Table of contents

The hyperfine structure of 6snd ³D₂ (11<or=n<or=27) Rydberg states of barium is reported. Between n=19 and 24 the hyperfine structure is dominated by perturbations due to the near-degenerate 6snd ³D₁ fine structure component, masking the resonance in singlet-triplet mixing. The hyperfine structure can be analysed quantitatively to yield the singlet-triplet mixing parameter for the ³D₂ Rydberg series, provided the fine structure splittings of the ³D states are known. Good agreement between the singlet-triplet mixing parameters of the ¹D₂ and ³D₂ series was observed.

A simple cross section formula for the impact ionisation of atomic hydrogen, analytic in both the projectile charge state (q) and speed (v= beta c), is presented. The formula rigorously approaches the Bethe asymptotic cross section at high speeds, and accurately summarises the recent data of Shah and Gilbody (1982) for all highly-stripped (q>¹/₂Z) ions with E/A>or=30 keV amu^-1.

Transfer ionisation processes are studied for the N⁷⁺+He and O⁷⁺+He systems by means of ion beam spectroscopy at 7 keV. The autoionising states (3l3l') of N⁵⁺ and (1s3l3l') of O⁵⁺ are identified.

In theoretical studies of potential scattering of beam electrons in the presence of a magnetic field it is necessary to obtain wavefunctions inside the magnetic field which satisfy the free-wave boundary condition outside the magnetic field. An explicit solution having this property is constructed using the eigenfunctions inside the field and assuming that the field vanishes sufficiently rapidly across the boundary at any finite distance. The result is independent of the distance and the geometry of the boundary. Further, it shows how the preparation of the initial states of a given energy inside the field may be controlled by adjusting the momentum of the incident beam.

An electron transmission experiment is used to study the low-energy total cross sections in Na, K, Rb, Cs and Hg. A shape resonance is observed in Na at 0.08+or-0.02 eV and in Hg at 0.42+or-0.03 eV. In Rb evidence is found for the existence of a shape resonance below 0.05 eV. The results are compared with the available experimental results and theoretical predictions.

The connection of the asymptotic form recently obtained by Altick (1980, 1982) with earlier works on this problem has been considered. It is shown that the Altick asymptotic form represents the partial-wave expansion of the plane-wave asymptotic form.

Attention is drawn to collisional ion-ion recombination X⁺+Y^-+X⁺ (or Y^-) to XY+X⁺ (or Y^-) which can be important when the ion density n_i is very high. If, for example, X⁺ is Kr⁺ and Y^- is F^- the recombination coefficient alpha _i is 3.0*10^-7 (300/ theta )^4.5(n_i/10¹⁵) cm³ s^-1 where theta is the temperature.

Relations between magnetic radial integrals are derived and used in the discussion of symmetry properties of mutual spin-orbit matrix elements.

X-ray spectra of foil-excited fluorine and silicon ions at tandem energies have been observed with a curved-crystal spectrometer. The higher spectral resolution compared with previous beam-foil work on F has permitted the identification of satellite lines to transitions in F IX and F VIII arising from F⁴⁺-F⁷⁺ core-excited ions. In addition, the K_{alpha alpha} ^h two-electron one-photon transitions of F and n,n'=2,2 to 1,3 shake-up transitions of F and Si have been observed.

The quasi-classical approximation is used to investigate the dynamics of a Rydberg atom in the field of intense electromagnetic radiation whose frequency is close to that of an atomic transition. The conventionally periodic four-frequency classical motion of an atomic electron along the invariant tori arising due to the radiation is described analytically. The applicability conditions of the theory developed are discussed.

The vibration-rotation spectra of nine H₂O bands obtained experimentally and falling in the region from 5300 to 6050 AA are analysed by applying Watson's Hamiltonian for asymmetric semirigid rotors together with the study of their subband structures. These bands are: 104, 123, 203, 222, 302, 321, 401, 411 and 500. The quantum assignments are further confirmed using frequency combination differences and the theoretical rigid-rotor intensities.

Superfluorescent emission from carbon tetrafluoride was generated by a single-mode CO₂ TEA laser whose frequency was swept over 30 MHz of the 9R(12) laser line centre. The results confirm the spectroscopic identification of the CF₄ pump transition while the dependence of the threshold pump energy on the exact frequency of the CO₂ laser is found to agree with the theoretical estimates.

The complex poles of the S matrix are determined by solving the Siegert eigenvalue problem using a variational method. Virtual states as well as resonances have been calculated. A boundary condition at a finite distance R is introduced, as proposed by Siegert. This requires that the potential be truncated at r=R. The error due to the fact that the potential does not vanish for r>R is corrected by a perturbative method. An analysis of the cut-off problem in the complex momentum plane is given.

The technique of excited-state degenerate four-wave mixing, ESDFWM, has been used to monitor the absorption of light at two wavelengths in the extreme wings of the pressure broadened Na D lines. The relative slopes of the ESDFWM signals as a function of pressure of rare gases He, Ne and Ar are compared with predictions of the quasistatic theory of line broadening using theoretical interatomic potentials.

Absolute absorption coefficients for the Ba resonance line at 5535 AA perturbed by rare gases below one atmosphere were measured between 1050 and 1150K in the spectral range of about 1 to 100 cm^-1 on both sides of the line centre. The difference interaction potentials Delta V(r) between Ba and rare-gas atoms were reduced using two line profile theories; the unified Franck-Condon (UFC) method of Szudy and Baylis (1975), and the auto-correlational method of Kielkopf (1982). As in the authors' previous measurement on Sr, for the heavy rare gases Xe, Kr and Ar, Delta V(r) has shallow wells with depths 50, 30 and 18 cm^-1 respectively at an interatomic distance of r=6.2 AA. For light rare gases Ne and He, on the other hand, Delta V(r) has a sharp positive peak with heights 70 and 80 cm^-1 respectively between r=4 and 4.5 AA. However, part of the profiles were not well explained by the present analyses, and the Delta V(r) deduced should be further refined in the future.

An experiment is described for measuring differential cross sections of Li⁺ ions scattered by Na atoms at laboratory energies between 25 and 134 eV. Pronounced structure in the elastic cross sections is seen arising from rotational 2 Sigma -1 Pi interactions. The data are compared with a consistent 'quantum' calculation that includes the three lowest molecular states and is based on theoretical potentials and matrix elements taken from the literature. Details of the calculation are given in an accompanying paper. Good agreement is found between experiment and theory.

For pt.I see ibid., vol.15, p.3695 (1982). Differential cross sections for elastic Li⁺-Na collisions in the energy range 17<E_CM<103 eV have been calculated from theoretical potentials and couplings by combining the impact parameter method with (a) a semiclassical and (b) a quantum mechanical cross section formula. Results of both calculations deviate significantly from each other. The quantum result is discussed in terms of an analytic model showing the general features of the specific case. The semiclassical formula is shown to be inapplicable to scattering at low energies (below say 200 eV) for systems exhibiting large inelastic cross sections. In an accompanying paper the quantum result is demonstrated to agree well with experiment.

The mechanism for atomic recoil excitation caused by strongly interacting nuclei is analysed using the quantum impulse approximation. The energy and momentum transfer between an active atomic electron and the colliding nuclei is considered self-consistently. A unitary operator that transforms wavefunctions and observables with a change of relative coordinates is presented to aid in the evaluation of transition matrix elements. This treatment provides a common framework for analyses of atomic recoil excitation for nuclear collisions ranging from Coulomb scattering to resonance reactions.

The angular distributions of ejected electrons in the energy range 5-70 eV have been measured at angles from 30 to 150 degrees for 20 keV He⁺ impact on He. The angular dependence of excitation cross sections of autoionisation states 2s²¹S and 2p²¹D+2s2p¹P are in good agreement with previous data measured by Bordenave-Montesquieu et al. (1982). The continuous parts of the electron spectra show symmetrical angular distributions around 90 degrees in the laboratory frame for low-energy electrons (<or=30 eV). These angular distributions are discussed in connection with the molecular autoionisation mechanism.

Angular distributions of autoionisation electrons from Ne(2p⁴3s²)¹D due to He⁺+Ne collisions at 2 keV were measured in coincidence with the scattered projectiles and compared with those of electrons from He(2p²)¹D, measured by Kessel et al. (1979). A theoretical analysis of such angular electron correlations in terms of a density-matrix description is given. The authors find that the Ne excitation occurs in a nearly pure m=0 sublevel with respect to an axis in the scattering plane which coincides with the internuclear line of the collision partners on the outgoing part of the trajectory at a separation of R approximately=2.4a₀. This is interpreted as being due to an excitation at small R by a radial coupling of two Sigma -potential curves, followed by a rotation of the quasimolecule as a whole up to R approximately=2.4a₀. A quantitative estimate of the energy separations between Sigma , Pi and Delta potential curves correlating to He⁺+Ne(2p⁴3s²)¹D indicates that a rotational coupling between these curves is responsible for the decoupling of electronic and nuclear motion at R approximately 2.4a₀.

Energy spectra of radiative electron capture (REC) X-rays have been studied for 110 MeV Ne ion bombardment on gaseous targets of H₂, He, CH₄, N₂, O₂ and Ne. A Si(Li) detector placed at 90 degrees to the beam direction was used for the measurements. The width and peak energy of the REC peak are compared with the impulse approximation calculation and fair agreement is obtained. The REC X-ray yield and the peak energy have been measured for the H₂ target as a function of the gas pressure and the results are discussed in relation to the charge equilibration process of the ions in the gas.

Differential elastic scattering of electrons by neon atoms has been investigated using the X_alpha potential approach in the energy range of 10-50 eV. The s, p and d phaseshifts are also presented. The relative dependence with energy of the differential cross section at 30 degrees scattering angle has been calculated in the range of 30-200 eV. Satisfactory agreement between the present calculation and experimental measurements is achieved.

The R-matrix method for electron-molecule scattering using a Slater continuum basis is applied to scattering from hydrogen and nitrogen targets using the static exchange approximation. Comparison with results of previous single-centre calculations allows the effectiveness of the computational techniques to be assessed. It is found that accurate results can be obtained very efficiently at low electron impact energies but that a numerical continuum basis will be necessary to obtain accurate results at higher energies.