Table of contents

The perturbation of an autoionizing series by a broad, intruding window resonance is treated within the framework of analytic K-matrix theory. Explicit formulae are given for lineshapes; symmetry reversals and width fluctuations are studied, and interesting properties of the resulting profiles are illustrated and discussed.

The oriented and aligned metastable 6²P_3/2 thallium atoms produced in photodissociation of TlBr at 266 nm have been observed. The effect has been registered by a variable dichroism of the atomic vapours which is caused by a precession of the angular momenta in an external magnetic field. The application of the effect to study the dynamics of TlBr photodissociation has been discussed.

Partial cross sections were measured for the ionization Ar+e to Arⁿ⁺+(n+1)e for n=2-5 as a function of electron energy (0-660 eV). Although recent investigations are in agreement with regard to the absolute cross sections for single ionization, large discrepancies still exist for multiple ionization. The results support the measurements of Stephan et al (1980) and Wetzel et al. (1987), but disagree with other recent studies which invariably obtain larger cross sections. These discrepancies are attributable to the dependence of electron multiplier gain on ion impact energy and, therefore, ion charge. The cross sections presented for Ar⁴⁺ and Ar⁵⁺ have not been previously measured.

The virial relations for the Dirac equation are generalized for the case of the non-diagonal radial matrix elements and for the case when the author integrates over an arbitrary finite interval of the radial variable. The case of the Coulomb field is considered separately. Recurrence relations between the solutions of some class of inhomogeneous equations appearing in the relativistic calculations of atoms according to perturbation theory are derived. Some applications of the obtained relations are discussed. In particular, a possibility of their application to the calculations of hyperfine structure of multiply charged ions in the first and higher orders of perturbation theory is demonstrated. The corresponding non-relativistic analogues of the relations obtained are also considered.

Experimental evidence is presented for the change of Einstein's A coefficients for spontaneous transitions from the upper laser level of an argon in laser discharge due to the presence of a high-intensity laser flux. To demonstrate that this quenching effect cannot be attributed to a reduction in self-absorption of the strong spontaneous emission line, absorption and line profile measurements have been performed. Computer modelling of the reduction of self-absorption due to Rabi splitting also indicated that this effect is too small to explain the observed quenching of spontaneous line emissions.

Relative intensities of the bands of the Delta nu =-1 sequence of the C ² Pi ₁2/-X ² Sigma subsystem are measured. The effective vibrational temperature is estimated. The variation of an electronic transition moment R_e is determined which yielded the relation R_e(r)=constant*(r-2.458) in the range 2.5439<or=r<or=2.4787 AA. The smoothed relative band strengths are derived.

The influence of coupling between vibrational bound and continuum states on vibrational transitions among bound states of H₂ molecule in the electronic ground state caused by H atom impact is theoretically investigated by solving the vibrational close-coupling equations under the rotational IOS approximation. It is found that the coupling between the bound and continuum states causes a noticeable change in the integrated cross sections for the vibrational transitions with Delta nu = mod nu _i- nu _f mod mainly larger than 4 or 5, and has a non-negligible influence on the total vibrationally inelastic scattering cross sections for the molecule in the high-lying vibrationally excited states, although the vibrationally elastic and the total scattering cross sections are little affected. This coupling generally makes the de-excitation cross sections small and the excitation cross sections large compared with the results obtained by the calculations without the continuum states.

The intermolecular potential between the ground electronic state of H₂O (¹A₁) and the H⁺ projectile is obtained from SCF calculations of near-Hartree-Fock quality for the rigid rotor (RR) target molecule and within the Born-Oppenheimer (BO) approximation. The symmetry-adapted, multipolar expansion of the potential energy surface (PES) produces the leading coefficients which are further extrapolated via the perturbation expressions for the induction and polarization forces in the long-range region. The rotationally inelastic collision processes for this asymmetric top rotor are computed, at the energies examined in recent molecular beam experiments, in order to estimate the contribution of (T,R) energy transfer in the direct channels. The quantum dynamics was treated within the infinite-order-sudden (IOS) approximation and both rotationally summed and partial inelastic cross sections are analysed and discussed.

The effect of electron momentum transfer is included in the description of the charge exchange process governed by the interaction (crossing or avoided crossing) of quasimolecular potential curves at large internuclear separation. The present approach does not resort to the two-state approximation but employs the exact expression for the charge exchange amplitude as a surface integral. Asymptotic analysis gives explicit expressions depending on initial and final atomic states. Generally the importance of electron momentum transfer is measured by the ratio zeta of collision velocity to the typical electron velocity in the partners. In the case of sigma to sigma transitions the effect is proportional to zeta ². The theory applies uniformly also to the transitions between the quasimolecular states with different symmetry when the adiabatic potential curves cross.

Calculations of triple differential cross sections for the process where impact of a moderately energetic electron on a two-electron target simultaneously ejects one of the electrons and excites the other are made in the framework of a first Born approximation without exchange. Results obtained using ejected electron wavefunctions calculated with Coulomb potentials of different effective charge, with the static potential due to the unperturbed residual ion, or with Klapisch-type parametric potentials, are compared with experimental data. Strong dependence on the final state wavefunction is found, notably for the ratio between contributions from excitation of the 2s₀, 2p₀ and 2p_+or-1 residual ion states. The cross sections, both globally and for the individual contributions, are however insensitive to the amount of correlation in the initial state wavefunctions.

Using the R-matrix approach electron impact excitation of C VI has been studied. Collision strengths for all transitions between the lowest fifteen excited states up to the n=5 level have been computed in the energy range below 50 Ryd. All partial waves with angular momentum L<or=12 have been included and the contributions of L>12 have been obtained from the recently developed no-exchange program. The resonances have been delineated and collision strengths are compared with the earlier available data. Additionally, they have been integrated over a Maxwellian distribution of electron velocities to obtain the effective collision strengths which are tabulated at a series of temperatures below log T₃=6.2 K.

Angular correlation measurements of the two final electrons following primary ionization of the Ar(2p) orbital are performed at 2 keV and 3 keV in a wide range of momentum transfer revealing a strong dependence of structural parameters on the kinematical conditions. Characteristic features are a dominant recoil structure at small momentum transfer, a breaking of symmetry around the direction of momentum transfer and a deviation of the recoil peak maximum from the axis of momentum transfer towards smaller deflection angles. The electron momentum density of the Ar(2p) orbital was extracted out of experimental data for the first time. Within experimental errors good agreement was found with results of Hartree-Fock calculations. Extension of the technique of electron momentum spectroscopy performed in slightly asymmetric coplanar kinematics to study electronic structure has been achieved.

Results of an R-matrix (close-coupling) calculation, including explicitly spin-dependent effects such as the spin-orbit interaction, are compared with those obtained from the 'standard approach; where such effects are neglected. Hence, for the first time, the importance of the relativistic effects is estimated quantitatively for the electron-sodium scattering system. The agreement between the two calculations is very good if both the initial and final projectile energies are large compared with the relevant fine-structure splittings. Experimental evidence about the importance of explicitly spin-dependent forces for this system is confirmed.

A parameter-free modelling of the exchange and correlation-polarization forces which play a fundamental role in describing the scattering of slow electrons from molecular targets is applied here to the calculation of total integral cross sections and differential cross sections for electron collisions with polyatomic targets: H₂S and NH₃. Several features of the results are discussed and analysed to try and shed more light on the physical details of the processes at hand. The present computation constitutes one of the first examples of such an analysis for the systems and appears to provide, among other things, much original information on the possible existence of low-energy, metastable negative ions of NH₃ and H₂S in their ground electronic states.

The R-matrix method is used to calculate low-energy electron scattering by CO. Nuclear vibration is included using a non-adiabatic approximation. The author obtains excellent agreement with experiment in the description of the ² Pi resonance. The author also finds some evidence for a very weakly bound state of CO^- having ² Sigma symmetry, but no sign of any non-adiabatic effects in the corresponding vibrationally inelastic cross sections.

A time-of-flight technique has been used to characterize the components of a positronium (Ps) beam generated by charge-exchange reactions of positrons in helium and argon gases. Ground and excited states of positronium have been resolved and the ratios of the signal rates of Ps to both scattered and incident positrons have been quantified in the Ps energy range 7-41 eV. By observing the pressure dependence of these ratios estimates of the Ps total scattering cross sections have been extracted for both gases at energies in the same range.