We introduce an analogy between the theory of autoionizing states as described by Fano and the tbeory of certain states ("pseudo-autoionizing states") of an irradiated atom. These "pseudo-autoionizing states" are used to obtain a number of qualitative results concerning resonant multiphoton ionization probabilities for several experimental arrangements. This method makes clear the importance of nonresonant processes in determining resonant multiphoton ionization line shapes.

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The object of this review is to describe and to interpret the basic phenomena recently observed in the interaction between free atoms and laser radiation. Some of these have been known for a long time in magnetic resonance but, in the optical range, the situation is strongly modified because of spontaneous emission and the Doppler effect. Therefore, particular attention is paid to resonance fluorescence and Doppler-free spectroscopy, but without dwelling on applications. Transient effects are also discussed, but we ignore all the phenomena which characterize, not specifically the interaction between atoms and fields, but the free evolution of atoms after coherent excitation. On the other hand, since propagation effects lead in the visible range to completely new aspects of the role of saturation in the response of atomic systems to resonant monochromatic excitation, pulse propagation experiments are described in detail. Finally, multiphoton processes are considered, with special emphasis on Doppler-free multiphoton excitation and resonant multiphoton ionization.