Since 1981 there has been an attempt in Europe to organize a series of small meetings/workshops/tavole rotonde with the aim of bringing together physi cists and chemists interested in problems concerning atoms or molecules in teracting with external photons where the continua are investigated. The number of problems that fall into this category turns out to be vast. However, it is not possible to make a strict separation into problems con cerning atomic and molecular collisions and those related to the usual spec troscopy. This admixture of two disciplines, discussions on the role of photons and on the interaction of external electromagnetic fields with the continuum provided a central motivation for these workshops. The fourth of this series of meetings was held at Cortona between June 16 and 20, 1986. It was attended by about 100 researchers in the field and there were 43 presentations, all having equal time. These talks form the subject matter of this volume. The idea of publishing the proceedings of these meetings is not new. It allows one to have small meetings in which the subject matter can be discussed at length in a lively atmosphere. However, after the meeting is over, the speakers can collect their thoughts and produce articles in which the results of their interaction with the other participants can be incorporated.

Inhalt
I Photons and Atomic Continua.- Microwave Excitation and Ionization.- Nonlinear Electron Dynamics Within the Stretched Hydrogen Atom in Intense Microwaves.- One-Dimensional Atomic Physics.- Is Subthreshold Ionization a Chaotic Phenomenon?.- Stochasticity in Hydrogen in Presence of dc and ac Fields.- The Problem of Laser-Induced Tunnel Ionization.- An L2 Method for Continuum States.- Collective Effects in Multiphoton Ionization: Application to Two- Photon One-Electron Ionization of Xenon.- Permanent Quantum Beats from Coupling of Two Atomic Bound States to a Single Autoionizing State.- Electric Field Effects on Structure in the Continuum.- II Above-Threshold Ionization.- Above-Threshold Ionization with Linear and Circular Polarizations.- Dressing of Continuum States and Manipulation of Intermediate Resonances in Multiphoton Ionisation.- Theoretical Models for Above-Threshold Ionization in Atomic Hydrogen.- Non-Perturbative Theory of Above-Threshold Ionization.- Multiphoton Ionisation Towards Multiple Continua: The Role of Space Charge.- Two-Photon Above-Threshold Ionization: Study of a Schematic Model Atom.- Multiphoton Absorption Above the Ionization Threshold.- III Molecules in Laser Fields.- Bound-Continuum Transitions of Na2 Studied by Doppler Photofragment Spectroscopy.- Mechanism of Laser Induced Decomposition of Gaseous Polyatomic Molecules.- Coupled Equations and the Bloch-Nordsieck Representation for Molecules.- The Franck-Condon Principle.- IV Atomic Collisions in Laser Fields.- Reactive Collision Dynamics of the Collision System Mg + H2 Using Far Wing Laser Scattering.- Laser Pulse-Induced Molecular Collisions: Effect of the Temporal Width of the Pulse.- Charge Transfer in Collisions and in Complexes Involving Excited Atoms.- Laser-Induced Charge Transfer in Ion-Atom Collisions.- Collisional Ionization in Laser-Excited Barium Vapor.- Laser-Assisted Ionization in the He*(21S, 23S) + He(l1S) Collision System.- The Collision Dynamics of Associative Ionization.- Production of Atomic Rydberg States in Optical Collisions.- Energy Transfer Collisions in Laser-Excited Mixtures of Alkali Vapors.- The Far Wing Behaviour of the LICET Lineshape.- High-Resolution Lineshape Study of a LICET Process in the Far-Wing.- Collisionally Aided Coherent Emission at an Optical Frequency.- V Electronic Collisions in Laser Fields.- Close-Coupling Solution of Radiative e-H + Scattering in a Laser Field.- Electron-Proton and Electron-Hydrogen Atom Collisions in the Presence of a Radiation Field.- Electron Scattering in Intense, High-Frequency Laser Fields.- Applications of Generalized Coherent States.- High Intensity Effects in Multiphoton Free-Free Transitions.- New Results for Perturbative Calculations of Free-Free Scattering.