The formation of ground-state positronium in collisions of positrons with hydrogen and helium atoms is considered within the distorted-wave Born approximation. In particular the authors analyse the eikonal and continuum-distorted-wave models in both their post and prior forms. Differential and total cross sections are calculated over a broad range of intermediate energies extending from 40 to 500 eV. The results confirm that the total cross section decreases rapidly with increasing energy, as predicted by the simple first Born approximation, and do not agree with experimental evidence of a slower decay. The computed total cross sections for helium targets agree with those of the close-coupling model for energies above 100 eV. Below 100 eV the authors find a large post-prior discrepancy. The calculated differential cross sections show evidence of a Thomas peak at high energies for the CDW models but not for the eikonal models.

The development of lasers capable of producing high-intensity pulses has opened a new area in the study of light-matter interactions. The corresponding laser fields are strong enough to compete with the Coulomb forces in controlling the dynamics of atomic systems and give rise to multiphoton processes. This book presents a unified account of this rapidly developing field of physics. The first part describes the fundamental phenomena occurring in intense laser-atom interactions and gives the basic theoretical framework to analyze them. The second part contains a detailed discussion of Floquet theory, the numerical integration of the wave equations and approximation methods for the low- and high-frequency regimes. In the third part, the main multiphoton processes are discussed: multiphoton ionization, high harmonic and attosecond pulse generation, and laser-assisted electron-atom collisions. Aimed at graduate students in atomic, molecular and optical physics, the book will also interest researchers working on laser interactions with matter.

We propose a unified R-matrix-FLoquct method which can he used to analyse both the multiphoton ionization of a t o m and laser-assisted electron-atom collisions. Our treatment is nompertwhative and can be applied to an arbitrary LII"'II.

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