Table of contents

The CASSCF and contracted CI methods have been used to calculate the potential curves of the X³ Pi , B³ Sigma ⁺ and C³ Pi states of the so far unobserved SiC molecule. The transitions between the ground state and the B and C states are predicted to fall in the wavelength range 4000-6000 AA. The Franck-Condon factors for the transitions and molecular constants for the X, B and C states are given.

When multiphoton ionisation occurs towards multiple continua, electrons with various energies are ejected. The author uses a simple model to describe space charge in order to show how the electron energy spectrum can be perturbed. In particular the author explains the complete vanishing of the first peak observed for strong intensities.

A simple mechanism responsible for H⁺-H^- associative ionisation is proposed which selects the (2p sigma _u)²¹ Sigma _g⁺ configuration state as the autoionising state relevant to this process. According to results on mutual neutralisation, the HH¹ Sigma _g⁺ and B"B¹ Sigma _u⁺ adiabatic states of H₂ are assumed to be the way of entering the transition region. Quantal calculations have been performed including all dissociation and ionisation channels. The calculated cross section is found to be in agreement with the previous experimental results of Poulaert et al. (1978).

It is shown for the first time that the theoretical model for the collisional l mixing in atomic intermediate Rydberg states proposed by de Prunele and Pascale (1979) can also be applied to J mixing in the intermediate Rydberg states with higher quantum defect delta _l values and not only to collisions with noble gas atoms and N₂ but also with ground-state alkali atoms. The model is applied to the calculation of J-mixing cross sections of nD Rydberg states of Rb (n=7-10) and Cs (n=9-23) perturbed by noble gas atoms and N₂. J mixing in Cs(nD)-Cs(6S₁2/)(n=11-23) collisions is also considered. The agreement with the available results is rather good.

Simple arguments are used to show that radiative decay causes negligible modification of near-threshold inelastic electron-ion cross sections.

The properties of the close lying 4f⁶6s6p ⁷G₁ and 4f⁶5d6s ⁷G₁ states in Sm were examined as a possible system for an experiment on atomic parity non-conservation. The authors find that the electron-electron matrix element is appreciably smaller than the Coulomb-induced electron-nucleus interaction. An estimate of the appropriate M1 transition element to the 4f⁶6s²⁷F₀ ground state suggests optical rotations as large as 10^-4 rad at one absorption length. However, in practice, the background from the allowed E1 transition will reduce the observable effect. An alternative scheme involving the electric-field-induced amplitude is suggested.

The inverted-term structure of the octet and sextets of f⁷ compared with that of the singlets and triplets of f² is related to the interchange of spin and quasispin. This interchange is formalised by an operator R. The R classification of the annihilation and creation operators for the electrons in an atomic l shell leads to several new Lie groups, among which is SO(6l+3). Racah's Coulomb operators e_i are broken up into their R-symmetric and R-antisymmetric parts. The limited nature of the generalisation of the term inversion to other f^N configurations is related to selection rules on the irreducible representations of SO(7) and G₂.

The effective-bounds formulae for dipole transition moments introduced by Anderson and Weinhold (1974) have been strengthened significantly without altering the approximate wavefunctions by two procedures, each of which modifies the transition operator. Results of simple calculations on the hydrogen atom suggest that highly accurate moments may be obtained from wavefunctions of only modest accuracy.

The authors analyse the motion of a classical hydrogen atom in a resonant electric field and obtain a criterion, based upon the resonance overlap theory, for the strength of the field required to produce ionisation: the dependence of this upon I_m, the angular momentum in the field direction, is obtained. For I_m=0 they present preliminary numerical results that are in reasonable agreement with theory. An averaging approximation is used to show that for weak fields the system decouples into two one-dimensional systems. The authors show, by direct comparison with numerically integrated orbits, that this decoupling is accurate for moderate field strengths; but they also show that the dynamical picture suggested by this averaging approximation is not correct in some regions of phase space. A tentative explanation of this difference is given.

The transition 6p³⁴S_3/2-6p³²D_3/2 at 876 nm in atomic bismuth has been studied by laser spectroscopy. The absorption spectrum and the Faraday rotation spectrum have been measured over the frequency range occupied by the hyperfine structure, giving the following values for the interaction constants: A(²D_3/2)=-1226.7(1.2) MHz, B(²D_3/2)=-617(10) MHz. From the Faraday rotation spectrum the ratio of the reduced electric quadrupole and magnetic dipole matrix elements, (⁴S_3/2//( omega /4 square root 3)*(-2er²C⁽²⁾)//²D_3/2)/(⁴S_3/2// mu //²D_3/2), was found to be 0.117(8). When the interaction constants are combined with data for the other levels of the 6p³ configuration, there is disagreement with the parameterisation of the fine and hyperfine structure given by Dembczynski et al. (1975, 1983).

The G0_u⁺ state in Hg₂ 'excimer' molecules (correlated with the 6¹P₁ atomic state) was populated by stepwise excitation through the metastable A0_g⁺ state, and the fluorescence emitted in the 0_u⁺ to X0_g⁺ decay was recorded with a spectrometer and a transient digitiser. The time evolution of the fluorescence indicated a very short ( approximately=1 ns) lifetime of the 0_u⁺ excimer. The band corresponding to the v"=0(A0_g⁺) to v'=0(G0_u⁺) transition was identified in absorption and the energy separation of the two potential wells was determined as was the wavelength of the UV fluorescence emitted in the decay of the v'=0 level of the G0_u⁺ state to the repulsive X0_g⁺ ground state. A determination was made of the vibrational frequency omega '_e and the anharmonicity omega '_e chi '_e of the 0_u⁺ state.

The absorption profiles of the far wings of rubidium 5S to 6S and 5S to 4D forbidden transitions in the presence of a rare gas perturber (argon or helium) are experimentally investigated. The quasistatic interpretation of the temperature dependence of the profiles gives information about the excited state potential curves and the oscillator strengths of the various transitions. The experimental results are compared with calculated potentials and, particularly in the case of helium, with the l-dependent pseudo-potentials available for alkali-helium pairs.

Vibrationally and rotationally resolved cross sections and asymmetry parameters for the photoionisation of H₂ by 584 and 304 AA radiation are studied for the vibrational states of H₂⁺ from 0 to 12 (14 for the 304 AA line). Vibrationally summed cross sections ( sigma ) and vibrationally averaged asymmetry parameters ( beta ) are evaluated from these results. The calculated ( sigma ) agrees well with the experimental data. The values of ( beta ) are 1.93 and 1.86 for the 584 AA and 304 AA lines, respectively.

The dependence of the parameters (width, intensity and shape) of the cusp in the processes of electron capture to the continuum (ECC) and electron loss to the continuum (ELC) has been studied as a function of impact velocity (from 2 to 5 au) for the simple collision systems H⁺-He, He⁺-He and He²⁺-He. An approximately linear increase in the width (FWHM) is found in both cases. There is no difference in this respect within the error limits for He²⁺ (ECC) and He⁺ (ELC). Regarding cusp shape, similar symmetry (ELC) and asymmetry (ECC) have been observed as in the case of heavy-ion projectiles. None of the theories satisfactorily describes the experimental absolute cross sections of ECC cusp (He²⁺), while in the case of the He⁺ projectile (ELC) rather good agreement with theory can be seen in the higher-velocity region. The deviation at lower velocities in the latter case could be explained by an ECC effect present even in the case of He⁺. Concerning the dependence of the cusp yield on the Z_p value of the projectile, Z₀^2.5+or-0.3 was obtained from the yield in H⁺-He and He²⁺-He collisions.

The triple differential cross section for the ionisation reaction e^-+He(1¹S) to He⁺(1s)+2e^- is analysed in the case of the coplanar, asymmetric geometry within the framework of the eikonal-Born series theory. Detailed calculations are performed for incident electron energies of 256 and 600 eV, and compared with recent absolute measurements. The authors find that second-order effects are crucial in explaining both the position and magnitude of the binary and recoil peaks.

Double differential cross sections for electron impact ionisation of helium have been measured in a crossed-beam experiment for primary energies E₀ of 100, 200, 300, 400, 500 and 600 eV. Angular dependences of the cross sections are presented for the scattered (primary) as well as for the ejected (secondary) electrons. The energies E_B of the ejected electrons were chosen to be 2, 4, 10, 20 and 40 eV. The cross sections have been put on an absolute scale by extrapolating the generalised oscillator strength for the primary electrons to the optical limit. The authors also used the fact that the integrated cross section of a scattered electron of a given energy loss E_O-E_A corresponds to the integrated cross section of a secondary electron with kinetic energy E_B=E_O-E_A-IP where IP is the first ionisation potential of He, when double excitations and ionisation are ignored. Cross sections for double excitations and ionisation of He by electron impact are negligible (<2%) compared with single excitation and ionisation cross sections.

Low-energy inelastic e-Hg collision calculations are carried out using the relativistic R-matrix method based on the Breit-Pauli Hamiltonian. It is found that including the mass correction term and the Darwin term does not significantly improve the results obtained including just the one-body spin-orbit term in the Hamiltonian. However configuration interaction between the 6p²6d and the 6s6p² configurations must be included to obtain the observed energy differences between the (6s6p²)⁴P_1/2,3/2,5/2 and the (6s6p²)²D_3/2,5/2 resonances. It is confirmed that calculations where the (6s6p²)²D_3/2 resonance occurs at an incident electron energy of 5.2 eV give good agreement with experiment.

Ozone yield is a good indicator of atomic oxygen formation in gas discharges. Through exact measurement of the ozone formed in a silent discharge per unit energy it is possible to determine the rate coefficient for electron impact dissociation of molecular oxygen and the efficiency of the dissociation process. The agreement of the measurements with calculations is within 30%.

The total ionisation cross sections of the lithium atom by positron impact are calculated by using two distorted wave models. The lithium atom is considered to be a one-electron atom. The results using the first Born approximation (FBA) have also been reported. The distorted wave results differ very little from those of the FBA.