Table of contents

The radial density D(r) is computed from analytical Hartree-Fock wavefunctions for the first 54 elements of the periodic table. The calculations exhibit the familiar picture of spherical shells of electron density for the first three periods but for the fourth and fifth periods an ambiguity in the shell structure is observed. This ambiguity is analysed in terms of orbital contributions.

The connection established between 1/Z perturbation theory and the Thomas-Fermi energy E_TF(Z,N) for heavy ions with N electrons by March and White (1972) has been developed quantitatively by Dmitrieva and Plindov (1975). Their work is adapted here to yield the approximate analytical expression for 0<N/Z<or=1. A similar approximate formula is given for the exchange energy.

A two-step optical resonance experiment, which realizes the optical analogue of the Autler-Townes experiment, has been performed on a sodium atomic beam excited by two CW narrow-band dye lasers. An intense laser radiation was locked to a component of the D₂ resonance line while a weak laser radiation probed an upper connected transition. A dynamic Stark splitting of the probe transition was clearly observed for various intensities of the strong resonant radiation.

The angular distribution parameter beta has been measured for the neon 2p electrons in the energy range 0-80 eV above threshold using a synchrotron light source. The absolute photoionization cross section of neon 2s electrons has also been determined in the 0-130 eV region above threshold. Both sets of data are compared with recent theoretical calculations and with experimental data using conventional light sources.

An experiment is reported on two-step photoionization of Cs via the 6P_3/2 intermediate state using circularly polarized radiation from a GaAs laser for the first step (8521 AA) and from an Ar-ion laser for the second step. Values for the ratio of ion production rates for right- and left-handed circular polarization in the second step and for the spin polarization of the photoelectrons are presented. The measurements are interpreted on the basis of a calculation which takes into account the hyperfine splitting of the levels involved in the first transition. A description in terms of fine-structure splitting of the 6P state is shown to be totally inadequate, even when allowing for possibly large spin-orbit effects in the continuum. The process is evaluated as a possible source of a spin-polarized electron beam.

An error appearing in a previous paper by the author (see ibid., vol.8 p.2798 (1975)) is pointed at the Glauber additivity t matrix is rederived for scattering off two fixed centres. The results of applying this formula to scattering in a liquid metal is also reported.

The threshold ionization of atomic hydrogen by electron impact has been studied using the classical three-body collision theory. The calculated energy distribution of the two outgoing electrons appears to be in agreement with the behaviour predicted by Wannier and Rau (1971). The energy dependence of the orbital momentum acquired by the two electrons is found to be proportional to E^-0.5, in disagreement with results obtained by Cvejanovic and Grujic (1975).

The differential cross sections for the scattering of low-energy electrons by helium atoms have been calculated using the R-matrix method with partial waves up to L=4. Results for the elastic collision and excitations to the 2³S and 2¹S states of He are presented and compared with experiment.

There have recently been a number of measurements of the differential elastic scattering of electrons by rare gases, in the energy range 20 to 200 eV. The authors present a phaseshift analysis of some of the argon data, the phaseshifts determined producing curves which closely fit the experimental data. Using the fact that the relative values of the differential cross sections as a function of scattering angle (i.e. the shape) are much more accurate than the absolute values, it is shown that a set of most probable absolute values can be determined. The phaseshifts also enable the total elastic cross sections to be determined, giving values which are probably more accurate than those deduced by arbitrary extrapolation of the experimental data and numerical integration.

For pt.I see ibid., vol.8, p.442 (1975). Computational errors of the authors in a previous paper on electron loss from fast hydrogen atoms passing through atomic hydrogen are rectified. The revised cross sections considerably exceed the results of the standard first Born approximation given by Bates and Griffing (1953). The loss cross sections due to Bates and Griffing are correct. Their neglect of discrete states of the target hydrogen atom with principal quantum number n>3 produces an error not greater than three per cent at any impact energy. The corrected closure approximation is significantly improved taking full account of the continuum of the target atom. It is shown that the corrected closure approximation converges more rapidly that the usual procedure in the first Born approximation. Present upper and lower bounds to loss cross sections are in excellent agreement with the results obtained by the first Born approximation without use of the added closure approximation.

The collision broadening and shift of the lithium resonance line, formed in absorption under low pressures of helium, have been measured. The (half) half-width gamma and shift beta are found to vary with helium density N according to the following relations: gamma /N=(1.73+or-0.10)*10^-9 rads^-1 atom^-1 cm³ beta /N=(0.00+or-0.05)*10^-9 rad s^-1 atom^-1 cm³. A recent theoretical calculation is in reasonable agreement with these results.

New emission bands centred at 4700 AA have been observed following optical excitation of the cadmium 5³P₁ level in a mixture of cadmium and mercury vapour. Radiative decay times of 4.0 and 2.8 mu s have been measured for two states which are shown to be collisionally mixed. An excimer-excimer quenching rate of 2.8*10^-12 cm³ s^-1 and a bi-excimer mixing rate of 4.0*10^-11 cm s^-1 have been observed. Morse potentials for the states are derived from the spectra.

An empirical relationship is given for the calculation of the higher-order contributions to the viscosity of binary gas mixtures in the Chapman-Cowling approach. A maximum error of five parts in ten thousands is observed.

For pt.I see ibid., vol.9, p.357 (1976). A theory of the optical pumping of high-J molecule levels with a broad-band CW laser has previously been developed. The linewidth of the Hanle effect is analysed in the present paper for two particular cases in which an exact integration is possible. These cases are: (i) gT (where g is the Lande factor and T the lifetime) is the same for the two levels; and (ii) one of the Lande factors vanishes. In general, the Hanle resonance is power-broadened for high laser intensities. However, in the case of a zero Lande factor of the ground state, the theory predicts a radiative narrowing of the Hanle effect. It is shown that this most unusual feature is connected to the fact that, in the latter case, the physical observables of the resonance level may get the ground-state lifetime, when the transition is saturated.

The theoretical behaviour of radiofrequency transitions with Delta m>or=1 in an atomic vapour under the simultaneous action of a longitudinal optical pumping, a static field introducing small decoupling between I and J and a relaxation mechanism is investigated. It is shown that the information contained in the eigenvalues and eigenvectors of the Hamiltonian of the atom 'dressed' by the RF field can be used to simplify significantly the equations of the time evolution of the atomic density matrix. Although an analytical solution is obtained only in a few simple cases a numerical solution is possible for any RF polarization. Several previous treatments are shown to be limiting cases of the general equations presented here. From the analysis of the recent experimental results of Arimondo and Corbalan (1974) on the ground state of ⁸⁷Rb, it turns out that the observable quantities are strongly dependent on all the parameters describing the simultaneous action of the pumping cycle, relaxation and the RF interaction. Thus accurate descriptions of the pumping cycle and the relaxation mechanism are required for any comparison with theory of experimental results for the widths and shifts of multiple-quantum transitions.

A set of operators, which act on the eigenstates of the atomic Hamiltonian in the presence of a partial I-J decoupling in a way analogous to the way the usual irreducible tensor operators act on the eigenstates of the total angular momentum, is introduced. Starting from these operators a set of operators, analogous to Haroche's pseudotensor operators, is obtained for the dressed-atom Hilbert space. They are thus particularly suitable to the theoretical treatment of the Delta m_F>1 multiple quantum transitions within the Zeeman levels of a hyperfine state of an alkali atom. As an example, this treatment is applied to a system having a 'fictitious' spin F=1, and it is shown that the contribution of the single and multiple quantum transitions to the multipole components of the dressed-atom density matrix is separated.

Starting with well known formulae for nth-order atomic susceptibilities, irreducible spherical tensor techniques and Racah algebra are used to put these in a form suitable for the treatment of complex nonlinear processes, e.g. where fine and hyperfine interactions can be included and the interacting fields can have quite general polarizations. By way of illustration the susceptibilities are calculated in detail for third harmonic generation and Raman scattering in alkali vapours, taking into account the effect of spin-orbit coupling.

Ground-state energies of two-electron atoms H^-, He and Li^P, embedded in a very strong magnetic field, are calculated using the Hartree-Fock method. The one-particle wavefunction contains a transverse part, which is determined by the magnetic field, and a longitudinal part, which is determined by the Coulomb interactions. The self-consistent field method is used to adjust the longitudinal function. For the strongest fields considered in this study, the improvement over previous variational estimates is between ten and twenty per cent. For lower fields, the author's results contradict those by Mueller et al. (1975) as a somewhat higher ground-state energy for the H^- ion is obtained.

The Breit interaction which gives the leading correction to the Coulomb potential in quantum electrodynamics, is required for precision calculations of binding energies of inner electrons in heavy atoms for calculations of multiplet structure, and for other applications. The correct form for use in multi-configuration calculations is discussed, and the angular momentum reduction to a sum of radial integrals is carried out completely in an approximation valid for all problems except those involving very tightly bound electrons. The coefficients are presented in a form suitable for computation; a computer program for matrix elements of the Breit interaction for general atomic states is at present under test.

Various descriptions for the primary high n-state population in a hydrogenic ion emerging from a solid foil were investigated and power lay dependences in time were found for the delayed emission of 'prompt' photons due to cascading.

L X-rays were produced by the impact of protons between 200 and 900 keV energy on thin Ag, Sb and Au foils. The spectra were measured by a Si(Li) detector and analysed by least-squares techniques to yield X-ray intensity ratios as a function of energy. From these, ionization cross section ratios for the subshells L_I, L_II and L_III were calculated. For Ag and Sb, X-ray emission rate ratios were also determined which agree with Scofield's values in many cases, but not in all. For Au, X-ray intensity ratios are generally in agreement with published results; for Ag and Sb, the ratio L alpha /Ll is smaller than Scofield's values. The ratio sigma ₂/ sigma ₁ is generally well described by the plane-wave Born approximation theory; for the ratio sigma ₃/ sigma ₂, a decision between different theories is difficult.

Exchange excitation has been directly observed in the excitation of the 6³P_1,2 states of mercury by transversely polarized electrons. A significant change of polarization of the electrons scattered in the forward direction has been measured for energies between threshold and 7 eV for both states. Above 7 eV the influence of exchange excitation of the 6³P₁ state decreases rapidly. This in in qualitative agreement with theory. From the measurements, the relative contributions for triplet (exchange) and singlet ('direct') scattering in the excitation of the 6³P₁ state in mercury were obtained. These can be explained in terms of intermediate coupling.

For pt.I see ibid., vol.9, no.5, p.791 (1976). The ¹S₀ to ³P excitation of a 'two-electron atom' by polarized incident electrons is analysed in general terms of the scattering amplitudes using density-matrix techniques. Intermediate coupling for the excited atom is included. The expressions obtained can be used to explain the experimental results for the 6¹S₀ to 6³P_1,2 excitation of mercury. Polarization effects are found if a strong spin-orbit interaction within the excited atom, which gives rise to fine-structure splitting, is taken into account during the collision process.

The absolute photoionization cross section of potassium is reported from the 3p ionization limit at 50 nm down to 32 nm. Measurements on the shapes of the autoionizing resonances at 65.3 and 66.2 nm are also described. Comparisons are made with existing experimental results and recent calculations of potassium photoionization cross sections.

The many-body Green's function theory of Yarlagadda and Taylor (1975) is applied in the lowest order to the coincident ionization of the helium atom. Significant agreement is obtained with the experimental results of Ehrhardt and coworkers (1972). The success of the two-time model of inelastic processes is demonstrated.

Absolute elastic differential cross sections for electrons in argon have been measured for electron energies between 60 and 150 eV in the angular range from 5 to 150 degrees . Total elastic cross sections are calculated by integrating differential cross section values. The results are compared with other experimental and theoretical data available in the literature.

Some forms of asymptotic expansions are suggested for wavefunctions of charged particles scattering on systems with charge, multipole moments and inner degrees of freedom. The recurrence relations necessary for the calculations are obtained. Formulae are written for collisions of electrons with ions or atoms. The methods suggested may also be used in the investigation of collisions between other particles.

The CO⁺(A² Pi to X² Sigma ⁺) and CO₂⁺(A² Pi _u to X² Pi _g) fluorescence produced by impacting CO and CO₂ with synchrotron radiation of wavelength in the 175-750 AA region was investigated and the production cross sections were measured. For primary photon wavelengths longer than 630 AA, the fluorescence cross sections show structure at the positions of the Rydberg series for both gases. For primary photon wavelengths shorter than 500 AA, the cross sections decrease monotonically with the wavelengths. The present measurements are compared with the various results reported by previous authors.

Absorption by Cs₂, Rb₂ and RbCs of radiation at the nine wavelengths of an Ar-ion laser leads to production of molecular and atomic ions via two-photon processes. The authors report absolute probabilities for these processes in Cs₂ and Rb₂ (and relative numbers for RbCs) and discuss the spectral behaviour in the light of spectroscopic information on the potential energy diagrams. For Cs₂, values for the photoionization cross sections of the E state, the intermediate state in the two-step process, are derived.