Table of contents

By constructing wavefunctions of the form Psi (H₂^-)= Psi (H₂, e)+ mu ( Psi (H^-H)+ Psi (HH^-))+ nu ( Psi (H^2-H (HH^2-)) for H₂^-, it is calculated that the ground state for this species, with ² Sigma _g⁺ symmetry, is bound relative to the ¹ Sigma _g⁺ ground state of H₂ and a free electron. The Psi (H₂, e) accommodates an electron in a diffuse 1s orbital, which is centred midway between the protons. When the Weinbaum wavefunction with (STO-5G) 1s orbitals is used to describe H₂, the minimum energy for each of these species is calculated to occur at an internuclear separation of 1.43 au. The resulting electron affinity (D_e(H₂^-)-D_e(H₂)) for H₂ is 2.0 kJ mol^-1. A similar calculation with nuclear-centred 2p sigma orbitals also included gives an electron affinity of 6.5 kJ mol^-1.

Energy level positions and branching ratios for radiative decay have been calculated for a number of doubly excited states in beryllium-like Ne VII using the SUPERSTRUCTURE code. These are compared with weak unidentified features observed in spectra obtained from foil-excited neon beams at energies of 6, 13 and 18 MeV.

The Stark effect on diamagnetic Rydberg states of hydrogen is investigated. In the perturbative regime, re-ordering of levels within a hydrogenic n manifold leads to level repulsion when the ratio of field strengths is varied. When the fields become strong enough very narrow avoided crossings between levels from different n manifolds appear. In addition the authors study the Stark effect on recently observed quasiLandau structure in the irregular part of the spectrum.

It is argued that modifications suggested by Cohen and Harrigan (1985) in a recent treatment of the ECPSSR theory for direct inner-shell ionisation by Brandt and Lapicki (1979) are not needed. The ECPSSR theory accounts for the projectile energy loss (E) and Coulomb deflection (C) as well as the perturbed-stationary-state (PSS) and relativistic (R) effects in the treatment of target inner shells. In fact, it is demonstrated that an insertion of a relativistic mass factor in the argument of the energy loss function results in more than a factor of two disagreement with ab initio calculations of the energy loss function.

The R-matrix method is used to calculate rates for electron excitation between the ground state of Se XXV and the 26 levels (2p⁵) 3s, 3p and 3d. Configuration interaction wavefunctions are used in the calculations. For certain transitions resonance effects increase the electron excitation rates significantly. The rate coefficients are compared with earlier distorted-wave results.

R-matrix calculations are presented for elastic and rotationally inelastic scattering of positrons from CO at energies below the positronium formation threshold. The calculations are performed within a number of models that represent charge polarisation effects, the rotational motion of the target and the asymptotic potential. The computed total and momentum transfer cross sections are found to vary widely between the different models employed. A comparison is made between the experimental results for this system and the recent calculations of Jain (1986).

The electron wavefunctions for two states of the PbF molecule are calculated. For the ground state of the molecule, tensor components of the hyperfine interaction of the valence electron with the nuclear spin of Pb, electric and magnetic dipole moments and electron matrix elements of P-odd and P, T-odd interactions are calculated. In accordance with the calculation an effective spin-rotational Hamiltonian for the molecule is given. The Hamiltonian includes the parity-non-conserving terms and the interaction with the external fields. The spin-rotational spectrum of the PbF molecule is found to be rather complicated; this is connected with the large anisotropy of the hyperfine A tensor. The results of the present work can be used in planning future experiments on the search for P-odd and P, T-odd phenomena in molecules.

A formal relationship between the Dirac-Hartree-Fock method and the relativistic Hartree-Fock scheme of Cowan (1976) and Griffin has recently been established. Following these developments the authors present in this paper a detailed numerical comparison of both the approaches examining the total energy, the orbital energies and the one-electron and two-electron integrals. It is also shown why the RHF method is able to yield accurate values of the relativistic transition energies. Moreover employing uniquely accurate values of the correlation corrections for p- and d-electron systems, they demonstrate the usefulness of the RHF method in evaluation of relativistic corrections to the differential term energies. Advantages offered by this method for positron scattering on heavy systems are also discussed.

The authors report results on the wavefunction representation of doubly excited states of He and on their autoionisation probability amplitudes in momentum space. These are presented in terms of conditional probability density and differential transition amplitude plots. The correlated momentum wavefunctions are obtained by Fourier-transforming position wavefunctions computed by multiconfigurational Hartree-Fock and variational procedures. Special techniques are developed for the Fourier transform of the numerical Hartree-Fock scattering orbitals. The differential transition amplitude plots lead to the same conclusions regarding the mechanism of autoionisation as did the previous work in position space.

Measurements of I(K beta )/I(K alpha ) for proton-induced X-rays from thin targets in the 22<or=Z<or=32 region confirm a previously suspected systematic discrepancy between experimental data and the theoretical predictions for de-excitation of a single K vacancy in a neutral atom. The present paper is the first proton work to pay specific attention to the KMM radiative Auger satellite and to use a measured resolution function in unfolding K alpha -K beta peak overlaps. A discrepancy is common to electron-, photon- and proton-induced ionisation but sufficient precision is not yet available to reveal any small differences among the three processes, which might arise from differing multiple ionisation.

Evaluating the K-shell fluorescence yield of an X-ray detector material like Si or Ge from the escape to parent peak ratio is essentially an integration problem; e.g., side wall effects make a contribution of the order of 1% for uncollimated beams. The transition from atomic X-ray fluorescence to detector emission is physically governed by relative absorption coefficients, and it is the uncertainty in these values which restricts the numerical reliability of the results: omega K_.Si=0.0481+or-0.0014 and omega K_,Ge=0.532+or-0.016. Both the values with their associated error limits are not sufficient to decide whether effects of the outer-shell configuration are numerically relevant.

Laser polarisation spectroscopy has been used to measure broadening and shift of the helium 587.6 nm and the neon 588.2 nm lines in a low-pressure glow discharge. A least-squares computer routine accounting for competing broadening contributions extracted the pressure broadening parameters from the raw data. The results are compared with those of Cahuzac and Damaschini (1977), (1980) for the helium line and with those of Kuhn and Lewis (1967) and Jabr and Bennett (1980) for the neon line.

Synchrotron radiation was used to excite an inner-shell electron into a Rydberg orbital at the Ar 2p to ns, nd, Kr 3d to np and Xe 4d to n p resonances. The resonant decay into shake-off channels was studied by three different electron measurements. Firstly, threshold electron scans were obtained over the resonances and thresholds. On the first resonance for each atom, photoelectron spectra were collected. The intensity distribution of low kinetic energy electrons was also determined for a few resonances. Finally, a shake calculation was carried out to compare with the experimental shake-off probabilities. Shake-off is observed to be a strong decay channel for these resonances.

The semirelativistic R-matrix method based on the Breit-Pauli Hamiltonian is used to study the photoionisation of neutral zinc atoms ((3d¹⁰4s²)¹S₀) in the wavelength range 1300-250 AA or photon energies from the ionisation threshold (9.39 eV) up to 50 eV. Theoretical results are presented for the total cross section, the angular distribution and the spin polarisation of the photoelectrons, and finally for the orientation and alignment of the residual Zn⁺ ions. Good agreement is obtained with the available experimental data and other theoretical calculations for photon energies above the (3d⁹4s²)²D_5/2,3/2 thresholds.

Two-laser multiphoton ionisation under the autoionising-like resonance condition is studied theoretically within an extended atomic model dictated by the experiment of Feldmann et al (1986). Ordinary coupling between a selected pair of bound states via the continuum only is supplemented in the author's model by Raman-type coupling involving the atomic spectrum below the ionisation threshold. In the case considered this additional Raman coupling enhances the ionisation rate to a much greater extent than the coupling via the continuum can do. The Raman coupling modifies the autoionising-like shape parameter and is decisive as far as the resonance peak profile is concerned. Exploiting this Raman coupling within the atomic model with four effective bound states, the author gives explanations for all the details of the ionisation spectrum detected by Feldmann et al, i.e. the number of resonance peaks, their strength, fine structure, shape, relative heights and sensitivity to the laser field.

The motion of an electron of a classical hydrogenic atom in an oscillating electric field is studied theoretically. An analysis is provided, based on the iterative (mapping) forms of the classical equations of motion in perturbation theory and the adiabatic approximation. This greatly facilitates the numerical investigation of stochasticity and the ionisation process and allows the approximate analytical estimation of the threshold field strengths for the onset of chaos and of the diffusion coefficient of the electron in energy space. The method is asymptotically exact at high field frequencies and gives a good approximation for medium and low frequencies. The adiabatic approximation describes well the approach of the stochastic ionisation threshold field strength to the static field ionisation threshold. From the quantum mechanical point of view the ionisation is a result of the great number of one-photon transitions in the strongly perturbed spectrum of the atom. This results in the diffusion of the electron in energy space identical to the diffusion due to stochastic classical motion. The estimation of the mean time of diffusive ionisation is also given.

The collision-induced two-photon forbidden 3s²¹S-3snp ¹P transitions (n>11) and collisional enhancement of 3s²¹S-3sns ¹S transitions (n>8) of Mg I have been observed and studied as functions of Mg vapour pressure and perturbing gas (Ar) pressure by using two-photon resonant ionisation and thermionic diode techniques. The occurrence of forbidden transitions and collisional enhancement of 3s²¹S-3sns ¹S transitions is qualitatively attributed to the formation of a Rydberg Mg₂ excimer and a Mg_xAr_y Fermi complex. Quantitatively, the above phenomena are interpreted as collisional mixing of high Rydberg states of Mg. The mixing coefficients are estimated by using perturbation theory and the semiclassical (JWKB) approximation. The theoretical estimation is in good agreement with the experimental result.

The energy and angular distribution of secondary oxygen ions produced in He⁺-O₂ collisions has been investigated within the energy range 3-25 keV. Dissociative charge transfer processes dominate and involve many exoergic and endoergic channels. Cross sections for capture into the c ⁴ Sigma _u^- and B ² Sigma _g^- predissociating states of O₂⁺ have been separately determined. Angular distributions of the fragment ions exhibit anisotropies characteristic of momentum transfer and short predissociation lifetimes.

The vibrational populations of CO(X ¹ Sigma ⁺ and a ³ Pi ) and N₂(X ¹ Sigma _g⁺ and B ³ Pi _g) produced in collisions of He⁺ with CO(X) and N₂(X) in the E=100-1000 eV energy range are studied, using a high-resolution energy loss spectrometer, as a function of the scattering angle theta of the He⁺ particles. The results for CO(a ³ Pi ) show dramatic deviations from the Franck-Condon distribution and exhibit contrasting behaviours as a function of both E and theta . The measured vibrational distributions for CO(X) and N₂(X) deviate strongly from the often invoked Poisson distribution and show peculiar dependences as functions of E and theta . The possible bond-length-dependent phenomena that could influence the vibronic excitation into CO(a, nu ') are discussed. A simple model is set up to explain the main effects that govern the dependence of the vibrational distribution in the electronic ground states of CO and N₂.

The authors derive a simple non-perturbative expression for the united-atom limit of radial couplings between one-electron molecular orbitals. The formula explicitly exhibits the dependence on the nuclear charges and the choice of the origin for the electronic coordinates. The coupling is non-zero only for the optically allowed combination of united-atom quantum numbers l'=l+or-1.

The relativistic matrix continuum distorted-wave model is extended to electron capture from any arbitrary shell of the target to any arbitrary shell of the projectile. A general analytical simple formula for the scattering amplitude is obtained. Applications for the cases of charge exchange from the K shell and L shell of the target are presented. Comparisons with experimental data and other theoretical approximations are given.

After the single-electron capture collision Ar⁷⁺+He to Ar⁶⁺+He⁺ a measurement of Ar VII transition intensities in the region 100< lambda <1000 AA shows that the projectile captures the electron into the n=4 level of the Ar VII ion. A configuration interaction calculation of Ar VII energy levels is used to interpret and identify observed transitions. It is underlined that population of (Ar⁶⁺)³L states rather than (Ar⁶⁺)¹L states is favoured.

The transmission of longitudinally polarised electrons through a vapour of optically active isomers was investigated. The transmitted intensities, I(P) and I(-P), measured for the same isomer with electrons of opposite helicity, define a transmission asymmetry A=(I(P)-I(-P))/(I(P)+I(-P)). The experiments were performed at 5 eV electron energy with D(+) and L(-) camphor. The measurements confirm the theoretical predictions based on general symmetry arguments: (i) an asymmetry exists; (ii) the asymmetries observed with targets to opposite handedness are of the same magnitude but of different sign. With an electron polarisation P=-0.28, A(D+)=(23+or-11)*10^-4 A(L-)=(-50+or-17)*10^-4. The problem of relating these results to theoretical estimates of helicity dependence in scattering from chiral molecules is discussed.

Phase shifts and total and differential cross sections are calculated for the elastic scattering of unpolarised electrons by argon atoms in the impact energy range of 0.1 to 40 eV. The target atom is represented by a frozen core in a continuum relativistic Hartree-Fock calculation with dipole and quadrupole polarisation corrections. Exchange is calculated exactly and the parameters of the model polarisation potential are fixed by independent calculations. The results are in good agreement with experiment and with some of the non-relativistic coupled Hartree-Fock calculations; they demonstrate the importance of core polarisation effects as well as of relativistic effects at low incident energies.

Absolute total cross section measurements are presented for e^--Ne and e^--Ar scattering. The covered energy range spans from 100 to 3000 eV. The overall systematic uncertainty was +or-2.4%, the random uncertainty was +or-1.5% and the angular resolution error was less than 0.5%. A comparison with existing experimental and theoretical data is given. This comparison suggests that the semi-empirical data of de Heer et al (1979) for Ne are possibly overestimated by 5% at 100 eV.

Theoretical results for differential cross sections for the elastic scattering of electrons and positrons from xenon atoms are presented. For electrons two full relativistic calculations are shown where exchange effects are treated (i) by completely antisymmetrised wavefunctions and (ii) by an approximate local exchange potential; for positrons full and semi-relativistic results are compared. In general the agreement between the different approaches is quite satisfactory, however, the inclusion of a complex absorption potential further improves significantly the agreement with the available experimental data. Furthermore a simple semiclassical model for the local minimum in the differential cross section for positrons at small scattering angles is proposed.

The total cross section for electron scattering from vibrationally excited (principally 010) CO₂ molecules has been measured in the energy range 0.12-2.0 eV with a time of flight electron spectrometer. The vibrationally excited molecules were produced by thermal excitation of the gas contained in a scattering cell at a temperature of 300 degrees C. The cross section for scattering from these excited molecules was deduced by comparison with similar measurements performed at room temperature. At energies below 2 eV the excited-state cross section is considerably larger than that for scattering from ground-state CO₂. The ground-state measurements are in excellent agreement with other recent experimental and theoretical work. No other experimental or theoretical data exist for the excited-state cross section.

The M=0 components of the P^o and D^e states of two continuum electrons are frequently produced by electron impact ionisation (or double photoionisation with polarised photons). Some experiments are able to measure the triple differential cross sections (TDCS) of such processes as functions of the two electron energies and spherical angles theta ₁, phi ₁, theta ₂, phi ₂ with respect to the incident beam direction (or polarisation). Here the authors use the Wannier theory and proper coordinates to derive theoretical expressions for these TDCS near threshold. As already shown by Peterkop (1983) different energy behaviours result from different nodal properties at the Wannier point. They show that the ¹D^e state has a mixed character, i.e. it contains a first part with no angular node and a second part with a second-order angular node which, however, cannot be neglected. Consequently the resulting TDCS includes two different energy behaviours associated with two specific functions of angles. Finally they take advantage of these P^o and D^e particular states to establish a link between some approaches using different methods and systems of coordinates which have been proposed so far within the frame of the Wannier theory.

Using a coincidence technique in a coplanar geometry, triple differential cross sections (TDCS) for electron impact ionisation of helium are measured in the 0.5-2 eV energy range above threshold. As a few (LS Pi ) states (0<or=L<or=2) of the two outgoing electrons are obviously involved in the process, their respective intensities appear as unknown parameters in the theoretical TDCS as deduced in the frame of the Wannier theory. We show that almost all these parameters can be determined through normalisation to the measured TDCS in two specific geometries: in the first one the two electrons are kept in opposite directions while in the second one they remain symmetrical with respect to the incident beam. A comparison with the complete set of data is then performed. The measured TDCS are in agreement with the Wannier theory for the lowest energies (0.5 and 1 eV). The angular correlation predicted by this theory is confirmed but a precise determination of the angular correlation width cannot be achieved. At 2 eV the overall agreement becomes poorer, although some predictions of the Wannier theory still apply: in particular the measured TDCS are almost independent of the energy sharing between the two electrons. Finally specific measurements at 8 eV clearly show from considerations of symmetry that the Wannier theory no longer applies at this energy.

Triple differential cross sections for the ionisation of helium by fast (8 keV) electrons are calculated using the first Born approximation. Generalised oscillator strengths are also obtained, as a function of momentum transfer, for ejected electron angles of 0 and 180' with respect to the momentum transfer direction. Comparison is made with experiment and with previous theoretical calculations. The results are very sensitive to the choice of wavefunctions for the initial and final target states.

Hartree-Fock calculations have been performed for linear and ring-shaped carbon clusters. Magic numbers have been identified and compared with other theoretical and experimental numbers.

An electrodeless microwave-excited discharge in argon has been used to examine the population densities of some seventeen Ar I and six Ar II atomic levels at pressures between 6.0 and 0.02 mbar. Electron density measurements in the range 10¹⁷ to 10¹⁸ m^-3 were made from the microwave impedance and these were compared with the ion density measurements from double floating probes. The results show the importance of ion loss to the tube walls and the necessity of allowing for ionic diffusion when using the probe method. The atomic system is shown to be in the (quasi-) saturated phase and the population coefficient r_m⁽¹⁾ is determined for seven Ar I atomic levels from 6s to 9s. The influence of microwave power upon the plasma quantities, electron density, ion density and electron temperature, and the atomic level populations has been demonstrated. It is found that the input power and pressure are dependent variables.