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The theoretically predicted 1s²2p³⁴S-1s²2s2p²⁴P optical transition in Be^- has been observed experimentally. The ⁴S state decays at 2653.01+or-0.09 AA to the metastable ⁴P state with tau =1.25+or-0.10 ns. Thus the electron affinity for Be(1s²2p²)³P is 4.2 meV larger and not 10 meV lower than the electron affinity for Be(1s²2s2p)³P as predicted by the most accurate electron correlation calculation available.

Xenon was ionised by 100 fs UV pulses. The electron spectra showed a decreasing energy of the photoelectrons with increasing light intensity. No resonances were observed in the electron spectrum. The absence of resonances is understood in terms of the ratio of the pulse duration and the characteristic time (2 pi n³ in au) of the intermediate state. From the observed spectra it appears that the four-photon ionisation of xenon at 309 nm is not saturated at an intensity of 1.5*10¹³ W cm^-2.

A phenomenological correspondence between model potential theory and pseudopotential theory is given in the framework of supersymmetric quantum mechanics.

The angular distribution of fast delta electrons from K-shell ionisation is discussed within the framework of the impact parameter Born approximation (SCA theory). Analytical expressions for the triply differential cross section are derived based on simplifying the peaking approximation. These expressions are easily evaluated and the structures in the momentum distributions of the ejected electrons are discussed. The most dominant feature appears to be the so-called binary ridge, the shape of which shows only a weak dependence on the impact parameter of the collision.

Spectra from plasmas produced by focusing the output of a Q-switched ruby laser onto planar targets of metallic strontium have been photographed in the 40-500 AA region. Lines arising from 4p-5s transitions in Sr VI and Sr VII have been identified. From the line profiles it is possible to estimate the mean kinetic energy of these particular ionic species in the plasma.

The authors report on calculations of energy levels and electric dipole radiative transitions in O III. They obtain wavefunctions for bound and continuum states by solving coupled integro-differential equations in the close-coupling approximation employing two independent but equivalent numerical methods: the R-matrix method and the linear algebraic method. A detailed comparison of the two methods is made as a confirmation of the accuracy and consistency of the calculations for the Opacity Project. They include eight states of the O IV target with configurations 2s^x2p^y(x+y=3) and represent these by configuration interaction wavefunctions. All bound states of O III of configurations 2s^x2p ^ynl with n<or=10 and l<or=3 are calculated. Oscillator strengths for transitions between, and photoionisation cross sections from, these bound states have been obtained.

Strong Zeeman quantum beat signals have been observed in laser-induced fluorescence from atomic vapours of Zr and Yb generated by cathodic sputtering in a low-pressure rare-gas discharge. The results have been used to determine Lande g factors for ten levels of the z ³F^o, y ³F^o and y ⁵D^o terms of Zr I and cross sections for the collisional destruction of alignment of Yb I 6³P^o₁ in He, Ne, Ar, Kr and Xe.

The authors analyse the effect of a high-frequency harmonic field on a one-dimensional system whose natural frequency is a decreasing function of the classical action. The show that for weak to moderate fields the classical and quantal excitation mechanisms are quite different: the quantal motion is dominated by relatively few 'quasiresonant' states so it cannot follow the diffusive or chaotic classical motion. They approximate the motion near the initial state by a standard map and show that the system is characterised by an effective Planck constant which needs to be very small for classical dynamics to be valid. They apply this analysis to the one-dimensional hydrogen atom and compare calculations performed using a full basis and the quasiresonant basis and show that there is good agreement.

A master equation is developed, analogous to the Lehmberg-Agarwal equation, for the study of electric quadrupole transitions in atomic systems. This equation allows one to study the evolution in time of a system of atoms interacting with and via the radiation field in terms of the average values of atomic operators only.

By focusing an excimer pumped dye laser on an atomic beam, the authors have observed multiphoton multiple ionisation of the neutral strontium atom in the tuned wavelength range 532-41 nm. Sr⁺ and Sr⁺⁺ yields were measured with a time of flight spectrometer. At least three photons are required to produce Sr⁺ and eight photons to produce Sr⁺⁺. As a function of the wavelength, the Sr⁺ yield exhibits resonances which correspond to two-photon excitation of bound states (5s²-5p²¹D₂ and 5s²-5p²¹S₀) and three-photon excitation of autoionising states 4d5f J=1 and J=3 of the neutral atom. The Sr⁺⁺ yield exhibits four broad smooth resonances that they interpret, from known spectroscopic data, as two-photon (5p_1/2-4f), three-photon (5s-6p_{1/2, 3/2}) and four-photon (5s-7d) resonances in the singly charged ion. The latter transitions start from a singly charged ion in the 5s or 5p_1/2 state reached after three- or four-photon ionisation of the neutral ground state. The variation of the Sr⁺⁺ pattern as a function of the dye laser beam intensity shows that resonances are shifted and broadened when the intensity is increased. These features have been interpreted by a perturbative treatment of Sr⁺ ionisation based on the use of an effective Hamiltonian which takes into account higher-order terms involved in the calculation of resonance effects. Calculated multiphoton ionisation probabilities are in good agreement with experimental data.

All independent geometric factors are derived for the possible polarisation dependence of optical activity and linear birefringence effects bilinear in a static electric and magnetic field, for fluids. The extension to oriented systems is illustrated. Differences between the methods used and results obtained with those of earlier workers are noted and where possible resolved. Several optical effects predicted to exist by earlier workers are shown to be forbidden, and vice versa. In particular the Jones birefringence should appear in E1/M1 coupling for all fluids, irrespective of the centrosymmetric or non-magnetic character of the system, and there exist field-dependent refractive index contributions which reverse with the direction of the light beam and are detectable by a ring laser.

The L² solution for a model two-channel problem is derived employing an L² discretised Schrodinger equation. In the elastic scattering region, the finite-basis L² solutions can be identified after a renormalisation, with the true scattering functions, in the same manner as for single-channel models. For energies above the inelastic scattering threshold it is shown that this renormalisation procedure is no longer valid. It is pointed out that the difficulty can be resolved by use of Lippmann-Schwinger techniques for the finite basis. The implication of the results for the construction of regularised optical potentials is discussed.

The mixing of state multipoles for an atomic state with unresolved fine structure in a magnetic field is investigated in its simplest case: the conversion of alignment to orientation. The combined action of the external field and the spin-orbit coupling results in a build-up of orientation and hence in the emission of circularly polarised light although the excitation process is axially symmetric and is invariant with respect to the reversal of the symmetry axis and thus the system could only show alignment initially. The case investigated is the collisional excitation of the He 3³D state by He⁺ ions. After correction of some errors in the theoretical literature the agreement between the calculated and the observed circular polarisation of the fluorescence radiation from the collisionally excited He atoms is very good. Owing to possible objections in two previous experiments this is the first unambiguous demonstration of the alignment to orientation conversion effect.

Angularly resolved (0-0.15 deg) energy gain measurement provides a detailed analysis of the dominant processes involved in low-energy charge transfer reactions of O⁶⁺ and N⁶⁺ on a He target. Influence of short distance interactions for population of minor one-electron capture channels is demonstrated. Special emphasis is given to two-electron capture processes. The authors' data are compared with available theoretical and experimental results concerning C^6-, N⁶⁺ and O⁶⁺ ions colliding with a He target.

The absolute total electron scattering cross sections for NH₃, OCS and N₂O were measured using a linear transmission technique. The presented results cover the range of impact energy from 1 to 80 eV for NH₃ and from 40 to 100 eV for OCS and N₂O. The most prominent feature of the cross section for NH₃ is the very broad hump centred at about 10 eV with a maximum value of 23.5*10^-20 m². Above 10 eV the present results on NH₃ are in excellent agreement with recent calculations of Jain (1988).

Absolute measurements have been made of cross sections for the ionisation of Li²⁺ ions by electrons with energies which range from threshold (122.45 eV) to 1000 eV. The results are in good agreement with available distorted-wave exchange calculations. The scaling behaviour of ionisation cross sections along the hydrogen isoelectronic sequence is discussed.

The ortho-positronium, o-Ps, quenching reactions promoted by (NH₄)₂(Ce(NO₃)₆) and (Gd(H₂O)₆)Cl₃ were studied in aqueous solutions at 278, 288, 298 and 308 K. The results obtained, together with those reported in a previous paper of the authors', suggest that the effective reaction radii of 3d and 4f complexes in o-Ps quenching reactions depend on the reaction types. In particular, the effective reaction radii are longer for o-Ps oxidation (Ox) reactions (Ce⁴⁺, Fe³⁺, CrO^2-₄) than for Ps spin-exchange (SE) reactions (Gd³⁺, Fe²⁺, Cr³⁺). As a consequence it is suggested that the Ox reactions occur by 'hopping mechanism', so that effective and crystallographic radii of these ions in Ox reactions are practically equal even if the Ps electron orbital and metal-accepting orbital do not significantly overlap at their closest distance. On the other hand the effective reaction radii of SE reactions, shorter than the crystallographic ones, seem to imply that the magnetic interaction promoting the SE reactions studied here occurs between distant localised electrons. The results are discussed with reference to the Smoluchowski equation which relates the rate constants of diffusion-controlled reactions to reactant radii.