Table of contents

Energy- and angle-resolved photoelectron spectroscopy has been used to investigate the spectral dependences of the angular distributions of electrons photodetached from a beam of metastable He^- ions.

Measurements of the asymmetry in the scattering of unpolarised electrons from optically pumped spin-polarised sodium atoms have been carried out at incident electron energies of 20, 54.4 and 70 eV and for a range of scattering angles between 20' and 140'. This asymmetry, which was first proposed by Farago (1974) as a means of measuring spin-orbit effects in the presence of a dominant spin-exchange interaction, is less than 1% at all energies and scattering angles studied.

A key process in muon-catalysed fusion is the de-excitation of the dt mu 'nucleus' of the resonant muonic quasi-molecule, ((dt mu )dee), by emission of an Auger electron. The dt mu in the quasi-molecule is initially in a weakly bound excited state with J=1 and v=1. A preliminary calculation is carried out of the rate of the dominant transition to the state with J=0 and v=1. Use is made of the dipole matrix elements calculated for this transition by Bhatia et al. (1989) and Scrinzi and Szalewicz (1989) and, for the first time, full account is taken of the molecular nature of the quasi-molecule. Reasonable agreement is found with the results of earlier calculations.

Recent developments in the field of alignment and orientation studies of electron impact excitation of atoms are discussed. The article reviews experiments which have a well defined planar symmetry, i.e. in which the initial and final momenta of electrons are determined. The experimental results obtained by different groups during the last four years are presented after a brief theoretical introduction followed by a description of typical experimental arrangements.

The authors present accurate (relative errors approximately=10^-7) eigenvalues for the 1s²¹S₀ eigenstates of the Dirac-Coulomb Hamiltonian for helium-like atoms. A novel non-perturbative method, based upon a sequence of multiconfiguration Dirac-Fock calculations together with an extrapolation procedure, is used to arrive at the numbers given. In conjunction with perturbation theory, the same technique may be extended to calculate higher-order contributions to the energies of two-electron systems to high precision. As an example, they have obtained estimates of the energies of the interelectronic transverse electromagnetic interaction.

The excited potential curves of Na₂ are computed up to the states dissociating into Na(3s)+Na(4d) by a two-electron model potential method which uses an expansion on screened two-centre orbitals. The agreement between the computed excitation energies and the experimental determination is better than 200 cm^-1. A good agreement is also found with previous calculations of Jeung (1983,1987) up to the 3s+5s dissociation limit, but not with previous calculations of Jeung and Ross (1988) for the states dissociating into 3p+3p. The analysis of the results in terms of molecular quantum defects reveals some avoided crossings characteristic of two-electron effects.

Measurements of the Stark widths of twelve Ar IV lines in the near UV, emitted by a gas-liner pinch plasma (electron density 2.4*10¹⁸ cm^-3, electron temperature 7 eV), are reported, and compared with calculations of electron impact broadening. Good overall agreement is obtained with the semiclassical Gaunt factor approximation derived earlier by two of the present authors. When experimental factors are accounted for, no evidence is found for deviations of the Stark profiles from symmetrical Lorentzians. The spectral lines are also found to be practically unshifted. The importance of taking careful account of the structure of the radiating ion in the calculation is shown. The applicability of the impact approximation to such a dense but relatively cool plasma is also considered.

Molecular hydrogen in the X ¹ Sigma _g⁺ (v_X=0,J=1) state was resonantly excited to the E,F ¹ Sigma _g⁺ (v_E=0,J=1) state by two-photon absorption. The use of a laser with low pulse energy and narrow bandwidth reduced the importance of the photoionisation of this state by the absorption of another photon, and thereby enhanced the role of collisional quenching. The molecular ions and vacuum-ultraviolet fluorescence that resulted from the laser pulse were measured for hydrogen densities from 10¹⁵ to 10¹⁹ cm^-3. An analysis of the rates of excitation and ionisation, and a comparison of the spectrum of the fluorescence with a computed simulation, suggest that bound states in the outer well of the double-well E,F potential were involved in the excitation transfer to the excited states of B ¹ Sigma _u⁺ that produce the fluorescence. Neutral and electron collisions, ionic processes and higher-order radiative interactions are discussed as possible explanations of this phenomenon.

The authors have calculated cross sections for the excitation of Na(3s) and Li(2s) atoms by proton and antiproton impact within the framework of the optical potential model. They also present results for capture into hydrogen as well as loss and ionisation cross sections and compare their results with experimental data available.

Results of an electron spectroscopy study of ionisation processes in collisions of F atoms with He, Ne, O₂ and CO₂ at low keV energies are presented. It is shown that in case of the He and Ne targets ionisation is partly due to population of singly-excited autoionising states of the ¹D nl series and partly due to another mechanism which results in the production of a continuous electron energy distribution and becomes particularly important at high energies. A discussion in terms of the molecular orbital model is presented and indicates that quasimolecular Auger ionisation must be the other F⁺ production channel. In the case of molecular targets the authors observe a direct ionisation channel and also dissociative ionisation via the population of ²D nl autoionising states of fragment O atoms.

Differential elastic (vibrationally) scattering cross sections of CH₄ by electron impact have been measured using a modulated crossed-beam method. The energy and angular range covered were from 5 to 50 eV and from 12 to 156', respectively. The integrated and momentum transfer cross sections were obtained from the differential cross sections. The present results are compared with the earlier data of Tanaka et al. (1982) and with theoretical results of Lima et al. (1985) and Jain and Thompson (1982). Some discrepancies were found in the measurements and theoretical results.

Absolute electron impact cross sections for single ionisation leaving H₂⁺ in its electronic ground state and double ionisation leading to dissociation into all four constituents have been determined by a direct method in the energy range 0.15-2.6 keV. The values for single ionisation are in perfect agreement with, but the double-ionisation values are much larger than, the earlier determination by Edwards et al. (1988,1989). The authors' results exhibit a close relationship between the H₂ system and its atomic counterpart, helium, with respect to their response to single- and double-ionisation processes.

Measurements are reported for the circular polarisation of the L_alpha radiation arising from the cascade of the 3²D_j states of atomic hydrogen to the 2²P_j states detected in coincidence with electrons with n=3 energy loss. Data for the atomic orientation parameter L_{perpendicular to} are presented at incident electron energies of 54.4 and 100 eV and at scattering angles theta _e of 20' and 25'. These data, together with previous measurements for this transition, constitute a determination of the complete parameter set ( gamma ,P_l, rho ₀₀,L perpendicular to ) for 3²D_j excitations in this dynamic range. Calculations for P₃ and L_{perpendicular to} using distorted-wave approximation theory are also performed. The theoretical results are presented and compared with the experimental data.

The authors consider the rotational spectrum of the hydrogen molecular ion in a magnetic field. It is shown that the Berry phase factor for this system acts like an additional magnetic field which effectively reduces the external field by about 5%.

The authors report optical saturation of the 1³S-2³P transition in positronium using ultraviolet light from a frequency-doubled, pulsed dye laser. The increase in ground-state annihilation radiation due to magnetic triplet-singlet mixing in the excited state is used to monitor the transition. From the variation of the annihilation rate as a function of magnetic field, laser power, wavelength and polarisation, the authors verify that the transition is optically saturated, in agreement with model calculations of the system.