction divergency at the origin and the resulting normalization problem are taken care of by introducing a numerically determined cut-off radius (a function of j) inside which the wavefunction is chosen equal to zero. The method has been applied to the Rydberg states n_min<or=n<or=10, 0<or=l<or=5 of Be II (n_min= 3), and Mg II (n_min=3), and the stability with respect to cut-off radius is examined for transition probabilities, lifetimes and branching ratios.

Theoretical K-shell fluorescence yields for various electronic configurations of neon are presented. These fluorescence yields are averages of the level fluorescence yields with a statistical population.

A Stark effect experiment is presented, which involves the recording of transients in radiation resonantly scattered by a vapour, following the sudden application of an electric field pulse. The theory of this effect is developed from the case where a general step-function perturbation is applied. The transients have the form of damped sinusoidal oscillations: the damping is determined by the rate of radiative decay; the modulation frequencies depend directly on the energy-level splittings caused by the applied field. This technique has been to measure the magnitude and sign of the differential Stark shifts in the 6³P₁ level of ¹⁹⁸Hg and ¹⁹⁹Hg. These lead to a value for the tensor polarizability which is in good agreement with recent measurements. Results are presented of a calculation of the polarizabilities of the 6³P₁ and 6¹S₀ levels based on experimental oscillator strengths.

Using a high resolution Ge(Le)-Si(Li) spectrometer, coincidences between K and L X-rays were recorded, in order to measure all the L₂ and L₃ subshell yields of bismuth (Z=83).

De-excitation of He(3¹S, 3³S) states by collisions with Ne, Ar, Kr and Xe atoms is mainly attributed to chemi-ionization. For He(3³P) states relaxation is attributed to chemi-ionization and a collisional radiative process to He(2³S) states.

Elastic scattering of electrons by hydrogen atoms has been investigated in the plane wave approximation in 50-200 eV energy range. A general expression has been derived for the non-adiabatic potential in a Hermitian form. First-order perturbed wavefunctions and pseudo-states are employed to construct dynamic polarization potentials. The theory underestimates the differential cross sections, but the shapes of the theoretical curves are in agreement with the experimental data. The results are also in fair agreement with more sophisticated calculations.

For pt.VIII, see ibid., vol.8, no.7, p.1088 (1975). Differential cross sections for the excitation from the ground state to the 2¹S and 2³S states of helium and to the (2s) state of atomic hydrogen by electrons are calculated for incident energies of 50 to 200 eV. A wavefunction calculated by Winters (see abstr. A47875 of 1974), is employed to describe the incident channel, making allowance for absorption, polarization and exchange effects. Final state scattering is taken into account through a distorted wave formalism. The results provide a generally satisfactory representation of experiment, although it is considered that improvements could be obtained by including the coupling between the final states and the neighbouring 2P states.

Apart from the well known ²S negative ion state found in e^--He scattering, additional p and d wave resonances below the first excitation threshold have been reported by several authors. A careful experimental investigation of the differential cross section does not confirm these observations. If such resonances exist, their width should be smaller than 10 mu eV.

Cross sections of such processes as double photoionization and photoionization accompanied by excitation of He-like atomic systems are investigated in the limit of high, but still not relativistic, photon energies. In addition to total cross sections electron energy and angular distributions are also obtained. Coulomb wave and Green functions were employed in the momentum representation. Results for the total double ionization cross section as a function of photon energy are close to those obtained by predecessors with much more complicated ground state wavefunctions. The cross section of ionization with simultaneous excitation differs considerably from those obtained before. Angular and energy distributions of ionized electrons, are, to this extent, investigated for the first time.

Recent measurements of the angular differential cross sections for the elastic scattering of electrons of 100 eV energy in helium, neon, and argon over an angular range 10 to 150 degrees are discussed. The observed relative differential cross sections for neon and argon have been converted to an absolute scale by measuring differential scattering in 10% and 10.3% mixtures of neon and argon respectively in helium and using available cross section data for helium for angles of 30 degrees and 50 degrees . Differential cross sections for 100 eV electrons in helium are also deduced from the mixture experiments. The results are compared with other measurements with the absolute data of Jansen et al. (1974) for angles of 50 degrees and below is found to be excellent and extends the data to 150 degrees .

A coincidence technique has been used to measure angular correlations between electrons which have excited the first excited state of argon and the photons arising from the decay of this state to the ground state. The ratio of differential cross sections for the excitation of the degenerate substates and the relative phases have been deduced from the correlations for incident electron energies of 80 eV and 113 eV and electron scattering angles of 8.5 degrees . The angular correlations show poor agreement with those predicted by the Born approximation.

The problem of excitation of the positive ions by electron impact is discussed using analytic properties of the Green function for the Coulomb field. The solution obtained has the correct asymptotic expansion in powers of (1/Z)ⁿ, where Z-1 is the ion charge. Illustrative calculations for the 2s-3s and 2p-3s transitions in O VI show large resonance contributions to the excitation cross sections.

S-wave phase shifts for the positron-helium elastic scattering problem are presented in the coupled static approximation ( i.e. the trial function contains the ground states of both the target atom and positronium but no other terms). Various models of the target atom wavefunction are employed, two of which confirm qualitatively the marked effect on the results as reported by Wardle (1973) for these models. While none of the target models used in this approximation appears to lead to an adequate description of the system, as is also the case in positron-hydrogen scattering in the same approximation, a qualitative comparison of the best results presented here and the corresponding approximation in the hydrogen case suggests a manageable calculation with two open channels (i.e. with real positronium formation) could produce meaningful results.

Differential cross sections for 4³S and 4¹S excitation of He in He⁺ on He collisions have been measured in the energy range 94 eV-120 eV and in the angular range 8-17 degrees (in the laboratory frame) by detecting, in delayed coincidence, the scattered ions and the emitted photons ( lambda =5047 AA, 4713 AA). Relative values of the corresponding total cross sections have also been determined, from 60 eV up to 3 keV. Below 400 eV they oscillate in opposite phase as functions of energy. This behaviour can be explained by the Rosenthal model a long range crossing (at 36 bohr) makes the inelastic amplitudes produced by radial couplings interfere at the inner crossing points (1.3 bohr). A double system of oscillations is observed in the differential cross sections. It is interpreted as Stueckelberg interference, coming from inner crossings, combined with a nuclear symmetry interference effect. Calculations of the differential and total cross sections have been carried out and satisfactory agreement between calculated and measured cross sections is obtained.

A unique set of normal coordinates is constructed in a Cartesian basis for a non-planar molecule XY₄ belonging to the group T_d, using the method proposed by Sheth (1975), and hence establishing the usefulness of the method for cubic groups.

High dispersion emission spectra of the CuTe molecule have been observed. Apart from the previously reported A-X system, there is a new B-X system at wavenumbers about 1700 cm^-1 greater. The vibrational analysis of the A-X system has been performed. The rotational analysis of several bands of this system allows them to be assigned to a Delta Omega =0 transition and to give specific constants From a vibrational analysis, the green system is assigned to a B-X transition with the same lower state as the red system.

Electron-electron angular correlations have been measured for the ionization of molecular hydrogen and nitrogen by electron impact at 100 eV incident electron energy and also at 250 eV for hydrogen. A total of 18 cases were studied at various secondary energies and primary scattering angles. Strong binary peaks were observed in all cases resembling what would be expected from the ionization of atoms having similar electronic structures. The recoil peaks which were observed were, however, generally weak, especially with nitrogen. A first attempt at determining part of the six-fold differential cross section for molecular hydrogen was made by varying the energy of the lower energy secondary electron while all other scattering parameters were held constant. The result was obtained in the binary peak and shows the vibrational excitation of the residual ion. The observed structure is in reasonable agreement with a computed curve based on Dunn's calculated Franck-Condon factors (1966) for the ionization of H₂.

The angular distribution expression for scattering of a structureless particle from a rigid rotating diatomic molecule has been simplified by using the angular momentum recoupling method. The formulae are derived by using this expression to compute the cross sections for transitions among various rotational levels of a diatomic molecule when the e^--molecule problem is solved in a body-fixed frame of reference in the fixed-nuclei approximation. The energy dependence of the differential cross section for rotationally elastic and inelastic e^--N₂ scattering has been calculated in the adiabatic-nuclei approximation. The results for the momentum transfer cross section are also given.

A study of the autoionization of the SF₆^- ion has been performed using the technique of ion cyclotron resonance (ICR). This study utilizes the motion of charged particles in the trapping plate plane to effectively separate the SF₆^- ion and electron currents, thus enabling the direct evaluation of the ion current signal as a function of time. The autoionization process was observed as a function of both the electron-neutral interaction time and the density of the neutral molecule. It was found that the 'apparent' autoionization lifetime, tau _a, of SF₆^- varies as a function of observation time from 50 mu s to 10 ms. Estimates are also made of the effective collisional stabilization frequency of the SF₆^-*-SF₆ collision pair as well as the vibrational fluorescence lifetime of the excited ion.

The experiments described show that the decay rate of an afterglow in rare gases energized at low energy is sensitively dependent on the angle theta between the axis of the long glass tubes housing the afterglow, and the direction of a uniform magnetic field. The results are for magnetic field strengths in the range 100 to 1000 G. Also described are experiments which show the dependence of the decay rate on the electron number density and theta at given field strength, and that diffusion along the field lines is not an important loss mechanism affecting the decay. Explanations for the phenomena are advanced on the presence in the afterglow of an ordered assemblage of electron pair oscillators.

A microwave radiometric technique has been used to investigate the decay of electron average energy, U_e, in the afterglow of pulsed N₂ discharges. The technique involves first determining the generally non-Maxwellian electron velocity distribution function, f( nu ), from radiation temperature measurements, and then integrating f( nu ) to obtain U_e. Theoretical analysis shows that the decay of U_e in the afterglow is controlled by the vibrational temperature T_v, attained at the end of the active discharge. For a quasisteady state value of U_e>or approximately=0.5 eV in the afterglow, close thermal equilibrium exists between the electrons and the vibrational levels of the N₂ ground state. Comparison of the experimental U_e decay rate with that predicted from coupling theory yields an additional heating term which is believed to arise from mutual ionizing collisions of the a ¹ Pi _g state.