Table of contents

The authors report experimental results of the ionization of D₂O vapour by intense 10.6 mu m pulses. The processes leading to the products of D⁺, D₂O⁺ and (D₂O)⁺₂ are qualitatively described by tunnel ionization from a reduced atom picture of the molecule, which is enlarged due to dressing by the strong laser field.

The cross section for H⁺₂ dissociative recombination with slow electrons is calculated using the multichannel quantum defect theory, with the set of molecular data used for the recent study of H+H* associative ionization by Urbain et al. Excited vibrational levels of s sigma _g and d sigma _g Rydberg states show up as broad resonances, mostly dips, in satisfying agreement with the vibrationally selected measurements of Mitchell et al. The authors analyse the mechanisms for resonance formation and discuss the adequacy of the theoretical treatment.

Photoionization cross sections for the 2s²2p³⁴S^o ground state of the nitrogen-like Cr¹⁷⁺ ion have been obtained using the R-matrix method. Calculations are carried out in the LS coupling scheme with two sets of target states. In the first, the six target states, ³P^e, ⁵S^o, ³D^o, ³P^o, ³S^o and 2p⁴³P^e, are constructed by using 1s, 2s and 2p atomic orbitals. In the second set these are represented by the extensive configuration interaction using 3s, 3p and 3d correlation orbitals. The initial bound and the final continuum state wavefunctions are represented consistently in terms of the R-matrix basis functions. The partial and total cross sections are obtained. The cross sections are dominated by Rydberg series of resonances converging onto higher thresholds. Some of these resonances have been analysed and their positions and widths are listed.

The authors present an exact expression for the energy levels of a rigid symmetric top molecule in an external electric field of arbitrary strength. The solution of this Stark splitting problem is presented in the form of a rapidly convergent continued fraction, well suited to numerical evaluation for any state.

The analytical properties of the scattering matrix in the adiabatic formulation of the multichannel scattering problem in a three-body system are investigated as functions of the coupling constant and energy in the framework of the adiabatic hyperspherical approach. The localization and the character of pole motion in the complex coupling constant plane and on the Riemann energy surface are studied. On this basis a method for obtaining the resonant state parameters by extrapolation in the coupling constant using Pade approximants of the second type is suggested. The developed approach is applied to calculate the position and width of the ¹P^o shape resonance in the e^--H scattering. The position E_r=0.015 eV and width Gamma _r=0.012 eV of the H^-1P^o shape resonance calculated in the simple one-channel adiabatic approximation are in good agreement with other theoretical calculations.

Polarized electrons were scattered elastically and inelastically from unpolarized thallium and lead atoms. The left-right scattering asymmetry has been measured for energies ranging from 2.5 to 14 eV at scattering angles between 35' and 125'. The influence of the electronic configuration of the valence electrons on the results of the asymmetry function is discussed. For elastic collisions no clear-cut conclusions about the significance of the 'fine-structure' effect compared with the Mott scattering effect can be drawn. Numerical results from R-matrix calculations show good agreement with the experimental data for thallium, whereas the theoretical predictions for lead are less satisfactory.

The complete set of generalized STU parameters are presented for elastic and inelastic scattering of polarized electrons from unpolarized thallium atoms. Some of these parameters can only be measured in 'triple scattering experiments', since the change of the initial polarization vector has to be determined. Numerical results from a semirelativistic Breit-Pauli R-matrix calculation are compared with experimental data obtained by the Munster group for the asymmetry function S_A. The agreement is very satisfactory, and the overall structure of the results indicates the importance of the fine-structure effect-even for very heavy target systems.

A relativistic formulation of the distorted-wave method is presented for electron impact excitation of xenon. Calculations are performed for xenon 6s(3/2)⁰₁ and 6s(1/2)⁰₁ states with both the target atom and scattered electron treated fully relativistically. The wavefunctions representing the distorted waves are fully antisymmetrized to account for electron exchange. Differential cross sections and collisional alignment and orientation parameters are presented and compared with recent experimental data. The overall agreement with experiment is very satisfactory. The occurrence of changes in the spin orientation of the scattered electron due to both exchange and relativistic effects is predicted. The possibility of experimentally observing these spin changes in the collision frame through electron-photon coincidence measurements is pointed out.

The total cross sections for electron and positron collisions with sodium and potassium are calculated in the modified Glauber (MG) approximations, employing the Szazs-McGinn wavefunctions within the frozen-core approach. While the present values of total cross sections for electron and positron collisions are found to be consistent with those previously obtained in the model-potential approach, the positron and electron MG cross sections of the present calculation merge with each other at a rather low energy, as was observed in experiments by Stein et al. (1988-90).

In a previous paper (Boston et al., 1990) the authors discussed how P- not T-violating electron-nucleon forces give rise to static local multipole moments. The lowest of these is the rank 1 anapole moment. They now show that a self charge-conjugate system such as positronium can have a non-zero anapole moment, although its normal magnetic moment is identically zero. They calculate non-relativistically the anapole moments of positronium in both the ground and first excited ³S₁ states.

Three-photon resonantly enhanced third harmonic generation in a collisionally self-broadened gas of two-level atoms, appropriately buffered to allow phase matching, achieves maximum output when the input frequency is sufficiently detuned from exact resonance to avoid excessive self-absorption of the third harmonic. For a given active gas density or laser detuning there exists a relation between the buffer gas density, active gas density and laser detuning which maximizes the third harmonic intensity. The optimal combination depends only on the physical characteristics of the tripling medium and the buffer gas. When the third harmonic is maximized the absorption coefficient alpha and the sample length L combine so that alpha L is of order unity. The characteristics of three-photon resonantly enhanced third harmonic generation in a buffered gas are analysed in detail.