Table of contents

The J. Phys. B: At. Mol. Opt. Phys. plays an important and prominent role in the atomic, molecular and optical physics community around the world. This special role is a consequence of many factors. The primary reason is, of course, the sheer excellence of the scholarship that is published within its pages, a scholarship which has a particularly strong base in Europe, to the great benefit of the Journal and which is manifest in the contributors, the Editorial Board and our distinguished past Honorary Editors. This combination has led to people in the atomic, molecular and optical community across the globe seeing publication in J. Phys. B: At. Mol. Opt. Phys. as an excellent way to communicate their research to a broad and influential readership.

In taking up the Honorary Editorship, I am most conscious of the importance of the Journal to our community. I very much hope, therefore, that I can approach the success of our past Editor in helping to keep the Journal on the sound path it has taken to-date.

We have, of course, entered into a time when the technological side of publishing will be changing rapidly. This is a great opportunity to make the scholarship in these pages more readily and speedily available to you - the reader - and details of online access to the Journal are printed overleaf. I am also very much aware of developments that continue within our field. I am sure, however, that the principles that make J. Phys. B such an important journal today will enable it to exploit these opportunities in the very best fashion for the atomic, molecular and optical community in the future.

K Burnett

The formula due to Trees for calculating the matrix elements of two-electron operators within the configuration has been generalized from to other groups. This is done to account for regularities appearing in the partial sums over parents when the Trees formula is applied to spin - spin and spin - other-orbit interactions within the f shell. Several examples are described.

A realistic three-parameter correlated wavefunction is used to construct an exact analytical expression for the expectation value of the helium-atom Hamiltonian expressed in the interparticle coordinates. The parameters determined variationally are found to satisfy the orbital and correlation cusp conditions to a fair degree of accuracy and yield a value for the ground-state energy which is in good agreement with the exact result.

It is demonstrated that the oscillator strength of the resonant inner-shell excitation in a noble gas atom is considerably smaller than that in its alkali neighbour because the effective charge acting upon the excited electron in the alkali is much larger. With an increase of the excitation's principal quantum number, the difference between line intensities in noble gases and their alkali neighbours rapidly disappears. The calculations are performed in the hydrogenic and in the generalized random-phase approximation with exchange including rearrangement effects due to inner vacancy creation and Auger decay. It is shown how the change of nuclear screening can manipulate the inner-electron discrete excitations.

Results of an argon e, (3 - 1)e double-ionization experiment with an incident energy of 5.5 keV are presented. The two emitted electrons, with energies of 96 and 17.5 eV, are observed in coincidence, and several angular distributions were measured simultaneously using a dual-position multidetection system. The observations provide clear indication for a shake-off mechanism being responsible of the double-ionization process under the kinematics considered.

We present a study of tunable high-order sum and difference frequency mixing in the 7 - 70 eV range, in xenon, argon and neon. We mix the light from a terawatt, sub-picosecond titanium - sapphire laser and that from a tunable optical parametric generator. The radiation generated through mixing processes involves absorption or emission of one or two photons from the weak tunable source. The relative strengths of the sum and difference frequency processes depend strongly on the wavelength range of the generated radiation. The sum frequency processes are stronger at low photon energies in xenon, whereas the difference frequency processes dominate at high photon energies, in neon, being almost comparable to harmonic generation.

We have used photographic spectroscopy of a beam - foil source to measure the vacuum ultraviolet 1s2s - 1s2p transition wavelengths in helium-like sulphur. The results are Å and Å, respectively. These measurements, together with our earlier measurements in helium-like silicon and phosphorus, are compared with other precision measurements for Z = 14 to 18, and with recent relativistic and quantum electrodynamic calculations.

A theoretical and experimental study of the behaviour of the correlation satellites arising during 5s photoionization of xenon is presented. Many-body perturbation theory and configuration-interaction techniques have been applied to calculate the wavefunctions of the Xe II ionic states. An expression for the angular distribution parameter of the photoelectrons taking into account final-ionic-state configuration interaction is derived. Photoionization cross sections and angular distribution parameters were calculated for the 5s main line and the majority of the satellite lines and compared with our high-resolution measurements and earlier lower resolution measurements.

The differences in the angular distribution parameter dependence on the photon energy for the 5s main line and satellites were analysed in terms of their origin. The most important mechanisms are: interference of several photoionization channels characterizing different orbital momenta of the photoelectron, the mixture of terms with different total orbital momenta in the final ionic state, and the dependence of the photoelectron wavefunctions on the total momentum of the photoelectrons.

A method to make ab initio predictions of electronic autoionization rates in atoms and molecules by use of many-body theory in the discrete basis set approximation is presented. The method is applied to the autoionizing states and in He and Ne, respectively, and to the state in the HF molecule. The many-body expansion is carried through the third order in the electronic interaction, which means that the leading correlation terms are included. Correlation effects were found to be important in all the three cases investigated, and very good agreement with experiment was obtained for He and Ne where experimental values are available.

Absolute values for the energies, natural halfwidths and excitation cross sections for the Kr I resonances and absolute partial widths for the spectator Auger, participator Auger and Auger shake decays of the Kr I and the Kr I resonances into Kr II states have been collected and critically discussed or have been extracted from already published relative literature data. By means of photon-induced fluorescence spectroscopy a cascade population of the Kr II state following the excitation of the Kr I autoionizing resonances was observed and quantified. It is ascribed to a cascade process involving a correlative radiative transition of the type within Kr II. The probability of this process is explained within a very simple final-state configuration interaction calculation.

Autoionizing 4fng J = 0 - 3 levels of barium in the energy range 88 240 - 90 460 have been studied, both experimentally and theoretically. Excitation spectra, recorded in a three-step pulsed laser experiment based on an isolated-core excitation scheme, are confronted with eigenchannel R-matrix calculations. Series converging to the upper and lower thresholds are found to interact strongly, in particular for J = 1. As a result the observed 4fng resonances vary widely in width, shape and intensity. Although the complete R-matrix calculations involve many interacting channels for each J (up to 49 for J = 3), good agreement is found between experiment and theory and all observed resonances can be unambiguously assigned.

We have measured the energy dependence of the dynamical spin-polarization parameters for photoelectrons from zinc atoms in the wavelength region from 100 nm to 115 nm. The pronounced structure near the resonances found in previously measured absorption spectra also occurs in the spin-polarization parameters. From the experimental results the dipole transition matrix elements including the relative phase shift difference have been determined. The data are also compared with an existing R-matrix calculation and a new calculation employing semi-empirical Slater - Condon parameters.

The exact solving of the continuum radial wave equation for the one-electron two-centre problem is achieved through the characteristic exponent parameter which is determined in a rigorous way by a simple mathematical method. Its derivative with respect to the intercentre distance R involved in matrix elements calculations between discrete and continuum states is also given. Illustrative results are presented for several states of , for up to 10 and for and momentum K = 1 au. As an example, the calculated expansion coefficients of the radial continuum wavefunctions expressed in series of the Legendre functions of the first kind with separated exponent are given for the state of . Representative curves of the separation constant as well as the characteristic exponent are displayed.

Ionization and capture of an electron from elliptic H(n = 4) states in collisions with protons have been studied by means of close-coupling calculations for relative velocities in the range of 1.0 to 3.0. Within this velocity range we show that ionization and capture cross sections behave oppositely to each other as either the eccentricity, e, or the orientation angle, , between the beam and the classical plane of the elliptic orbital is varied. Thus the total electron loss is shown to depend only weakly on e and . Also the dependence of the electron loss cross section on the alignment angle, , between the major axis of the elliptic state and the beam axis is studied. The theoretical calculations for the dependence of the relative capture cross sections on e and are shown to be in good agreement with experimental observations for collisions at the same relative collision velocities. We suggest that the dependence of the capture cross section as well as the dependence of the ionization process on e, and should be measured.

A semiclassical close-coupling equation is formulated for the electron-capture process in ion - atom collisions under the influence of an external magnetic field. The direction of the magnetic field is chosen to be parallel to an initial velocity vector of a projectile. As we deal with the case for the magnetic field strength less than the critical field strength , the second- and higher-order terms for the magnetic field are neglected. Eigenfunctions in zero magnetic field are employed as a basis set for expansion. The equation obtained is the same in structure as that at zero magnetic field except that the relative velocity v is replaced by the scaled velocity . Effects due to the magnetic field are: (i) dynamical couplings, similar to rotational couplings and (ii) the Zeeman splitting of energies. The phase factor due to the gauge transformation is similar in form to the electron-translation factor. We apply this close-coupling equation to a study of electron-capture processes in a singlet system at , for which the zero-field results have been reported earlier (Shimakura et al 1993). Calculated results show that the total electron-capture cross section increases by about 30% at the magnetic field T compared to that at zero magnetic field at collision energy E = 122.5 eV/amu. At low collision energies below 500 eV/amu, the effect of the magnetic field becomes dominant over that of rotational couplings at . Because the magnetic field removes the degeneracy of electronic energies for (and ) states by the Zeeman effect, transition probabilities for and (and also and ) states differ significantly. The transition probabilities oscillate with time even if the collision pair separates considerably, particularly for and states because of the magnetic field.

The velocity dependence of state-to-state integral cross sections for electron transfer and excitation in collisions is studied in the 0.05 - 0.3 au velocity range using the impact parameter semi-classical method and a 28-state molecular orbital basis model including a common translation factor. The initial orbital alignment dependence of electron transfer is in fair agreement with recent experiments and with atomic orbital model calculations. The main electron transfer channel from is to the states. The integral cross sections from an aligned or oriented state to an aligned or oriented state and vice versa and the corresponding alignment and orientation parameters are presented as a function of the impact velocity.

This paper reports experimental results for the influence of target excitation and orbital alignment on the electron transfer process for the collision. The impact energy range is 1 - 45 keV. The total cross section parameters are found to depend sensitively on collision velocity. Agreement is good with theoretical predictions based on the 26-state AO model of Nielsen et al, and the 28-state MO model of Courbin and Machholm, except at the highest energies.

The recent double-ionization experiments (e, 3 - 1e) on argon by electron impact are, for the first time, investigated theoretically. Simple qualitative models and more elaborate theoretical calculations are proposed and discussed in order to understand the mechanism of double ionization at high electron impact.

The triply differential cross section (TDCS) for electron-impact ionization of atoms is analysed in terms of tensorial parameters representing orientation and alignment in electron - electron coincidences. Particular attention has been paid to excited alkali states because of experimental feasibility.

Three dynamical models for the calculation of the TDCS have been considered. The plane-wave impulse approximation (PWIA), the first Born approximation (FBA), and the use of a correlated double continuum wavefunction as employed by Brauner et al, hereafter referred to as BBK. It is shown that the PWIA forbids any orientational effect, but allows for an alignment. The FBA generally shows an orientational dichroism, the structure of the TDCS within the FBA has been worked out for non-statistical M-state populations. Finally, final-state correlation is shown to modify drastically the FBA results even at intermediate to higher energies. Numerical calculations for H(2p) are presented for the purpose of illustration.

Detailed calculations are made using the Kohn method of positron scattering by the hydrogen molecular ion below the positronium formation threshold at 9.45 eV. Phase shifts from the two-centre Coulomb value are obtained for the lowest partial wave of symmetry using a very flexible trial function containing a large number of short-range correlation functions. The convergence of the results with respect to both the linear and non-linear parameters is explored.

The calculation of the energy absorption in a strong radiation field where the electron quiver energy is much larger than the thermal or photon energies is performed using the Born approximation for the collision. The multiphoton sums are evaluated with the aid of asymptotic forms to yield analytic expressions. Comparison with earlier results from the impact (or classical) model shows disagreements. The origin to this difference is ascribed to the breakdown of perturbation theory in this limit, and it is concluded that the impact approximation is more reliable at very high fields.

In continuation of our earlier work on binary mixtures (1992 J. Phys. B: At. Mol. Opt. Phys. 25 679), an empirical relationship to estimate the higher-order contributions to thermal conductivity of dilute monatomic multicomponent gas mixtures has been developed. This relationship is able to reproduce the exact contributions to within 0.2%. Finally, a method of estimating accurate values of thermal conductivity is outlined.

The following misprints in the above paper should be corrected:

Furthermore, the statement that a `perfect' experiment can be performed without observation of the projectile spin after the scattering process is incorrect. The equations for the relative phases of the scattering amplitudes are linearly dependent. As a result, the phase difference v₁ - v₂ cannot be extracted from a generalized Stokes parameter measurement alone. A way to determine all relative phases unambiguously is to combine the information from generalized Stokes parameters with that from generalized STU parameters. For example, linear combinations of the form and allow for a unique determination of the complex numbers and β₁β_2e^i(v1-v2), thereby providing the crucial phase difference v₁ - v₂. Details of the procedure can be found in Andersen et al (1996).

References

Andersen N, Bartschat K, Broad J T and Hertel I V 1996 Phys. Rep. in press