Table of contents

Using two-step optogalvanic spectroscopy via the intermediate 5d6p ¹P₁⁰ level, the position of the 6p²¹S₀ level has been identified to a broad profile ( approximately 300 cm^-1) about 44800 cm^-1. An MQDT analysis of the J=0 spectrum including mixing with the 6s epsilon s ¹S₀ continuum has been performed. The predicted relative oscillator strength densities between 44000 and 45400 cm^-1 are in fairly close agreement with the experimental data. The Fano profiles of the 5d_5/28d_5/2, 5d_3/2 9d_3/2 J=0 resonance, the broad 6p²¹S₀ profile and the hole profile for the 5d_3/2 10d_3/2 resonance are explained.

The XUV spectrum of a laser-generated xenon plasma is found to be dominated by intense 4d to 4f transitions which appear both in emission and absorption for a range of ion stages. Term-dependent Hartree-Fock calculations were performed to enable the classification of the strongest of these and to study the behaviour of 4f electronic states in xenon over a number of ion stages. With increasing ionisation the 4f eigenfunction is seen to contract from (r)=17.9 au in Xe I to 0.78 au in Xe XVII. Evidence for the possible non-radiative decay of some of these states is also discussed.

A two-level model with inverse-power interactions (the Callaway-Bartling model) is considered. The magnitude of the excitation cross section is strongly dependent upon whether or not the model contains a potential energy curve crossing. It is shown that the second-order Magnus approximation represents a useful, non-perturbative approach toward such problems.

A numerical treatment of the hyper-Raman pulse equations confirms that ringing may occur in the Stokes pulse.

The hyperfine structure of the 6s14d configuration of barium has been measured by optical-optical double resonance. From the observed positions for the hyperfine components of the ¹D₂ and ³D_1,2,3 Rydberg states, the singlet and triplet character as well as the amount of admixture of the 5d7d perturbing configuration was deduced. Although agreeing qualitatively, the authors' results differ quantitatively from predictions made by multichannel quantum defect theory (MQDT).

The electronic spectrum of the CuF molecule has been photographed at moderate resolution in absorption and some regions have been studied in emission, using spectra excited at reduced rotational temperature in a composite wall hollow cathode discharge. Three new systems, all with the ground state as lower state, have been discovered: their upper states are a ³ Sigma ⁺, A(0⁺) and D( Omega =1). Constants for ⁶³Cu¹⁹F are given.

For pt.I see ibid., vol.15, p.3801 (1982). Following ab initio calculations, a model has been developed that accounts for all the observed excited states of the CuF molecule and assigns them to a single structure: Cu⁺(3d⁹4s)F^-(2p⁶). Thus the A complex is interpreted as components Omega =0(e) and Omega =1(e and f) of the ³ Pi state of this structure while the D state is shown to be the Omega =1 component of the ³ Delta state. It follows that the CuF bands arise from 3d to 4s transitions forbidden in the atomic limit. Deperturbation of the electronic term values with respect to spin-orbit interaction has been carried out assuming pure precession and some restrictions on the parameters. From this it has been possible to predict fairly accurately the location of the components which have not yet been observed.

Laser-excited fluorescence spectra of Bi₂ have been studied both by high-resolution Fourier transform spectrometry and by conventional grating spectroscopy. The line Ar⁺ 5145 AA excites extensive series in the system A O_u⁺-X¹ Sigma _g¹. Ultraviolet lines of both argon-ion and krypton-ion lasers excite a new transition, V O_u⁺-X¹ Sigma _g⁺, and V O_u⁺; radiates both to X¹ Sigma _g⁺ and to a new state B'O_g⁺. Levels of the ground state X¹ Sigma _g⁺ have been characterised in the range 0<or= nu <or=105. Evidence is given to show that B' is the 0_g⁺ component of the ⁵ Sigma _g⁺ state formed from ground-state atoms, Bi(⁴S₃2/^o)+Bi(⁴S₃2/^o). Constants for the new states are given.

Absolute velocity-resolved total integral cross sections have been measured for collisions of helium singlet (2¹S₀) and triplet (2³S₁2) metastable (He*) atoms with ground-state helium, neon, and argon atoms in the thermal velocity range of 1.0-3.5*10⁵ cm s^-1. Additional measurements on the He*-Ne system with a large input aperture at the detector to suppress the sensitivity of the apparatus to elastic scattering failed to show the previously predicted (Arathoon) sharply rising velocity structure in the inelastic excitation transfer cross sections.

Argon recoil ions were produced by heavy-ion collisions and the argon K X-rays and Auger electrons emitted were measured with a crystal spectrometer and an electrostatic electron-energy analyser. Ions of Kr, Xr, Pb and U of 2.5-9.0 MeV amu^-1 specific energy were used to bombard argon gas. This is the first systematic study of resolved K X-ray radiation and for K-Auger electrons of argon where the nuclear charge of the projectiles is larger than the nuclear charge of the target atoms (Z₁>Z₂). In these collision systems one-, two-, and three-electron spectra are observed which have been analysed in detail. From the kinematic broadening of Auger electron lines of Ar¹⁵⁺ the authors deduce a mean recoil energy of E_R=21+or-3 eV. A drastic charge state effect as well as the systematics in the K alpha satellites for primary charges 26<or=q₁<or=66 lead to a universal scaling of the L-hole probability p_L as a function of projectile charge and velocity.

Transitions induced by rotation of the internuclear axis in close collisions of atomic particles at low energies are investigated within the one-electron united-atom approximation. The three-state problem ( sigma to pi , delta transitions) is treated in more detail. Effects arising as a consequence of a different choice of classical trajectories of the nuclei are demonstrated in the example of 3d sigma to 3d pi , 3d delta transitions, contributing to the Li³⁺+H to Li²⁺+H⁺ charge-exchange reaction.

Taking particle-atom ionising collisions in the presence of a laser field as an example the authors investigate how the properties of real lasers modify the results obtained within models where the laser is treated as a homogeneous single mode in the dipole approximation. The following properties are considered: (i) the spatial inhomogeneity of the laser beam; (ii) the temporal inhomogeneity as represented by two different envelope functions; (iii) the multimode structure in the simplest case of zero laser bandwidth. Significant modifications are found, especially for moderate and strong fields; however neither the inhomogeneities, nor the multimode structure cancel or drastically alter the predictions made on the basis of the homogeneous, single mode, laser. Generally, the inhomogeneities reduce the probabilities of processes with large numbers of exchanged photons, considerably enhancing collision processes with no photon exchange. On the other hand, a multimode laser field modifies the results in a largely different, almost opposite, way compared with fields displaying inhomogeneity properties. Detailed calculations are reported and commented together with the available literature.

An expansion used in electron collision theory is Psi = Sigma _i Theta _i+ Sigma _j Phi _jC_j, where the Theta _i are products of target functions times radial functions for the colliding electron and are referred to as free channels, while the Phi _j have the form of bound-state functions for the whole system and are referred to as bound channels. Previous formulations of quantum defect theory have used expansions in which the bound channels are omitted. It is shown that inclusion of bound channels does not alter the formal structure of the theory but may lead to rapid variations with energy of quantities which would otherwise be slowly varying. For the case of one free channel and a number of bound channels the dependence of the quantum defect mu on the energy E is given by tan( pi mu (E))= alpha - Sigma _j beta _j/(E- Delta _j) where alpha , beta _j and Delta _j are slowly varying functions of E. It is shown that experimental quantum defects for the ¹S and ¹D series in Ca I can be fitted to expressions of this type, with one bound channel for ¹S and three bound channels for ¹D.

A systematic study is made of the unitarisation method which the authors recently proposed for the eikonal-Born series approximation, in the case of fast electron-atom and positron-atom collisions. The authors' unitarisation procedure is based on a generalisation of the potential scattering eikonal expansion of Wallace to the multiparticle case, suitably modified to take the long-range polarisation effects and absorption effects at small angles into account correctly. They also develop a new non-perturbative approximation for the exchange scattering amplitude, which is appropriate to use with the authors' unitarised EBS direct scattering amplitude for electron-atom collisions. The authors' then apply their unitarised EBS method to the elastic scattering of electrons and positrons by atomic hydrogen in the energy range 100-400 eV. A detailed comparison is made with other related theoretical methods and with recent absolute measurements of (e^--H) elastic differential cross sections. Total (integrated) elastic and total (complete) cross sections for both electron and positron impact are also discussed.

A comparative study of the excitation of Li(3s) by electron impact in several distorted-wave approximations is made. These include the distorted-wave first and second Born approximations with both first- and second-order distortion in initial and final states. Second-order effects representing both target and projectile distortion are found to give a large correction to the first-order DWBA results. The predictions of the authors' full second-order model are compared with absolute measurements of the differential cross section in the energy range 10-60 eV. Calculations have also been made in the plane-wave second Born approximation, and the results show the failure of a perturbative approach based on the plane-wave Born series at intermediate energies.

Electron impact excitation of the (4d⁹5s²5p) 12.81 eV autoionising level of cadmium vapour atoms has been investigated for an incident electron beam kinetic energy of 150 eV. The angular distribution of electrons inelastically scattered through angles of up to 80 degrees and the coincidence angular distribution of autoionised electrons for a set of scattering angles between 7 and 24 degrees have been measured using a microcomputer-controlled coincidence spectrometer. The data are compared with the DWBA calculations of Balashov et al. (1982). Good agreement is obtained for scattering angles of less than 20 degrees ; thereafter both sets of data diverge from theory, possibly due to electron-exchange effects.

The authors present distorted-wave cross sections for excitation of the A ¹ Pi , a ³ Pi , a' ³ Sigma ⁺, D ¹ Delta and d ³ Delta states of CO by electrons in the 20 to 50 eV energy range. In these studies both the initial and final distorted waves are obtained in the static-exchange field of the ground electronic state. Differential and integral cross sections are presented and compared with available experimental data and with other calculations. The calculated differential cross sections for the A ¹ Pi and a ³ Pi states agree poorly in magnitude, but better in shape, with available experimental data. In general the distorted-wave cross sections differ substantially from those of plane-wave-type theories such as the Born-Ochkur-Rudge approximation.

Orientation averaging is performed by exact orientation averaging (EOA_kappa ) and four approximate methods (two well known and two new) and expressions for the double differential scattering cross sections of thermal neutrons on water molecules are developed. In general, the results obtained by the newly proposed approximate orientation averaging method agree best with the corresponding results obtained by EOA_kappa . The biggest discrepancies between the EOA_kappa results and results of the approximate methods are obtained using the well known approximate Krieger-Nelkin averaging.

A three-level system driven by two coherent fields on two-photon resonance leads to the trapping of population in a superposition of initial and final states which is immune to further photoexcitation, manifesting itself in a narrow coherence minimum in the absorption spectrum. The authors analyse the effects of laser field bandwidths and cross-correlations on these phenomena. They find that the inclusion of laser bandwidths dephases atomic coherences which leads to the destruction of population trapping and of the narrow coherence minimum in the absorption spectrum. When the driving fields are not cross-correlated, the atomic coherences are similarly dephased in both lambda and ladder systems. If, however, the driving fields are critically cross-correlated, two-photon coherences in the lambda system are unaffected by laser fluctuations; trapping and the coherence minimum persist. For the ladder system, on the other hand, the cross-correlation does not restore the atomic coherences involved.

A quantum theory appropriate to describe the cooperative decay of an ensemble of sources having different resonance frequencies is applied to the superradiant emission of two equally populated groups of detuned sources. A set of two coupled equations is derived, which fully describe the behaviour of this system. Analytical or numerical solutions of these equations are presented as a function of the detuning and the differences between the quantum and the semiclassical behaviour are discussed.

Density corrections to the mobility and dispersion law for electrons in the medium of disordered short-range scatterers are obtained using the green's function method. The results of calculations are in good agreement with experimental data on electron mobilities in dense gases with low polarisabilities.