Table of contents

Three factors causing the rate of nuclear beta decay to depend on the chemical environment of the nucleus are discussed. The operation of these factors is demonstrated in the low-energy decay of ⁶³Ni by calculations on three common oxidation states of nickel. Each of the three factors causes changes in the total decay rate of the order of one part in 10³ between these environments.

In a recent article, Rzazewski and Florjanczyk (1984) calculated the resonance fluorescence of a two-level atom driven by a smooth excitation pulse. The authors offer a different analysis of the problem which allows one to arrive at a physical interpretation of the results which differs from these authors. Moreover, they indicate that there is an additional contribution to the spectrum not considered in their work.

In very recent calculations by Chung (1984), term energies have been calculated for doubly-excited terms in lithium-like ions. The calculations include relativistic effects and mass polarisation. Mass polarisation can be investigated experimentally by measurements of the isotope shift. A measurement of the isotope shift in the 1s2s2p ⁴P⁰-1s2p²⁴P transition in Li I is presented. An isotope shift of 0.69+or-0.15 cm^-1 was found, which is in agreement with the theoretical shift 0.79 cm^-1, which can be deduced from Chung's calculations.

Two-photon Hanle resonances were observed in a three-level atomic system interacting with two resonant light fields. The choice of suitable polarisation of the light beams and shape analysis of the resonances enabled one to distinguish the effects connected with two-photon coherent transitions from the effects due to stepwise transitions. The experiment was performed with Ne atoms simultaneously interacting with 632.8 nm and 3392 nm radiations from two He-Ne lasers. The mutual coupling of these lasers made it possible to observe the influence of the mutual correlation of both light fields on the two-photon Hanle effect.

An aluminum hollow cathode run with deuterium and Ar as carrier gas has given strong spectra of some hitherto unreported bands of AlD and AlD⁺. Term values in the nu =0, 1 levels of the A² Pi and X² Sigma ⁺ states of AlH⁺ and AlD⁺ have been fitted to their respective energy expressions. The fit is poor for AlH⁺, nu =1 and this may be a result of perturbation. Improved constants for the C¹ Sigma state of AlD are reported from measurement of the C-A system.

The authors present computed frequencies of the S₀(0) transitions between bound and pseudo-bound levels of the para-H₂-para-H₂ dimer. The calculations account for the interchange symmetry of the H₂ molecules. Analogous calculations of the S₁(0) spectrum and found to be in satisfactory agreement with the laboratory measurements of McKellar and Welsh (1974). The authors' results lend support to the proposed identification of features in the far infrared spectra of Jupiter and Saturn with S₀(0) transitions of the H₂-H₂ dimer.

The ejected-electron spectra from 5-25 keV He⁺ incident on neon have been measured at ejection angle 90 degrees with respect to the incident ion beam. Strong velocity-dependent features have been observed for both target and projectile autoionisation peaks. For neon lines, experimental results are compared with those for Li⁺ impact by Bisgaard et al. (1980) and discussed in the framework of the MO model.

Using low-energy recoil ions produced by 110 MeV C⁴⁺ projectiles the authors have applied time-of-flight techniques to identify simultaneously q, r and s, measuring cross sections for the Ar^q++Ar to Ar^r++Ar^s++(r+s-q)e^- reactions at 1.8 q keV. Cases with up to five electrons transferred were observed.

Differential scattering experiments have been performed at collision energies E_CM=79 and 93 eV around which electronic excitation due to a potential crossing begins to arise. The velocity of the scattered particles was analysed by a time-of-flight technique. Nearly the complete angular range of the differential scattering in the CM system was covered in these measurements (10 degrees < Theta <170 degrees ). The dominant reaction around the reduced scattering angle E_CM Theta =10 keV deg, at which a maximum of the inelastic cross sections appears, is the following charge exchange caused by the 4f sigma -3s sigma crossing: Na⁺+Ne to Na(3s)+Ne⁺-16.4 eV. The parameters characterising the crossing point determined by a semiclassical analysis of the experimental result are as follows: the internuclear distance is R_C=0.865 AA, the repulsive potential V(R_C)=63.8 eV and the interaction V₁₂=1.35 eV. The interaction V₁₂ at the crossing point is small owing to the quasi u-g character of the MO 4f sigma and 3s sigma .

Variational R-matrix calculations of elastic electron scattering from the ground state of H₂ are reported for the ² Sigma _u⁺ resonant scattering state at R=1.402a₀. The orbital basis set includes exponential functions centred on each nucleus, spherical Bessel functions and numerical functions obtained by integrating asymptotic coupled differential equations. Polarisation response is treated in terms of polarisation pseudo-states.

An asymptotic distorted-wave method is proposed for electron-molecule scattering calculations, designed to bridge the gap between accurate variational calculations, feasible only for low partial-wave l values, and partial-wave Born calculations, valid for large l. Calculations of K matrices in the ² Sigma _u⁺ scattering state of e^-+H₂ illustrate the method.

Several ways of parametrising results of coherence and correlation analysis of atomic excitation in planar scattering experiments have been suggested over the years. Recently, Beyer and Kleinpoppen (1983) introduced new scattering amplitudes related to contributions from predominantly the attractive and the repulsive parts of the electron-atom potential. The authors clarify their relation to the so-called natural amplitudes of Hermann and Hertel (1980) and pursue the ideas further for the example of 80 eV electron excitation of the He(2¹P) state. They demonstrate that this way of parametrising the data is directly related to the experimental observables, enables easy visualisation of the shape and dynamics of the charge cloud of the excited electron, and clarifies comparison between theory and experiment.

Total cross sections for the dissociative recombination of H₃⁺ and its isotopic variants are presented. Results for HD₂⁺ differ in form from those for the symmetric H₃⁺ and D₃⁺ ions, probably because of its different vibrational modes. The relevance of these cross sections to H^- formation is discussed.

The authors have measured the rotational envelopes of selected vibrational bands of H₂, D₂, N₂ and CO following excitation of cooled ( approximately 25K) gas beams. The observed broadening in N₂ and CO at low impact energies is attributed to a breakdown of the electric dipole rotational selection rule mod Delta J mod =1. No broadening is observed in H₂ or D₂.

A simple relation between hydrogen radial matrix elements (nl' mod r^k-1 mod nl) and (nl' mod r^-k-2 mod nl) (l'+l>or=k>or= mod l'-l mod ), previously obtained by Blanchard (1974), is derived more directly from the classical properties of generalised hypergeometric functions and discussed with reference to the SO(2,1) dynamical group of the hydrogen atom. This is an extension of a corresponding result for matrix elements diagonal in the angular momentum quantum number, first derived by Pasternack.

Relativistic g-Hartree equations are derived. Negative energy (Dirac sea) states enter. Their renormalised contributions can be interpreted as Casimir-like energy shifts in the total energy of atoms. The use of optimised g parameters leads to their unambiguous determination. The effect should be particularly important for large Z (Ze² to 1) and relatively small N, the number of electrons, if transitions between distant configurations, e.g. transitions from the ground state to Rydberg states, are considered. For simplified numerical computations approximate g-Hartree equations are proposed in which Dirac sea contributions are treated in a Thomas-Fermi version.

The relativistic Thomas-Fermi theory, with a finite nucleus, is used to study the variation of the chemical potential mu with atomic number Z and number of electrons N(N<or=Z). This is done by solving a suitable dimensionless form of the differential equation for the self-consistent field for the semiclassical radius of the positive ions. Integrating the relation mu =( delta E/ delta N)_Z the difference between the total energy of positive ions and that of the corresponding neutral atom has been obtained. The scaling predictions of Marconi and one of the authors are confirmed by their numerical calculations, since the dependence on the nuclear radius for any reasonable variation of this quantity is of negligible consequence. Finally, attention is given to bringing the first principles calculation of the relativistic Thomas-Fermi total energy of neutral atoms which they have carried out into contact with the Dirac-Fock results of Desclaux. To do so, it proves necessary to correct the energy contributions of the K- and L-shell electrons by means of the Sommerfeld formula, thereby generalising the non-relativistic boundary correction of Scott.

A scaled Thomas-Fermi potential including spin-orbit interaction and a core polarisation potential has been applied to determine a complete set of radiative lifetimes for all alkali-metal atoms up to the quantum number n=15. The oscillator strengths of the Cs and Rb principal series are presented up to n=25. Different forms of the electric dipole moment including core polarisation have been used. Cs transitions have been computed by a pure Thomas-Fermi potential without any polarisation terms to ascertain the influence of the spin-orbit interaction and the core polarisation in both the model potential and the transition matrix elements.

Photoabsorption by the subvalence s subshells of the heavy alkali elements K, Rb and Cs is reported. Observations are compared with ab initio calculations and it is shown that mixing of s-subshell excited configurations with doubly excited p-subshell configurations must be included to obtain a satisfactory explanation of the observed spectra.

Using the concept of the time-dependent physical spectrum introduced by Eberly and Wodkiewicz (1977) the authors develop the theoretical basis of time-resolved correlation spectroscopy. The corresponding measuring device consists of two coupled photodetectors with frequency resolving equipment in front of each. A formula is presented for the time-resolved correlation spectrum depending on two time and two frequency arguments. It may be used to gain insight into the time correlation of photons with given frequencies. The authors apply the theory to the resonance fluorescence radiation of a two-level atom irradiated monochromatically. It is shown that in the case of a weak irradiation with large detuning the photons are emitted in a definite time order into the two sidebands of the fluorescence triplet. Under strong resonant irradiation the photons show up bunching (antibunching) behaviour if they belong to different (equal) sidebands. In addition to these results which are already known the authors present new results concerning intermediate situations.

A theory of the changes in momenta suffered by the translational motion of a two-level atom due to interaction near resonance within uniform light, is presented. The leading changes in the translational energy are first derived and the changes in the momenta due to the irradiation are then deduced from these in a simple and transparent manner. The derivation depends on the inclusion of a quantum interaction term whose classical origin is the Rontgen current in the electric dipole approximation. Thus the theory presented differs considerably from conventional approaches which are normally based on considerations of the transition rates and do not include the effects of the Rontgen term. The momentum changes obtained can be expressed in terms of the susceptibility and its first derivative, and are shown to be independent of the state of the atomic translation. They depend, mainly in sign, on whether the atom is in its ground or excited state. These features lead to a simple derivation of the average momentum changes induced by the irradiation of an atomic system.

A theory appropriate for investigating the angular spectra of atoms scattered out of an atomic beam by resonance fluorescence due to interactions with an orthogonal uniform light beam is presented. Particular emphasis is made on the scattering in the plane containing the two beams. The atomic distribution is shown to be intimately related to the angular distribution of the spontaneously emitted photons. Both distributions show marked variations with the detuning and field intensity parameters, leading to the possibility of maximum scattering of atoms to occur on either side of the atomic beam, while the accompanying photons are always emitted on one particular side of the light beam. Since the angular distributions reflect the momentum changes suffered by both the atoms and the electromagnetic field, this lends insight into the basic mechanisms involved in resonance fluorescence.

Polarisation of photoelectrons produced from unpolarised alkali atoms is studied analytically for a wide range of the strengths and durations of the light pulse. In the particular case of long pulse durations, where the rate equation for the ionisation probability is approached, the photoelectron polarisation is shown to be a monotonically decreasing function of the pulse strength. In the case of weak pulses, usually dealt with by perturbation theory, the photoelectron polarisation is found to decrease continuously when the pulse duration is shortened. It is, however, found also that if neither the long pulse duration or the weak-field condition is fulfilled then an increase in pulse strength and a shortening of the pulse duration need not always affect the photoelectron polarisation destructively.

Light emission in the spectral region 1900-5000 AA due to the collision of CO⁺ ion beams with H₂ molecules was observed in the relative collision energy range 6-63 eV. Strong emission of Balmer lines was found in addition to CO⁺ comet-tail bands. The production mechanism of the excited hydrogen atoms which emit Balmer lines was examined with the pressure and energy dependences of the emission. The conclusion is that H₂ molecules in heavy particle collisions are excited into Rydberg states or doubly excited states and then dissociate into H(1s) and H(nl).

Translational energy spectroscopy, using a double-focusing mass spectrometer is used to study energy-loss/gain spectra for the single electron capture collisions of 6 keV N^2+. ions with atomic (He, Ne, Ar, Kr, Xe) and molecular (N₂, H₂) target gases. The product states are identified by energy defect analysis. With all collision pairs, reaction channels have been observed which indicate the presence of the long-lived metastable state of N^2+.(2p²⁴P₁2/) in the incident beam. Comparison with previous data has shown that the fraction of this metastable state is higher in the present data. The measured data show that the probability of electron capture is greatest where there is a crossing of the potential energy curves for the initial and final states which occurs at internuclear distances between 4 and 7 au. New data are presented for Kr, Xe and N₂ target gases.

Electron impact ionisation cross sections for Mg⁺, Ca⁺ and Sr⁺ have been calculated in the binary encounter approximation. The contributions of the excitation-autoionisation process have been included in the present work. Hartree-Fock velocity distributions for the bound electrons have been used throughout the calculations. The present calculations give a good account of the experimental observations.

Measurements of positron and electron-Ne total scattering cross sections are reported in the energy range 2-50 eV. e⁺-Ar total cross sections are presented in the energy range 2-20 eV. In both the e⁺-Ne and e⁺-Ar cases marked discrepancies are found with other recent measurements and these are discussed along with possible systematic errors which may affect the data. The e^--Ne measurements are found to be in excellent agreement with recent experiments.

For pt.I see ibid., vol.14, p.1525 (1981). Recent experimental observations of gains in the near XUV ( lambda approximately 182 AA) encourage the application of the recombination scheme to shorter wavelengths. The authors discuss model calculations of gain at 38.7 AA, in laser-irradiated ribbons, based on the Al XIII H_alpha transition. The calculations used a more accurate description of the atomic physics than the earlier study. The scaling of the optimum gain with the ribbon composition and initial width is examined. The modelling indicates that an experimental demonstration of significant gain at 38.7 AA is possible with current technology provided about 10% of the pump laser energy, can be coupled to the target. The simulations including the atomic physics used the self-similar description for spatially isothermal expansions to evaluate the development of the bulk properties of the target. The authors have examined the validity of this approximation using a two-dimensional fluid code assuming constant ionisation. They conclude that the targets are well 'burnt through' in the conditions required for an experimental demonstration of gain at 38.7 AA, and that the atomic calculations should be associated with ribbons having half the width originally assumed but the same specific energy at the end of the pumping pulse.