Table of contents

A general formula has been established for the magnetic multipole moment integrals over Slater-type orbitals. These integrals with an arbitrary origin of the magnetic multipole moment operator are expressed in terms of the well known A and B functions.

A high-resolution K emission spectrum of O₂ is presented. Four bands are interpreted in a molecular-orbital description. Five satellite lines are found in the spectrum.

A recent letter by Albat and Gruen (see ibid., vol.9, p.L463, (1976)) is discussed. In that letter a solution was proposed to the difficulties due to the 'translational effects' in the molecular theory of atomic collisions.

The differential cross section has been measured for scattering angles less than 0.07 degrees with an angular resolution of 0.003 degrees FWHM. The differential cross section was found to decrease monotonically with scattering angle. In particular the node in the angular distribution, which had been predicted by some theoretical calculations, was not observed.

The cross section for charge transfer to fully stripped carbon ions in collision with ground-state hydrogen atoms is calculated in an adiabatic molecular-orbital basis. In contrast to previous work by Salop and Olson (1976), the cross section obtained by retaining only the dominant 4f sigma -5g sigma radial coupling is oscillatory. The inclusion of rotational coupling to other close-lying states damps out these oscillations to provide a smoothly varying cross section whose magnitude is considerably enhanced over that obtained from the two-state calculation.

Phaseshifts for e^-+He⁺ elastic scattering in the ¹P channel are calculated using perturbation theory. The position and width of the 2s2p¹P autoionising state of helium are determined.

The elastic-scattering cross section of electrons by atomic nitrogen in its ground state is calculated by a 'linear algebraic method'. Eight target states are included in the electron-atom wavefunction expansion, with target orbitals determined with a scaled Thomas-Fermi potential. This approximation is found to give no low-energy resonances, in agreement with the work of Berrington et al. (1975).

The yield of singly charged ions is considered for inelastic fast-electron scattering. The densities of the generalised oscillator strengths are calculated in the single-electron approximation, taking account of the many-electron inter-shell correlations in the RPAE framework and out of it. It is demonstrated that a special type of inter-shell interaction of the electrons is responsible for this effect. This interaction arises as a consequence of the rearrangement of the core self-consistent field due to Auger-decay of the 2p⁶ vacancy. Such interactions may be considered as a type of the so-called post-collision interaction and this permits us to achieve reasonable agreement with the existing experimental data.

Negative-ion resonances associated with inner K-shell-excited states of N₂ and CO have been observed in the yield of positive ions from electron impact excitation of these molecules.

A new and sensitive electron impact spectrometer has been used to observe pure vibrational excitation in H₂, NO and O₂. The excitation of vibrational levels of their ground states with over 4 eV energy has been observed. This excitation is produced by the decay of unbounded resonant states along the path which can eventually end in dissociation (dissociative attachment).

The possibility is discussed of using highly sensitive beam resonance methods to detect the nuclear-spin-dependent part of the weak neutral electron-nucleon interaction. In principle this can be achieved by inducing transitions with simultaneously applied electric and magnetic RF fields. However, no atom satisfies the criteria for optimum sensitivity and as a result the best experiment may only be marginally feasible. It is argued that the importance of the problem justifies further detailed calculations.

A formulation of many-body perturbation theory starting from the operator equation (H,Q⁺)= omega Q⁺ is presented. The method of solution is based on an operator scalar product, (X/Y)=Tr(X⁺Y), which allows the use of resolvent and partitioning techniques to establish Rayleigh-Schrodinger or Brillouin-Wigner perturbation theory for the excitation energy and excitation operator, Q⁺. The excitation operator contains all information about the two states involved in the transition. Specific results are given for removal or addition of an electron and for excitations of particle-hole type and comparison with the Green's function methods is made.

The effect of correlation on the one- and two-particle momentum distributions is analysed by using the natural expansion representation of a configuration-interaction space wavefunction. Several expectation values are reported with emphasis being given to two-electron properties. In particular, the angular and radial components of correlation are assessed by the determination of various correlation coefficients. The introduction of total angular correlation is found to increase the probability of the two momentum vectors being aligned-as is indicated by the positive values obtained for the angular correlation coefficients.

Data obtained in recent electron-photon coincidence experiments on helium excitation have been used together with differential cross section data to determine line polarisations independently of previous direct measurements. Polarisations have been determined at electron impact energies in the range 60-100 eV for the following lines: 2¹P-1¹S (58.4 nm), 3¹P-1¹S (53.7 nm) and 3¹P-2¹S (501.6 nm). Reasonable agreement is found with data from a recent experiment on the vacuum ultraviolet line polarisation of helium and with one experiment on the 501.6 nm line. The data are compared with recent theoretical predictions.

The photoabsorption cross section of sodium has been determined in the energy range 45-250 eV using a heat-pipe absorption cell and a synchrotron light source. The results are compared with previous relative values and with a single-electron calculation. The integrated oscillator strength for the n=2 shell is compared with that of neon. The problems of determining accurate cross sections for metal vapours in the extreme ultraviolet spectral region are discussed in some detail.

An experiment is reported on two-photon processes in molecular caesium in the 563-635 nm wavelength region. Ionisation into the X² Sigma _g⁺ ground state of Cs₂⁺ is the most abundant process in the region up to 600 nm. At shorter wavelengths ion-pair formation (Cs₂ to Cs⁺+Cs^-) is dominant. A qualitative discussion is given of the crossings of the (Cs⁺+Cs^-) potential-energy curve with the Sigma curves of (Cs+Cs*) systems. Only two out of this manifold are identified as giving rise to formation of the ion pair.

Reports on q quasi-molecular study of collisionsl processes in the Li⁺-He system in the laboratory energy range 0.3 keV<or=E_lab<or=50 keV. To handle the primary excitation mechanisms at small internuclear distances (R<or=0.5 a₀), diabatic states displaying quasi-symmetric properties (u, g) are introduced. The study of the excitation sharing between the 2p levels of He and Li is carried out within the Demkov-Meyerhof model, and excitation of He(1s2s) is explained in analogy with a similar process in the He-He collision. Analysis of the K-vacancy-sharing process leading to Li(1s2/2l') is undertaken within a many-electron framework. The necessity of creating a double 2p sigma vacancy prior to the Demkov-Meyerhof K-vacancy sharing is confirmed. The calculations are compared with available experimental data and previous theoretical results.

The closure approximation is used for calculating in the second Born approximation, angular distributions and excitation functions of electrons emitted from hydrogen in collisions with fast protons. Using the closure approximation, it is shown that contributions due to the principal value of the propagator can be neglected.

The authors are concerned with relatively simple semi-empirical models for quenching of excited atoms by stable molecules, i.e. the transfer of electronic energy into vibrational and translational degrees of freedom. The most notable example is the quenching for Na(3³P) to the ground state by N₂. The cross sections are first related to those for free-electron-molecule scattering in a 'first-order' theory and it is shown that this simple theory leads to numerical values which are far too small. Thus the authors invoke an ionic collision complex formed by partial charge transfer and succeed in obtaining reasonable total cross sections and final-state branching ratios from a hybrid theory. Calculations are carried out for excited Li, Na and K colliding with H₂, N₂ and O₂.

The continuous-slowing-down approximation (CSDA) and the modified discrete-energy-bin (MDEB) method are used to calculate a yield spectra U(E,E₀) in (eV s)^-1 of electrons with energy E resulting from a primary of energy E₀. The results of the two calculations are then compared. The MDEB method is found to produce consistently more ions per energy loss while at the same time producing less excitations of some of the low-lying states when compared with the CSDA. These discrepancies can be explained by studying the contributions from the individual generations of electrons to the yield spectra. The authors also present an integral equation for the solution of the yield spectra.

A new resonance profile parameter (w_l0) is proposed which can be considered as a complex number and a function of scattering angle. The argument (arg w_l0) and the absolute value ( mod w_l0 mod ) of w_l0 represent the asymmetry and the peak-to-dip height of the resonance profile. Fano's q-value-type parameter (Q) is expressed by w_l0 as Q=-cot(¹/₂ arg w_l0). The parameters w_l0 for the resonances of the electron-helium system are shown as an example.

A distorted-wave program is being developed for calculating the excitation of few-electron ions by electron impact. It utilises the exchange approximation to represent the exact initial-state wavefunction in the T-matrix expression for the excitation amplitude. The program has been implemented for excitation of the 2^1,3(SkP) states of two-electron ions. Comparison is made with other results where available. The first part of the paper discusses some of the astrophysical applications of these cross sections as well as the motivation and requirements of the calculational methodology.

The ejected-electron spectrum of magnesium vapour following autoionisation of two-electron excited states of the type 1s²2s²2p⁶3pnl is presented for the first time. Ejected electrons were energy analysed at 75 degrees to the direction of an incident electron beam of energy 17 to 500 eV. 46 lines are reported in the ejected-electron energy range 0.5 eV to 4.5 eV, of which 40 have been assigned to eight series. Comparisons are made with ultraviolet observations and theoretical calculations.

The ejected-electron spectrum of potassium vapour has been observed at 50 degrees and 75 degrees with respect to an incident-electron beam of 30 to 400 eV incident energy. Ejected-electron energies are in the range 4 to 21.5 eV. A total of 84 autoionising states of K I are reported, of which 49 have been identified by comparison with ultraviolet absorption data. Some of the remaining levels are assigned tentatively using available theoretical data. In addition, three states belonging to the K II autoionisation spectrum are identified.

The ejected-electron spectrum of caesium vapour has been observed at 75 degrees with respect to an incident electron beam of 30 to 400 eV incident energy. A total of 63 autoionising states, between 8.3 and 13.4 eV ejected-electron energy, are reported. Of the observed states, 37 have been identified by comparison with ultraviolet absorption data.

A study of the electron impact energy-loss spectrum of SF₆ under both optical (low scattering angle, high impact energy) and non-optical conditions (high scattering angle, low impact energy) has revealed a number of electronic excitation processes. With the help of theoretical calculations, several of these transitions have been assigned and approximate cross sections associated with four features have been determined. In addition, a strong resonance at 12 eV has been observed in both elastic and vibrationally inelastic ( Delta E=0.092 eV) channels.

The hyperfine structure of the 6p³²D_5/2, ⁰, 6P²(³P₀)6d ²D_3/2 and 6P²(³P₁)7s⁴P_3/2 levels of the Bi I spectrum has been measured on the 3397 AA and 3511 AA lines with a PGS-2 plane-grating spectrograph and on the 4308 AA line with a Fabry-Perot interferometer coupled to the spectrograph. The experiments led to: A(²D_5/2⁰)=2502.86(56) MHz, A(⁴P_3/2)=-60.9(1.4) MHz. B(⁴P_3/2)=-739(9) MHz, A(³D_3/2)=128.5(2.7) MHz and B(²D_3/2)=35(7) MHz. The hyperfine-structure analysis of the 6p³²D_5/2⁰ level was carried out according to second-order perturbation theory and no experimental proof was found that the B(²D_5/2⁰) constant differs from zero, as was postulated by some authors. For comparison, the theoretical values of the above hyperfine-structure constants were calculated.

The electron affinities of the molecules, C₂ and P₂ are calculated by the Green's function formalism which has been successful in determining accurate ionisation potentials of atoms and molecules. The adiabatic electron affinities obtained are AEA(C₂)=3.60 eV and AEA(P₂)=0.30 eV compared to the experimental values of 3.54 eV and 0.24+or-0.23 eV respectively. The vibrational structure in the radiative electron-attachment spectra of C₂ and P₂ is also computed. There is a close relationship between the vibrational structures in electron-attachment and photodetachment spectra.