Table of contents

Model potential calculations for the 4s to 9p Rydberg states of the NaNe molecule are presented. Four groups of sigma quantum defects are found. At 6.5 au, there exists, for all values of n, a pseudocrossing between the sigma (n+1)p and sigma nl, l>or=2 curves. This crossing can be predicted by the Fermi model of a free-electron-rare-gas interaction, provided one incorporates the Coulomb acceleration of the electron by the Na⁺ field.

Unstable H^- ions have been observed from the dissociation of H₂^-states using an electron impact spectrometer. These H^- ions are detected by collecting the electrons emitted by their decay to the ground state of atomic hydrogen.

The energy spectrum of electrons produced by multiphoton ionisation of xenon atoms has been measured, using a new type of electron energy analyser with an acceptance angle of 2 pi sr. Absorption of additional photons above the minimum number required for ionisation has been observed in the non-resonant as well as in the resonant multiphoton ionisation process. In the resonant proces the AC Stark shift is used to link wavelength detuning to the light intensity, in order to find the probability of extra photon absorption as a function of light intensity.

Cross sections have been measured for the formation of X²⁺ by collisions between X⁺ ions (X=Na, K or Rb) with centre of mass energies between 19.3 and 87.6 keV. The results were compared with similar measurements for Li⁺ and Cs⁺ previously made by Peart and coworkers (1981), but no simple empirical or theoretical scaling law could be found to relate results for the five single-charged alkali ions.

The eikonal-Born series approximation is unitarised by using a method due to Wallace (1975). Application is made to electron and positron elastic scattering from atomic hydrogen and to the excitation of the 2s state of atomic hydrogen by electron and positron impact.

The ground state of BeF₂^- has been studied from the standpoint of the ab initio spin-unrestricted Hartree-Fock scheme. The equilibrium geometry, force constants and vertical and adiabatic electron affinities have been calculated with several basis sets differing in the polarisation and diffuse functions in the Gaussian basis set. A comparison of total energies for the optimised ground states of BeF₂ (¹ Sigma _g⁺) and BeF₂^-(²A₁) indicates that the anion lies a reasonable distance above the neutral system. A study of the potential energy surface along the bending motion related to the electron capture e^-+X¹ Sigma _g⁺(BeF₂) to X²A₁(BeF₂^-) shows that the anion may still exist as a metastable species leading to resonances in the cross sections when electrons are scattered from BeF₂.

The importance of relativistic effects in the calculation of static dipole polarisabilities of atoms has been investigated using the approximation of Pople and Schofield. Comparison is made with previous studies. It is shown that the method can also be used to provide polarisation potentials for studies on electron and positron scattering by heavy atoms.

Reports new measurements of the wavelengths of the 1s2s ³S-1s2p ³P transitions in the helium-like ions of oxygen and fluorine, using a Tokamak as a light source. Results are compared with theory and it is shown that the author's results, and most of the experimental results of other groups for these transition wavelengths in helium-like ions with Z<or=10, may be brought into agreement with theoretical values if a small semi-empirical correction is applied. Possible implications of this for the theory of two-electron ions are discussed briefly.

The lifetimes of the 1s2p ³P₁ levels of helium-like magnesium and aluminium have been measured by the beam-foil decay curve technique. The 1s²-1s2p transitions were observed with a curved-crystal X-ray spectrometer. Decay curves taken under systematically varied conditions have been evaluated using several techniques including a cascade model. The results are in agreement with most of the theoretical predictions.

The Stark broadening of the Balmer line H_beta was studied in a high-current wall-stabilised arc at electron densities between 1.75 and 14.4*10¹⁶ cm^-3. The electron density was obtained by means of a two-wavelength Michelson interferometer. The reduced half-widths of H_beta were compared with the theories of Griem et al. (1962), Vidal et al. (1973) and Seidel (1977). A slight increase in the reduced half-widths with increasing electron density was observed, just as predicted by theory. The absolute values, however, were smaller than the theoretical ones. There was good agreement with the experimental data of Bassler and Kock. The results of Wiese, Kelleher and Paquette (1972) differ from the data by 5-10% depending on the electron density.

By using a perturbative treatment based on the central field approximation, the authors have calculated multiphoton ionisation probabilities in several cases of experimental interest (two-, three- and four-photon ionisation of the caesium and potassium ground states at the doubled Nd glass laser frequency and one-, two- and three-photon ionisation of some s, p, d and caesium Rydberg states). Some of these probabilities have been calculated beyond the minimum non-vanishing order. They have also performed some calculations for hydrogen. The comparison of their results with previous ones obtained by analytical methods provides a good test of the numerical techniques involved in the calculations.

A rigorous and practical time-dependent formulation of quantum decay processes is obtained by converting the Hermitian Schrodinger problem into an equivalent non-Hermitian Schrodinger equation. The properties of the original Hermitian and the reduced non-Hermitian Hamiltonians are compared and the non-Hermitian formulation is shown to be consistent with the theorem of Fock and Krylov for the description of truly decaying states. A self-consistent non-unitary algorithm is developed and used to obtain expressions for the time-dependent transition amplitudes of interest; they automatically preserve the total probability in the sum space at all future times tau >0. The formalism is coded for application to the important problem of ionisation decay of a ground-state hydrogen atom coupled to a strong radiation field with a sub-threshold photon frequency. The first quantitative information on the time dependence of two- and three-photon (resonant and non-resonant) ionisation of the hydrogen atom is obtained.

The gas-phase extinction-coefficient spectra of ten fluorosubstituted benzenes are presented and compared with benzene in the region 4.5-9.5 eV. The three singlet-singlet transitions analogous to the B_2u from A_1g, B_1u from A_1g and E_1u from A_1g transitions of benzene are presented and discussed. The appearance of the elusive ¹E_2g state is reported in 1,3,5-TFBz and two tetrafluorobenzenes, while in monofluorobenzene and o-DFBz the 3s and 4s members of the ¹E_1g Rydberg state are detected. From this work the authors conclude that they have the following order for benzene: ¹B_2u<¹B_1u<3s(¹E_1g)<¹E_1u<or=¹E_2g while the ¹A_2u( sigma , pi *) state seems to be located near ¹E_1u. They do not see any splitting of ¹E_1u in molecules of lower symmetry. The band system detected by variable-angle electron impact spectroscopy is also present in the optical absorption spectra of the heavily fluorosubstituted benzenes and its origin is attributed to a pi to sigma * transition. For hexafluorobenzene at least, they suggest that the first unoccupied orbital is sigma *.

The C² Pi -X² Pi _r transition in SiCl has been analysed rotationally. The C electronic state is a regular ² Pi state. Improved molecular constants have been obtained for the C² Pi state using the X² Pi _r molecular constants obtained in the B-X transition study.

For pt.I see ibid., vol.12, p.3005 (1979). The elastic scattering of atoms by molecules is treated by applying classical perturbation theory to scattering from an arbitrary anisotropic potential. General expressions giving the scattering angle as a function of impact parameter and molecular orientation are obtained. The problem of finding angles stationary with respect to these variables is simplified by symmetry considerations. The analysis is applied to a potential of tetrahedral symmetry. The total differential cross sections have a single peak which can be associated with particular orientations of the molecule. The scattering angle at which this peak occurs is not, however, stationary with respect to molecular orientation, in contrast with axially symmetric potentials. Body-fixed-type and in-plane approximations are also considered. While the latter is found to be excellent, the former is shown to possess serious shortcomings.

For pt. II see ibid., vol.14, p.3643 (1981). Using classical perturbation theory and the sudden approximation the authors investigate scattering in the rainbow region from potentials characteristic of atom-linear heteronuclear molecules. They find that cusps in the cross section occur at the stationary values of the deflection function these can be associated with stationary points in the potential surface. Other structures in the cross section can also be associated with certain features of the potential surface. They have investigated infinite-order sudden-type approximations and find them generally unreliable.

The one-dimensional integral representation of the inelastic scattering amplitude obtained in the curved-trajectory impact parameter method is discussed. The limit of this result when the sudden approximation is valid is investigated and a two-dimensional integral representation for the amplitude for scattering by a fixed anisotropic target is obtained. The relation with other sudden amplitudes is considered.

The linear and circular polarisation of the Li (2p to 2s) transition has been studied for 1 keV Li-He, Ne, Ar and Xe collisions as a function of the impact parameter in a photon-scattered-atom coincidence experiment. Microchannel plates were employed for position-sensitive detection of the scattered Li atoms. For He, Ar and Xe the excitation amplitudes (parameters lambda and chi ) are deduced from the measured Stokes vector. The results are compared with those from coupled-channel impact parameter calculations.

The differential cross section for electron capture in the semiclassical impact parameter treatment is written in terms of a classical elastic differential cross section multiplied by an electron transfer probability. The relationship between the phase of the transfer amplitude and the classical differential cross section is stressed. Comparison is made between the exact semiclassical result and the method described above, for the case where the internuclear interaction is purely Coulombic. The effect of electron screening in modifying the classical elastic differential cross section is also illustrated.

The authors extend the previous treatment of electron-atom scattering in the field of a low-frequency laser to the case of single ionisation. They expand in powers of the laser frequency neglecting second-order terms. They exclude the case in which there are scattering resonances in the absence of the laser and require that the energy of each of the final free electrons by several photon energies above threshold. Their result is similar to the case in which the atom is not ionised as the cross section in the presence of the laser can be expressed in terms of that in the absence of the laser, which slightly shifted initial and final momenta. The ionisation does, however, result in a change of the arguments of the kinematic Bessel function which appears in this kind of problem. The extension of their results to multiple ionisation and to inner-shell photoionisation by X-rays in the presence of a low-frequency laser field is also described.

An expression for the ionisation amplitude is obtained starting from Schrodinger's equation. Triple differential cross sections are calculated using the Born approximation with two different sets of effective charges for the outgoing electrons, and the results are compared with experimental data.

Electron impact ionisation cross sections for positive ions in the Li and Be isoelectronic sequences are calculated in the Coulomb-Born exchange and distorted-wave exchange approximations - using close-coupling wavefunctions to represent the initial and final states of the target system. The corresponding ionisation rate coefficients are also computed as a function of temperature. The results obtained are compared with experiments, with the infinite-Z method of Golden and Sampson (1979) and with the Lotz semiempirical formula. For C, N, O and Ne ions the mutual agreement is better than 25% and in some cases is of the order of a few per cent. For Fe²³⁺ and Fe²²⁺ the Lotz formula gives rates up to 40% higher than the Coulomb-Borm at high temperatures (>10⁸K), indicating that the formula probably gives an inaccurate estimate of the high-energy behaviour of the cross section for these ions.

The authors present a theory of electron-photon coincidence experiments with polarised electrons, taking spin-orbit coupling effects during the excitation into account. The influence of the initial polarisation vector on the angular distribution and polarisation of the emitted photons is discussed in detail. As an explicit example the excitation process ¹S₀ to ³P₁ in mercury is considered. Finally, the results are specialised to experiments where the scattered electrons are not observed. Using symmetry arguments and deriving additional selection rules they relate the degree of light polarisation to the initial polarisation. Two experimental results are presented.

The polarisation of molecular line radiation following threshold electron impact excitations is discussed in detail. Existing theories are summarised and the validity of various approaches to simplify the problem are assessed. A detailed comparison between experiment and theory is made for a number of lines of the Lyman and Werner systems of H₂ and it is shown that good agreement is obtained when known depolarising effects are allowed for.

Cross sections for electron impact ionisation of metastable rare-gas excimers (He₂*, Kr₂* and Xe₂*) are evaluated using a binary encounter method with orbital speed distributions determined by the appropriate molecular orbital. Gaussian basis sets are calculated for He₂* and Kr₂* and potential energy curves are provided for Kr₂*. The resulting ionisation cross sections are of order 1*10^-15 cm² and are about a factor of two higher for Kr₂* and Xe₂* than for He₂*. The cross sections for Kr₂* and Xe₂* are between 50% and 40% higher than the corresponding metastable atom cross sections.

The Hara (1967) and asymptotically adjusted versions of the free-electron-gas exchange (FEGE) approximation are compared with the orthogonalisation technique for electron scattering by CH₄ and H₂O. In general both FEGE approximations are more attractive than the orthogonalisation approximation but there is evidence that the asymptotically adjusted version overestimates the exchange effects at low scattering energies.