Table of contents

The direct and the charge-exchange excitation of the He 2¹P level in He⁺-He collisions has been studied in the impact energy range 150-1000 keV. The direct excitation cross section shows a maximum around 600 keV, whereas the charge-exchange cross section decreases monotonically with increasing impact energy by more than two orders of magnitude.

The experimental values of the positron-helium total cross section at 2 eV recently measured by Kauppila et al. (1976, 1977) and Burciaga et al. (1977) are compared with a theoretical value which is believed to be a conservative lower bound on the exact result. Both experimental values are found to be significantly lower than the theoretical value, and it is suggested that the discrepancy is due to the neglect of small-angle scattering in the experiments. The angle below which scattering would need to be neglected to explain the discrepancy is calculated for each experiment, and its value is consistent with the known or probably resolution of the apparatus.

The authors have used an apparatus which is a development of the extracting-field type of threshold electron analyser to obtain the threshold excitation spectrum of krypton atoms as a function of incident energy up to the ionisation potential. The improved resolution of this apparatus has permitted the observation of numerous levels that were previously unresolved in threshold excitation experiments.

The distribution of excited states of He atoms in the presence of plasma electrons is studied theoretically and experimentally. It is shown that (a) excitation transfer is important when the electron density exceeds a few times 10¹⁶ m^-3 and that (b) with increasing electron density, the relative population of levels with the same main and spin quantum numbers approaches a thermal equilibrium between such levels, even when the system is very far from equilibrium in other respects.

General arguments used for studying partial level density in nuclei, when applied to atomic levels, lead one to expect that the electronic states arising from some definite configuration would be distributed in a nearly Gaussian fashion. This feature is investigated for the 3d⁵ configuration and the 4f⁶ configuration of electrons in atoms, as well as for the even-parity states in Ta II and the experimentally observed states in the neutral Hf atom. It is found that their distribution is approximately Gaussian, although the departure from normality is not negligible.

An approximation scheme for the one-particle Green's function referred to as the two-particle-hole Tamm-Dancoff approximation (2ph-TDA) is introduced by means of a well defined infinite partial summation of the perturbation expansion for the so-called self-energy part. A spin-free formulation of the working equations is presented for molecular applications. It is demonstrated that the 2ph-TDA is a useful tool for a theoretical treatment of inner valence ionisation processes. A discussion of the physical content and the relationship to other approaches shows the central role of this approximation.

Ionisation potentials and spectral intensities for photoionisation of the valence electrons of CS, PN and P₂ have been calculated applying the so-called 2ph-TDA method (see ibid., vol.11, no.11, p.1889, 1978). Strong correlation effects are found for the two inner-most valence orbitals of all these species, showing that the independent-particle picture of ionisation fails completely. As far as available the results are compared to experimental data and to Green's function calculations previously performed for the outer valence orbitals.

The authors have observed transitions to the n²F_5/2,7/2 states of Cs with n<or=65 using CW dye laser excitation. These results are compared to those obtained previously using pulsed laser excitation. Transitions arise due to a sequential two-photon excitation involving a Cs₂ intermediate state. The limit of observed principal quantum number differs with CW and pulsed laser excitation and reasons for this effect are discussed.

An asymptotic method is presented for solving the problem of a hydrogen atom perturbed by a uniform electric field F considered as a weak field. An asymptotic expansion for the level width Gamma is obtained. A comparison is given of the values of Gamma obtained using the asymptotic formulae with exact numerical results.

Presents the first application to a physical case of the effective-operators method introduced by Armstrong, Beers and Feneuille (1975) to interpret resonant multiphoton ionisation. In particular, the four-photon ionisation of Cs I near the 6f resonance is considered. Atomic quantities needed for the evaluation of effective operators are obtained by using a non-relativistic central-potential model. Theoretical results are compared with experimental data recently obtained by Morellec, Normand and Petite (1976). Taking into account the uncertainty of the measurements, the main experimental features and, in particular, the shapes of the resonance curves are reproduced well by the calculation described here.

The hydrodynamic formulation of quantum mechanics is considered in the semiclassical limit and the relationship between quantum streamlines and classical trajectories is discussed. The flow pattern of the quantum streamlines can be described in terms of the interference of classical trajectories. The flow inside the classically forbidden regions is related to complex-valued classical trajectories. The analytically solvable cases of Coulomb and hard-sphere scattering are investigated in detail.

Rotational transition probabilities and cross sections for HD⁺ in collisions with He have been calculated for different initial rotational states and at collision energies below the threshold for vibrational excitation. To obtain more information about the different physical parameters influencing rotational transitions in molecular ions, a semiclassical strong-coupling method has been used instead of a fully quantal method. Due to the influence of the shorter range overlap forces and the presence of longer range induction forces the interaction for atom-molecular-ion collisions is much stronger than for atom-neutral-molecule collisions. This results in rotational transition cross sections which are of the same order for all energetically accessible states whereas in the case of neutral molecules the cross sections for higher order transitions decrease rapidly.

The small-angle partial cross section for the (2,0) to (3,0) transition of TlF molecules in collision with Ar atoms has been calculated and compared with the available molecular-beam experimental results. Both the semiclassical exponential approximation and first-order perturbation theory have been applied. An attempt has been made to take into account qualitatively the velocities of the target-gas atoms. It was found that the experimental results can be interpreted using, however, a much lower value of the Van der Waals interaction than is obtained from the London formula.

The polarisation degree pi of some prominent helium lines has been measured after collisional excitation by H⁺, H₂⁺, H₃⁺ and He⁺ ions in an energy range from 100-1000 keV. It was the aim of this experiment to investigate the influence of the different ion structures on the relative population of magnetic substates at high impact velocities. The proton data are compared with the first Born approximation. The effect of resonance-radiation capture on the measurement of pi (3¹P-2¹S) has been reinvestigated. It is concluded that the intensities I/sub /// and I_{perpendicular to} which enter into the definition of the polarisation degree are differently affected by resonance trapping at low gas densities.

The energy dependence of the total cross section for simultaneous excitation of the target and projectile atom in Ne+Ne collisions to states of the 2p⁵3p configuration has been studied for collision energies from 50 to 700 eV. It is estimated that simultaneous excitation occurs in about 25% of the inelastic events. Cross sections for the population of two individual 2p⁵3p states, Q(2p_n mod 2p_n') (Paschen notation), have been studied in detail for the case n=1. The relative magnitude of these cross sections depends on the population of the molecular states involved. Relative populations of these states have been determined from the reported measurements.

High-energy cross sections for the excitation of the 3s state of the hydrogen atom by simultaneous collision with electrons and photons are calculated under the second-order Bethe-Born approximation, both as functions of electron energy and of laser intensity. Resonant structures are shown to arise as a function of laser-photon frequency. The behaviour of these resonances is described and their origin explained. It is concluded that these resonances are characteristic of the simultaneous electron-photon excitation process itself and can occur for any atomic target, under suitable conditions.

The electron impact ionisation (e, 2e) spectrum of argon has been studied with a resultant energy resolution of 0.06 eV and an angular resolution of two degrees in a crossed electron beam and modulated atom beam apparatus. The previously observed 3s3p⁶, 3s²3p⁴3d and 4d(²S_1/2) states were identified; the spin-orbit splitting of 0.18 eV between the 3p_3/2 and 3p_1/2 states was revealed and the 3s²3p⁴4s(²P⁰) state was seen. Angular correlations between the scattered and ejected electrons in coplanar symmetric scattering were obtained for each of the above six states. The deduced electron momentum distributions for these states are consistent with the factorised distorted-wave impulse approximation model formulated by McCarthy and Weigold (1976). The characteristic electron orbitals are those appropriate for the total angular momentum and parity for each state. The cross sections for production of the 3p_3/2 and 3p_1/2 states are in the ratio of their statistical weights.

Normalised differential, integral and momentum-transfer cross sections for elastic scattering and for excitation of the 3³P, 3¹P, 3¹D, 4¹S, 4¹P and (3³D+3³P) states of magnesium have been determined at 10, 20 and 40 eV impact energies. Several autoionising transitions have been observed in the 8-12 eV region of the energy-loss spectra which correlate well with optical-absorption and ejected-electron spectra.

Electron scattering by manganese has been studied in the 10-100 eV impact energy and 10-130 degrees angular ranges. In certain energy and angular ranges cross sections for several inelastic processes exceed those associated with elastic scattering. The angular distributions which characterise optically spin-allowed and spin-forbidden transitions involving singlet and triplet states in the case of light elements seem to apply to transitions among states of high spin multiplicity. Differential, integral and momentum-transfer cross sections for elastic scattering and excitation of the y⁶P, z⁶P, a⁶D, a⁴P, z⁴P and z⁸P states have been obtained at 20 eV impact energy. The cross sections were normalised to the absolute scale by using the optical f value of the z⁶P excitation.

Experiments are reported in which a pulsed dye-laser beam nu _L was focused into a heat-pipe oven containing caesium vapour at densities of about 10¹⁷ cm^-3. Optically pumped fixed-frequency infra-red laser emission nu _IR and tunable stimulated electronic Raman-scattered radiation nu _S were observed. In addition the generation of ultra-violet radiation by coherent anti-Stokes Raman scattering nu _UV= nu _L- nu _S+ nu _IR was detected. Focused-beam phase matching appears to play a significant role in this process.

Presents new experimental data for the thermal diffusion factor of binary gaseous mixtures of helium and neon. The measurements have been performed in a two-bulb apparatus over the temperature range 60-300K, for mixtures of five different compositions. The uncertainty in the reported thermal diffusion factors is estimated as +or-3% at the higher temperatures, increasing to +or-6% at the lowest temperature. The experimental data are compared with thermal diffusion factors calculated with the aid of intermolecular pair potentials derived from molecular-beam scattering data and direct inversion of other gaseous transport properties.