Table of contents

The transient dynamics of a two-level atom in an amplitude-modulated laser light has been examined. The remarkable effect of constant intensity of the scattered light, in spite of the amplitude modulation of the incident light, is discovered. The effect of dissipation on the system is examined and a connection with known results in the steady-state domain is made.

Photoabsorption cross sections for the 8s' and 6d' autoionisation resonances of the krypton atom have been measured photoelectrically with a photon bandwidth of 7*10^-3 AA using the high-resolution spectrometer. The cross sections are parametrised with the aid of a lineshape formula which is based on multichannel quantum-defect theory and has a form analogous with the Fano resonance formula.

The asymmetry parameter beta of the photoelectron angular distribution of the process HI(5 pi ⁴) ¹ Sigma ⁺ ( upsilon "=0)+hv to HI⁺(5 pi ³) ² Pi _3/2 ( upsilon '=0)+e^- was measured at photon energies from 86440 to 87000 cm^-1. The data show resonance structure due to autoionisation that was not resolved before. The results are compared with predictions of an ab initio calculation by H Lefebvre-Brion and co-workers (1985).

By introducing wavefunctions consistent with the long-range nature of the Coulomb potential instead of conventional plane waves the authors show that the Lippmann-Schwinger equation for the Coulomb problem exists on the energy shell. The corresponding T matrix gives the well known exact result.

The authors report measurements of the P₁ coherence parameter for excitation of the spin-orbit coupled ns'(1/2)₁⁰('¹P₁') state of Ne (n=3) and Ar (n=4) by unpolarised electrons of intermediate energy (30-100 eV) which are subsequently scattered in the forward direction. The authors' experimental results verify the theoretical prediction of P₁=+1 when angular resolution effects are properly taken into account. This result clarifies the previous discrepancy between experiment and theory and demonstrates unambiguously that the excitation process under these conditions proceeds via the direct excitation of the LS-coupled singlet contribution of the excited states, i.e. via transfer of orbital angular momentum.

The authors propose a new method of solving the Schrodinger equation for a quantum system enclosed in a box with infinite potential walls. The method combines the variational technique with boundary perturbation theory and can be applied to a general non-separable case. When the Schrodinger equation for an enclosed system separates into an ordinary differential equation, the method gives the exact energy and wavefunction. As an application of the method the authors obtain the ground-state energy of the hydrogen atom placed off-centre in a spherical cavity. A generalisation of the variational boundary perturbation technique for finite potential walls is suggested.

The perturbation approach to the Dirac-Coulomb equation recently developed by the present authors is applied to the calculation of the energy correction at the first- and second-order level for a zero-order Hamiltonian taken as a symmetric sum of Fock operators. Results for several He-, Be- and Ne-like systems (up to Z=80) as well as for Zn²⁺ are presented and discussed.

Accurate values for the energies and linewidths of the 5dnf J=5, 4 (n=4-50) autoionising Rydberg series of neutral barium have been collected using either a single-mode CW ring dye laser or a Nd:YAG pumped pulsed dye laser and a beam of neutral atoms in the metastable 5d²¹G₄ and 5d²³F states. The interactions between series within the configuration converging to the 5d_3/2 and 5d_5/2 ionisation limits as well as the continuum interactions (for J=5) have been analysed using multichannel quantum defect theory (MQDT) in the 'phaseshifted reaction matrix' formalism. Good agreement between experiment and MQDT calculations is obtained; however, not all interaction parameters could be determined from the available data.

Spin polarisation of Auger electrons arising from photoionisation of atoms by circularly polarised photons is studied theoretically. Calculations show that in a wide range of photon energies the polarisation is large and can be measured in experiment. It is shown that the polarisation transfer can be used to study the dynamics of the Auger decay and in particular the influence of many-electron correlations on the Auger transition amplitudes. It is suggested that the polarisation transfer be used for measurements of the circular polarisation of X-rays.

The emission spectrum of the B ³ Pi _g to A ³ Sigma _u⁺ first positive system of N₂ has been studied under high resolution to investigate the predissociation occurring in the B ³ Pi _g state above the upsilon =12 level. The upsilon =13 and 14 levels are most strongly affected and detailed rotational analyses show that the process is not uniform and that in particular the ³ Pi ₀^- component of B ³ Pi _g is predissociated only weakly and in a J-dependent manner. The predissociation is interpreted by correlating the levels of both the ³ Pi _g state and the predissociating ⁵ Sigma _g⁺ state with their equivalent states in the Hund case (c). The ³ Pi ₀⁺, ³ Pi ₁ and ³ Pi ₂ components of the B ³ Pi _g state are homogeneously ( Delta Omega =0), and hence relatively strongly predissociated by the O_g⁺, 1_g and 2_g case (c) limiting equivalents of the ⁵ Sigma _g⁺ state. The ³ Pi ₀^- component (O_g^- in case (c)) can be predissociated only by the 1_g component of the ⁵ Sigma _g⁺ state and hence the predissociation, being heterogeneous ( Delta Omega =+or-1), is relatively weak and J dependent. This interpretation fully explains the occurrence of transitions from the ³ Pi _g^- levels and their decrease in intensity with increasing J and is in keeping with the interpretation of analogous behaviour in the PH molecule.

A quantum-defect theory of the Stark effect given recently by Sakimoto (1987) is applied to study the effects of an electric field on highly excited states of molecular hydrogen. The Stark energy levels are calculated. Also investigated are field effects on photoionisation through rotational autoionisation.

In charge exchange collisions of Ar⁷⁺ with He at 1.75 keV amu^-1, electron capture selectively populates the n=4 shell. Ar⁶⁺ (n=4) then decays radiatively. A full analysis of the spectrum observed in the VUV range 100-1000 AA allows identification of a series of unknown transitions including two-electron-one-photon transition electric dipole lines, with upper levels 3s4p and 3s4d. Their branching ratios are compared with calculated values. It is thus possible to deduce that they are mostly due to cascade paths from the initial level of direct capture rather than to a combination of excitation and capture.

Static exchange (plus correlation-polarisation potential) calculations are performed for elastic low-energy electron scattering from Be, Mg and Ca atoms with attention to the low lying ²P and ²D shape resonances. The correlation potentials are calculated with and without a scaling factor. The scaling factor is adjustable as the alpha parameter in the X alpha theory, and is adjusted to produce the ²P resonance energy of e-Mg scattering in agreement with the experiment. The predictions with the parameter-free correlation-polarisation potential are a ²P resonance at 0.15 eV with width of 0.19 eV for Be, a ²P resonance at 0.065 eV with width of 0.05 eV for Mg and a ²D resonance at 1.5 eV for Ca with width of 3.0 eV. The ²P resonances of Be and Mg and the ²D resonance of Ca are produced at 0.24 eV, 0.14 eV and 1.75 eV with widths of 0.33 eV, 0.17 eV and 3.2 eV respectively, by using the scaled correlation potential. No ²P resonances are produced in the calculations for e-Ca scattering using either the parameter-free or scaled correlation potential, i.e. the authors give evidence for the existence of a stable negative ion of Ca in the 4s²4p ²P state by using the theory used for electron scattering.

The ratio of distances of consecutive resonances in H^- converging to the nth threshold are determined for n<or=12 and L<or=6 by diagonalising the long-range centrifugal plus dipole potentials. The resonances for ¹S^e scattering converging to the n=5 threshold are calculated using a 15-state R-matrix expansion and are arranged into two series using the theoretically predicted ratios.

The polarisation fractions of radiation emitted following electron impact excitation of the n¹P (n=2-6) and n¹D (n=3-6) states of helium have been calculated using first-order many-body theory over the energy range from threshold to 500 eV. The predicted polarisation fractions are essentially independent of the principal quantum number (n), in agreement with the experimental results of Standage for n¹P (n=2, 3) and the Born results of Vriens and Carriere, but in apparent disagreement with the multichannel eikonal results.

Double ionisation of the noble gases Ne and Ar has been studied using beams of low-energy electrons and positrons. The results show that, except near threshold, the ratio of the double- to single-ionisation cross sections, R⁽²⁾, is greater for electrons than for positrons, with, at high velocities, the former merging with antiproton data and the latter with the proton values. These data confirm that the double-ionisation cross section is dependent upon the sign of the charge of the projectile.

Cross sections for the ionisation of Ca²⁺ by energy-resolved electrons have been measured for electron energies between 21 and 50 eV. Considerable structure was resolved in the ionisation function. At energies where autoionisation is dominant, the cross sections differ substantially from the results of recent calculations by Burke et al. (1983), Griffin et al. (1984) and Pindzola et al. (1987). The experiment indicates that many more transitions are significant than have so far been included in theory. Comparisons are also made with energies of autoionising levels deduced from theory, from ejected-electron spectra of neutral calcium and from the photoionisation of Ca²⁺.

Absolute differential cross sections for the electron impact rovibrational excitation of the upsilon =2 and upsilon =3 levels of HF and HCl have been measured in the energy range from threshold up to several eV in the angular range 15-135 degrees . Threshold peaks are found which show strong rotational excitation with non-isotropic angular distributions. In the upsilon =3 excitation function of HF a resonance structure occurs below the cusp of the opening of the upsilon =4 channel. This is the first experimental proof of the existence of nuclear-excited Feshbach resonances. In the case of HCl the threshold peak in the upsilon =3 excitation functions indicates the existence of a second non-dissociating negative-ion state at low energies.

The standard theory of collision broadening of a spectral line involving photon emission ignores photon recoil. An extension of the theory is considered for the case when the emission involves a heavy particle, implying large recoil. This development is required for collision broadening of resonances formed by the combination of (t mu ) atoms and D₂ molecules in meso-molecular studies. The small recoil in the photon case is shown to give negligible modification to the standard theory.

It is shown that while detecting positronium 3 gamma -decay quanta in a magnetic field one may observe counting rate oscillations at a frequency of triplet positronium level splitting. The oscillations may be observed in detecting the decay quanta by both three or one (two) counters. Based on this phenomenon it is possible to realise a novel matter research method, similar to the well known muonium spin rotation method.