Table of contents

Discusses the optimal choice of the basis for calculations of inner-shell excitations in the collision of heavy ions. Starting from a variational principle which minimises the time-integrated coupling strength, a relation between a model two-centre distance and the actual internuclear separation is derived. For 1s sigma excitations in Pb-Pb collisions it is found that the optimal basis is almost identical to the adiabatic one. For higher quantum states the adiabatic treatment becomes less adequate.

Discusses a non-perturbative ab initio theory for evaluating electron transfer cross sections from the individual subshells of multi-electron atoms by heavy projectile in the keV/amu to the MeV/amu energy region. By employing a two-state two-centre atomic eigenfunction expansion and an independent-electron approximation with a realistic Herman-Skillman potential for the target atom, the charge transfer cross sections from the individual subshells of argon atoms by protons have been calculated.

Earlier coupled-channels calculations on proton-argon collisions are extended to include 21 target-centred d functions in the basis along with the 27 s and p states used before (see ibid., vol.12, no.17, 1979). The range of impact energies is increased to 1-12 MeV. For K-shell ionisation, coupling the d states to the s and p states has only a small effect. However, the coupling to intermediate d states has a large effect for charge transfer.

The H₂^-2 Pi _u resonance is used in an estimation of the cross section for dissociative electron attachment (DA) to the metastable c³ Pi _u state of molecular hydrogen. The cross section is found to be substantially larger than that for DA to the low vibrational levels of the ground electronic state X¹ Sigma _g⁺. It appears that DA to c³ Pi _u could play an important role in the production of the relatively high densities of negative ions in low-density hydrogenic plasma.

Accurate calculations of several electronically excited states of H₃ which are important in dissociative recombination of H₃⁺ are reported. These surfaces allow discussion of the dissociation products. For electron energies above 1 eV the dominant product is expected to be vibrationally excited H₂ (X¹ Sigma _g⁺)+H(2s or 2p). The threshold for directly producing H^- from ground state H₃⁺ is predicted to be 5.8 eV.

Using a simple model it is demonstrated that the gradient of a graph showing the rate coefficient for mutual neutralisation as a function of the number density of the ambient gas tends to infinity as the number density tends to zero.

Oscillator strengths determined from single-configuration relativistic Hartree-Fock (RHF) calculations are reported for the lowest np²P_1/2,3/2 to (n+1)s²S_1/2 and np²P_1/2,3/2 to nd²D_3/2,5/2 transitions in Ga I, In I and Tl I. Core-polarisation effects are included in the RHF calculation by introducing a polarisation potential in the one-electron Hamiltonian as well as by employing the corresponding correction to the dipole moment operator in the transition matrix element.

The 3³S₁-3³P_0,1,2 transitions in helium-like oxygen have been observed in a foil-excited beam using a 'Doppler cancellation' light-enhancement technique applied to a grating spectrometer. The wavelengths of these transitions were measured at oxygen beam energies of 13.5, 18.0 and 23.5 MeV. The results are compared with calculations of the fine-structure splitting and with estimates of the Lamb shift of the 3³S and 3³P levels.

A theoretical study of cascade quantum-beat intensity profiles is presented for the case of fine-structure beats. An explicit expression for the beat profiles, in the form of a multipole expansion, is derived using the graphical methods of angular momentum theory. The calculations take into account the possibility for the initial state being polarised. Analogous results for zero-field hyperfine-structure beats are listed.

A fully quantum mechanical theory of cascade effects on Zeeman quantum beats is presented. The work is mainly concerned with the case of selective optical excitation of the upper level of the cascade, but beam-foil excitation is also considered. The analogous steady-state cascade process in a magnetic field is considered as well, focusing on the polarisation of the cascade light. Calculations are made for specific cascade processes in Ar II.

For pt.II see ibid., vol.11, no.11, p.1921 (1978). The problem of a hydrogen atom perturbed by a uniform electric field F is analysed. The stability of the level components with the same total quantum number n is investigated. In the limit of small F, energetically lower sublevels are more stable. However, when F increases, this well-known rule is violated for some sublevels at large n. A semiempirical formula for the ionisation probability, which is valid for any arbitrary quantum states, is suggested and discussed.

Describes use of the quantum-beat method to study hyperfine structure in the 5²P_3/2 and 6²P_3/2 states of ²³Na. A pulsed dye laser, frequency-doubled into the UV region, was used to excite sodium atoms abruptly in a beam. The fluorescent light was recorded with a fast transient digitiser, interfaced to a micro-computer. Theoretical calculations using many-body perturbation theory were performed for the entire P sequence measured so far, taking polarisation and correlation effects into account separately. Very good agreement between experimental and theoretical values was obtained.

The theory of resonant Raman scattering in the presence of intense incident and scattered light waves has been developed, using the quantum-mechanical master-equation approach. When the optical matrix elements between the resonant intermediate state and the initial and final states of the atomic system are larger than the relevant relaxation frequencies, it is shown that instead of the usual power broadening and shift of the Stokes or anti-Stokes line, each line can split into as many as five or seven lines, depending upon whether both the fields are exactly at resonance or otherwise.

The equations which permit the calculation by means of regular operations, of multiphoton photoionisation cross sections and the dynamic polarisabilities in an intense classical long-wave electromagnetic field, are considered for a hydrogen atom. The calculations have been performed for a circularly polarised field. A quantitative expression has been derived for the Lamb shift analogue which can be verified experimentally.

Approximate bounds for the phaseshift for non-zero-energy s-wave scattering problems are obtained. The model used is a simple extension of the theory for the zero-energy case.

The concept of pairwise additivity is extended to include one degenerate atom interacting with an arbitrary number of rare-gas atoms. The interaction is written as a matrix of dimensionality equal to the degeneracy. Adiabatic polyatomic energy surfaces are obtained by diagonalising the interaction matrix. The energy surfaces are compared to SCF calculations for the case of a lithium atom interacting with two helium atoms. Formulae are given for the addition of spin-orbit coupling.

The quenching cross sections of various highly excited states of rubidium have been measured for different perturbing gases (He, Ar, Xe, ground-state rubidium). The quenching of the hydrogenic nF states is shown to be mainly a l-mixing process. The behaviour as a function of n (9<or=n<or=21) and the magnitude of the l-mixing cross sections seem mainly to reflect the range of interaction between the highly excited electron and the perturber.

When He⁺ and Ne⁺ are injected into an Ar carrier gas at 300K using a selected-ion flow-tube apparatus, Ar⁺ and Ar₂⁺ are observed as reaction products. By observing the ratio of the sum of Ar⁺ and Ar₂⁺ concentrations to that of Ne⁺ as a function of the Ar pressure it has been possible to show that the rate-determining step for this reaction is a two-body process (Ne⁺+Ar) and to derive the charge-transfer rate coefficient.

The differential cross sections for the e-CH₄ elastic scattering at 205, 300, 410, 500 and 820 eV incident energies are calculated using the first Born, the eikonal and the two-potential approximations including the polarisation and exchange effects and are compared with the available experimental data and other theoretical results.

Detailed measurements of electron impact detachment cross sections have been made using inclined beams of electrons and either C^- or O^- ions. In the case of O^-, two structures were observed in the detachment function at energies of approximately 19.5 and 26.5 eV. No structure was found for C^-.

For pt.I see ibid., vol.12, no.12, p.1913 (1979). Electric dipole polarisabilities and electron-molecule static and adiabatic polarisation potentials for Li₂ are calculated using ab initio molecular orbital theory. Six different basis sets of contracted Gaussian basis functions, some including bond-centred functions, are employed Polarisabilities have been calculated at three internuclear distances. The dependence of the adiabatic polarisation potential on the distance to the electron and on molecular orientation is examined.

Considers the relationship between the impact, Born and 'straight-line path' approximations and the overlap of the regions of applicability. Particular emphasis is given to a perturbative fully classical model of the electron motion. It is shown that the perturbative fully quantum-mechanical approach (Born approximation) leads to a formally similar result. The impact approximation is shown to exist as an asymptotic limit. These results are used to study the role of cut-offs for Coulomb scattering.