Table of contents

After the single-electron-capture collision, Al⁷⁺+H₂ to Al⁶⁺+H₂⁺ at 2.6 keV amu^-1, a relative measurement of Al VII transition intensities in the region 185< lambda <750 AA shows that the projectile captures the electron into the n=4 and n=5 energy levels of the Al VII ion. A multiconfiguration Dirac-Fock calculation of Al VII energy levels is used to interpret observed transitions.

Be^- has been predicted to have three bound states of which the 1s2s2p²⁶S₅6/⁰ decays to a multichannel continuum via relativistic autoionisation. Application of a state-specific theory of electron correlation, relativistic effects and interchannel coupling has yielded tau =8.3*10^-3 s for the lifetime of this level.

The photoionisation of the L_2,3 subshells of Ar and the N₅ subshell of Xe is considered in the vicinity of their thresholds. The profiles of the Auger lines are calculated taking into account the post collision interaction. A comparison with the available experimental and theoretical data is carried out.

A set of coordinates suitable for the three-body problem is introduced. In this case the three-body wavefunction possesses the remarkable property of being compatible with the physical boundary conditions in the adiabatic representation.

It is shown that inelastic collisions of atoms with unfilled shells are accompanied, in the adiabatic limit, by spin co-orientation of the outer electrons and the electrons from inner shells with vacancies. The co-orientation disappears as the energy increases due to the violation of the adiabatic approximation. The proposed approach explains the experimental data on the fluorescence yield near the threshold for K-vacancy creation in 'N⁺+target atom' and 'Ne⁺+target atom' collisions.

Electron capture between the ground states of highly charged ions travelling at relativistic velocities is investigated. By extending the non-relativistic continuum distorted-wave approximation to the relativistic regime the authors obtain results for total cross sections allowing for spin-flip transitions. These results are compared with first- and second-order plane-wave perturbation theory.

A phenomenological model is given to account for the decay of intermediate states produced by simultaneous transfer and excitation in fast ion-atom collisions. Conditions are examined for which vacancy production and decay occur independently.

It is demonstrated that for electron scattering on any atom the forward scattering amplitude possesses the power-type singularity when the energy of the incoming electron is equal to the atomic binding energy. A new dispersion relation for the amplitude of e-He scattering is proposed.

A theoretical analysis is given of the parameters M₁+M₃, M₂ and cos delta which describe the coherent excitation of the n=2 states of atomic hydrogen by intermediate-energy electrons. At small momentum transfers, where they have recently been measured by Back et al. (1984), the parameters M₁+M₃ and M₂ are dominated by first Born s-p amplitudes, but outside the small-angle region higher-order effects are shown to be very important. The parameter cos delta is also very sensitive to higher-order effects. Unitarised eikonal-Born series (UEBS) results are presented for M₁+M₃, M₂ and cos delta at incident electron energies of 100, 300 and 350 eV.

Excitation of the 4p⁵(²P₃2/)5s³P₁ and 4p⁵(²P₁2/)5s ¹P₁ states of krypton by electron impact has been studied using the electron-photon angular correlation technique. Measurements are reported at an incident electron energy of 60 eV for electron scattering angles theta _e up to 30 degrees . The serious disagreement between two previous measurements at theta _e=20 degrees is discussed. The difficulty of obtaining a quantitative measure of the importance of spin-orbit coupling through the parameter epsilon using this technique is demonstrated.

Angular correlation parameters lambda , chi and epsilon for the 5s(³/₂)₁⁰³P₁ and 5s'(¹/₂)₁⁰¹P₁ states of Kr have been measured with an electron-photon coincidence technique at incident electron energies of 40, 50, 60 and 80 eV. These parameters were determined from two independent angular correlation measurements. The results are compared with the previous experimental and theoretical results.

The spin-orbit interaction and the Darwin term have been included in the treatment of two-electron systems using hyperspherical coordinates. The example of a ¹S-³P mixture within the symmetry J^pi =0⁺ is worked out. Results of numerical calculations are reported and discussed.

The Slater transition-state concept within the relativistic local-density R Xi method is applied to the study of shake-up energies in the core photoelectron spectra of alkali atoms and rare gases. The intensities of the shake-up transitions are calculated using the sudden approximation. It is found that the shake-up energies are well described by the present method and the intensities are in agreement with the experimental values. In these shake-up processes, the relativistic effects are found to be very small but the relaxation effects are important. The present simple one-electron picture seems to be adequate for the interpretation of the core photoelectron spectra.

This paper is an extension, to ions of higher charge, of an earlier paper which describes a collisional-radiative calculation for hydrogen-like ions of nuclear charge up to about 15. In the same way as before the authors take separate account of the (n,l,j) sub-levels up to n=4 and of each principal level up to n=50 assuming that the sub-levels within each of these principal levels are relatively statistically populated. Besides the usual radiative and electron collision processes they include heavy-ion (of any mass and charge) collisions among the sub-levels. In treating ions of nuclear charge greater than 15 it has been necessary (compared with the earlier paper) to modify the treatment of heavy-ion collisions, to include the possibility of radiative decay of the 2S metastable level by the magnetic dipole transition and to include the contribution of the helium-like satellite lines that lie close in wavelength to the individual components of the 1S-2P (Lyman-alpha) line. As before the authors concentrate on presenting the relative intensities of these two components. The present results show that it was justified to neglect the additional processes in the previous calculation.

The ratio of the intensities of the Lyman alpha (n=2 to 1), Balmer alpha (n=3 to 2), Balmer beta (n=4 to 2) and Paschen alpha (n=4 to 3) fine-structure components of hydrogen-like ions emitted from both recombining (ground-state density to 0) and steady-state plasmas of atomic number Z=2, 6 and 12 is examined. Separate account is taken of the (n,l,j) sub-levels up to and including n=5. Ion-induced collisional transitions between sub-levels of the same principal quantum number are considered for the two cases of (i) impurity of atomic number Z emission from a plasma composed mainly of hydrogen and (ii) emission from a plasma composed of full stripped ions of atomic number Z. It is shown that the fine-structure ratios vary from the statistical values and are highly dependent on the electron density and on the ground-state density of the hydrogen-like ion for electron densities below approximately 3*10¹⁴ Z⁸ cm^-3 for Lyman alpha, 3*10¹³ Z⁸ cm^-3 for Balmer alpha and 3*10¹² Z⁸ cm^-3 for Balmer beta and Paschen alpha.

The recently developed shifted 1/N expansion technique, where N is the number of spatial dimensions, is applied to the study of the energy levels of the hydrogen atom in presence of intense laser radiation. The analytic expressions for the energy eigenvalues E_n,l yield very accurate results for a wide range of n,l and the intensity-dependent truncation parameter lambda appearing in the effective laser-dressed binding potential. It is found that the level ordering satisfies E_n,l>E_n,l' for l<l' and the energy eigenvalues scale down roughly as lambda ^-1 for large lambda . It is also observed that the convergence of the perturbation expansion improves remarkably with increasing intensity of the laser field. Finally the scope for extending the method to other areas of atomic physics is briefly discussed.

The transition probabilities of the two-photon excitation from the ground electronic state to some low-lying excited electronic states in water are obtained in the approximation of vertical transitions and assuming a random orientation for the assembly of molecules. The calculations are performed by extending to the molecular system the methods previously employed for atoms, i.e. by exploiting the results of the RPA to compute the one-electron transition density matrices of both the initial-intermediate and the intermediate-final states. The effect of coupling of the excitation channels on the transition probabilities is estimated by comparing the RPA results with those obtained in the static-exchange separated-channels approximation. For each final state considered, the transition rate for linearly polarised photons together with a ratio of transition probabilities for different polarisation situations are reported. These quantities allow one to compute the transition rate at each frequency for every possible combination of polarisation and geometrical arrangement of the sources. A comparison is made between the results obtained using the length and the velocity forms of the perturbing operator.

The differential cross section for elastic e^--H scattering in the presence of a laser field has been calculated in the framework of the first Born and Born-exchange approximations following Ferrante et al. (1982). The effect of exchange is found to be sensitive to the direction of the laser field. For A₀//k_i, the effect of exchange is most pronounced.

The total cross sections of methane, ethane, ethene, propane, propene, cyclopropane, n-butane, isobutane and 1-butene were measured in a transmission experiment utilising positrons from a ²²Na source and a tungsten moderator and electrons from a thermionic cathode. As expected the cross sections for positrons are smaller than those for electrons and both grow with the size of the molecules. Above 30 eV the positron and electron cross sections decrease with increasing energy and approach each other. All measured positron and electron cross sections between 100 and 400 eV can be described by a formula containing the energy, the number of molecular electrons and only three fit parameters.

In the application of a generalisation of the Kohn method to low-energy positron-hydrogen-molecule scattering which was described in a recent letter, up to 31 short-range correlation functions, made up of one-particle functions of sigma symmetry, were included in the trial function. In the calculation described in this paper, the flexibility of the trial function is greatly improved by the inclusion of up to 64 short-range correlation functions of which 32 contain products of one-particle functions of pi symmetry. The inclusion of the pi functions has a very big effect on the calculated results, especially at very low energies. The behaviour of the phaseshift values with increasing incident energy is qualitatively similar to the behaviour of the corresponding S-wave phaseshifts in low-energy positron-helium scattering. Comparison with experiment indicates that the results reproduce qualitatively the experimental trend at very low energies but higher partial waves must make a significant contribution to the total cross section above about 0.1 eV. The improved results can be attributed to the much more satisfactory description of the polarisation of the hydrogen molecule made possible by the inclusion of pi functions.

The authors study the classical motion of an electron moving in a Coulomb potential and a resonant and near-resonant electric field, but constrain the electron to move only in the direction of the field. This simple model of an excited hydrogen atom in a microwave field is simpler to understand than the three-dimensional system, yet retains some essential dynamical features. They show how the Coulomb singularity can be removed by regularisation in order to produce a Hamiltonian suited to efficient numerical computations. They obtain accurate estimates of the field strengths at which ionisation occurs and show that for resonant fields this is well estimated by the second-order resonance-overlap criterion. They show that the frequency dependence of the ionisation probability is similar to that of the classical three-dimensional system and show how its behaviour depends upon particular stable periodic orbits which persist for quite strong fields. They discuss the relevance of this model to the three-dimensional case and to quantal calculations.

The authors investigate simultaneously two non-relativistic scattering problems from a unifying point of view, namely: (a) scattering of light by a free electron; and (b) scattering of electrons by a screened Coulomb potential. In both cases the electrons involved in the scattering process are embedded in an intense microwave field F(g) and in a strong homogeneous magnetic field B. Therefore these particles have to be described by exact field-dressed states. On the other hand, the basic scattering processes (a) and (b) will be treated to lowest order of perturbation theory. In particular the authors shall investigate the non-linear processes induced by the strong external fields in the classical limit of highly excited quantum states. In this case they shall be able to derive compact analytic expressions for the transition amplitudes and cross sections.

The theory of polarised light phase conjugation (PC) in non-degenerate four-wave mixing in a resonant medium has been developed in the irreducible tensorial formalism. The theory is applied to a wave with any polarisation in a resonant medium with levels of arbitrary momentum in the absence of pump-induced saturation effects. The formula for the non-linear susceptibility of the medium has been found and it is shown that the change of the polarisation at PC is essentially connected with the relaxation of the atomic multipole momenta (population, orientation and alignment). Wave equations were investigated for different pump-wave polarisations and three different physical mechanisms of the PC are discussed. The influence of the different relaxation rates of the atomic multipole momenta on the PC reflectivity has been investigated. The spectral dependence of the reflectivity and the spectral ranges which depend on the wave polarisation are found. For motionless atoms with small momenta (J₁=¹/₂ to J₂=¹/₂, J₁=1 to J₂=0), exact formulae for the non-linear susceptibility of the medium as a function of pump intensity were found. The spontaneous decay of higher level to the lower one was taken into account. Because of optical pumping, saturation effects can be shown at considerably smaller intensities than in the usual two-level system. The exact expressions for the PC reflectivity at different polarisations of the pump waves are given.

The steady-state population distribution of a four-level system resonantly excited by three laser fields of arbitrary strength was studied using density matrix formalism. The conditions for obtaining maximum population in the highest level as well as for population inversion between the highest and the lowest levels of the system are discussed.

The diffusion coefficient D of sodium in neon has been measured at 530K. In a cylindrical discharge tube filled with neon at densities N of 10²³ to 10²⁴ m^-3 a strong radial sodium density gradient is induced by means of a discharge current. In the afterglow of the discharge the decay rate of the fundamental diffusion mode is measured using dye laser absorption at the sodium D1 line. The value DN=(1.43+or-0.18)*10²¹ m^-1 s^-1 is found. The diffusion coefficient is calculated for temperatures between 300 and 600K using Lennard-Jones (n-6) interaction potentials with n=12, 8 and 7, and using a Born-Mayer potential. Potential parameters are taken from the literature. Best agreement with experiment is found for the Born-Mayer potential and for the Lennard-Jones (7-6) potential.