Table of contents

The projectile energy dependence of characteristic K X-ray and high-frequency quasimolecular X-ray production cross sections in Ni+Ni collisions has been measured. The extracted half widths for the frequency distribution of the continuum radiation above the united-atom limit increase as the projectile velocity to the 0.59 power. Theoretical descriptions of the collision-broadened quasimolecular radiation are inadequate to explain these results.

The analytic two-state model of Nikitin (1970) is examined with regard to its use in describing vacancy-sharing processes in slow ion-atom collisions. The range of applicability of the model is assessed by analysing the dynamic coupling equivalent to the exponential potential coupling. K-L vacancy sharing in Ne+Kr collisions is considered as an illustrative example.

An experimental approach previously used in this laboratory has been used to determine cross sections for one-electron capture by 4-25 keV H(2s) atoms in He, Ne, Ar and Kr. The results are compared with corresponding cross sections for one-electron capture by H(1s) atoms.

A multi-configuration optimised central potential model for atomic structure is described. In this model, the expectation value for the Hamiltonian is minimised by varying the central potentials in which are calculated the orbitals of the multi-configuration atomic state. Each potential satisfies a linear integral equation which is solved numerically. Results for the energies of some low-lying even- and odd-parity states of carbon are given and compared with experiment as well as with multi-configuration Hartree-Fock and close-coupling calculations. As in the case of the single-configuration optimised potential model, whose generalisation is presented here, the results compare favourably with those from other rather more complicated methods.

Dirac's equation of the hydrogen atom in a strong magnetic field is transformed to an infinite set of ordinary differential equations by expanding each component of the electron spinor in terms of the Landau orbitals. In the vanishing limit of the fine-structure constant, alpha to 0, the set of equations correctly reduces to the previously used non-relativistic form. The adiabatic approximation is introduced as an extension to finite fields of the form of the equations in the infinite magnetic field strength limit. The results of the numerical integration of the adiabatic equations are presented. The ground state experiences only a minor relativistic correction, but for the magnetically excited states the correction becomes prominent with increasing field strengths.

The authors apply the dressed-atom model to evaluate the spectrum of nearly resonant light scattering from atoms in a perturber gas. The intensity of the monochromatic incident light may be sufficiently high to cause saturation effects, and the detuning from resonance may be large enough to invalidate the impact limit. The results for a two-state or a three-state atom, which are valid in the limit that the lines in the scattered spectrum are well separated, generalise previous work that assumed either the impact or the low-intensity limit.

Two experiments have been carried out to show that the appearance of molecular lines need not lead to wrong interpretations of measured profiles. In a low-pressure plasma He₂ molecules are found to be practically absent, while at high pressure molecular lines occur but are identified easily. Thus they can hardly be confused with satellites of the Baranger-Mozer type (1961). Fast vibrational relaxation is supposed to be responsible for the strong population of the v=0 levels. Finally a quenching effect caused by an admixture of nitrogen is reported.

The authors have measured small asymmetries in the near wings of the self-broadened first resonance lines of Rb and Cs. The transitions from the impact to the quasi-static profile were observed. A comparison with recent calculations by Movre and Pichler (1975) reveals satisfactory agreement.

The bound-free photodetachment cross section of H^- is calculated in the length formulation. The bound-state and free-state wavefunctions are calculated to high accuracy using a close-coupling pseudostate expansion with the addition of Hylleraas-type correlation terms. At lower energies the results agree with those of Stewart (1978), to better than 1%, while at ejected electron energies just below the n=2 excitation threshold the results are within 1% of those of Broad and Reinhardt (1976). The shape resonance just above the n=2 excitation threshold is studied, and the partial photodetachment cross sections are compared with those of Hyman, Jacobs and Burke (1972).

Excitation transfer between the four adjacent 2p₂ to 2p₅ levels of Ne I due to collisions with ground-state He atoms has been investigated. Three of these levels were, in separate experiments, populated by pumping Ne 1s₅ metastables present in a flowing afterglow using a pulsed tunable dye laser; and the subsequent fluorescent decay of the four adjacent levels observed. Of the twelve possible cross sections for excitation transfer between these levels, ten have been determined and upper bounds established for the remaining two.

In a vapour cell Li₂ molecules were excited by a CW argon-ion laser. Optical pumping depletes the population of the Li₂(x¹ Sigma _g⁺, nu "=1,J"=24) level. In the presence of foreign inert gases (He to Xe) collisional relaxation from the adjacent rotational levels takes place, which is measured by means of CW dye-laser-induced fluorescence. From the pressure dependence of the fluorescence signal, rate constants for mod Delta J mod =2,4,6 transitions in the ground state were obtained by solving the rate equations numerically. The values for mod Delta J mod =2 are of the order of 0.5-1.5*10^-10cm³s^-1, corresponding to effective cross sections of 5-10AA². Effective cross sections for total collision-induced vibrational transfer out of the nu "=1,J"=24 level were also obtained, which increase from 0.8AA² to 12.9AA² (from He to Xe).

Accurate quantal values of photodissociation cross sections have been calculated for HeH²⁺ for the transition 2p sigma to 3d sigma over the accessible range of lambda lambda 800-5800 AA and an attempt has been made to explain whether this is the source of the well known ultraviolet flux deficiency observed by Stecher and Milligan (1962) in a number of early type stars. The cross sections were calculated for all the vibrational levels of the 2p sigma state for which the Boltzmann distribution has been assumed and for the two most probable rotational-level values at stellar temperatures of 20000K and 28000K. It has been concluded that HeH²⁺ cannot be the source of such stellar opacity.

Single-centre coupled-channel models based on sufficiently large pseudo-state expansions are expected to describe the proton-hydrogen system accurately above those energies for which charge exchange is large. Neither the first-order, nor the second-order, distorted-wave approximations to such a model can provide total cross sections which are an improvement on those of the first Born approximation in the energy region from 50 to 200 keV.

The K-shell excitation cross sections for C-C, N-N and O-O collisions measured at intermediate impact energies using Auger electron spectroscopy are compared with the MO model calculation. The observed excitation behaviors are consistent with theoretical prediction, but the cross section magnitudes show 20 to 40% discrepancies. The projectile-charge-state dependence of the cross section is observed as the cross section is approximately proportional to the number of total 2p vacancies initially existing in the colliding partners.

For pt.I see ibid., vol.12, no.21, p.3569 (1979). The K Auger electron yields from projectiles and target atoms are measured concurrently for C-N, N-O and C-O collisions in the intermediate energy range from 0.4 to 2.5 meV and the derived cross sections for K-shell excitation are compared with the MO model predictions of Taulbjerg et al. (1976) and Meyerhof (1973). Although some deviations are seen, the results support that 2p pi -2p sigma rotational and 2p sigma -1s sigma radial couplings successively proceed in these collisions. The projectile-charge-state dependence of cross section for the C-N and N-O cases reveals that the cross section magnitude is crudely proportional to the number of total 2p vacancies.

The neutral fraction Phi _{0 alpha} and the fraction with unit charge Phi _{1 alpha} produced when 6-24 meV helium beams emerge from C, Al, Ni, Ag and Au thin targets are reported. The velocity nu ₁ on emerging from the target ranges from 7.7 to 15.5 in atomic units of nu ₀ approximately=2.18*10⁸cms^-1. Phi _{0 alpha} varies from about 10^-5 to 10^-9 and Phi _{1 alpha} from about 10^-2 to 10^-4. Both clearly depend on the atomic number Z₂ of the material; Phi _{0 alpha} decreases approximately as nu ₁^-n with n varying between n approximately=12 for carbon (Z₂=6) and n approximately=10 for gold (Z₂=79), Phi _{1 alpha} decreases as nu ₁^-n with n' varying between n' approximately=6 for carbon and n' approximately=5 for gold. These results are compared with theoretical estimations using capture and cross sections by Bohr (1945) and Brinkman-Kramers (1930).

The influence of a long-range potential (other than the dipole one) on electron scattering by polar molecules is investigated. For this purpose the effective range theory for electron scattering by the potential with a long-range part -(D cos theta /r²)+( alpha /2r^s) is developed. To simplify the investigation the angular dependence of the alpha /2r^s potential is ignored. Formulae for the low-energy behavior of K matrix elements are obtained. These formulae are used to estimate the influence of the quadrupole interaction on the total and momentum transfer cross sections for electron scattering from CsF molecules.

The spin exchange cross sections for H-H and Mu-H collisions are calculated and compared. At low temperatures (<or approximately=100K), the thermally averaged cross section for Mu-H is as much as five times larger than the corresponding cross section for H-H. At high temperatures (>or approximately=400K), the thermally averaged cross sections are comparable and it is shown that the ratio of the Mu-H and H-H cross sections is given approximately by ( mu Mu-H/ mu H-H)¹(p-1)/ where mu is the reduced mass and the difference between the triplet and singlet potentials is approximately proportional to r^-p.

The thermal diffusion factors for six binary systems are measured in a two-bulb metal apparatus as a function of temperature and composition. The values are compared with the available values in the literature, wherever possible, and with the predictions of the Chapman-Enskog kinetic theory involving elastic collisions. The contribution of dipole and quadrupole moments of polar gases to the thermal diffusion factor is estimated according to the theories developed by Mason and co-workers. In general, the thermal diffusion data involving polyatomic and polar gases are not well represented by the available theories, and generation of reliable data may provide considerable guidance and much needed clues for the development of an accurate but tractable theory. A possible explanation of the minima and maxima observed in the thermal diffusion data is advanced.