ATOMS IN STRONG FIELDS AND THE QUEST FOR HIGH-INTENSITY LASERS

In recent years, novel techniques have been developed to achieve high laser field intensities by shortening the duration of the pulses after several stages of amplification (by first stretching and subsequently compressing mode-locked laser pulses). A laser system built according to these principles at Imperial College is described. It is capable o f reaching powers of similar to 10(18) W cm(-2) in pulse lengths of ab out 1 ps. The availability of field strengths 10 to 100 times as stron g as the field experienced by the electron of the hydrogen atom in the first Bohr orbit opens up a new regime in the study of the interactio n between light and matter. In particular, several effects, characteri stic of very strong AC fields such as ''above threshold ionisation'' ( ATI) high harmonic generation, etc are observed. Examples are given an d their implications for atomic physics are discussed. Although such e ffects are novel and interesting in their own right, an important ques tion which arises is the extent to which they may reveal new character istics specific to the atom under study. Alternative possibilities are that most of the atomic features (correlations, shell structure, etc) are swamped by the effects of the strong field, or else that the atom is ionised before the strong field regime can even be reached. Some a ccount of the controversies surrounding this question is given, and th e importance of the dynamics is stressed.A theory is described which a ccounts for multiphoton excitation and ionisation by pulses of intense laser light in a generalised Landau-Zener model within a dressed atom basis of Floquet states. Different regimes of the interaction are cla ssified, and a semi-classical limit is illustrated using action-angle plots of the Poincare section. These reveal how excitation takes place across a separatrix between two modes of motion, and how the growth o f chaotic trajectories near such a separatrix provides new paths leadi ng to ionization. Our approach stresses the significance of the match between pulse rise time and the magnitude of the avoided crossings bet ween dressed atom states for the atom or molecule concerned.