Introduction

In recent years a number of experiments on the ionization of Rydberg (highly excited) atoms and dissociation of molecules by a strong monochromatic field have been carried out [1-5]. A characteristic peculiarity of such processes is the large number of absorbed photons Nφ 100 and the excitation of many unperturbed levels. Due to this the dynamics of excitation may be described in the first approximation by the classical equations of motion. Such an approach was used for molecules in ref. 6 and for Rydberg atoms in ref. 7. The process of excitation obeys the diffusion law. The appearance of diffusion in the absence of any random forces is connected with the chaotic dynamics of the corresponding classical system. The nature and the properties of such chaotic motion in classical mechanics is now well understood [8–10]. At the same time an investigation of simple models has shown that the dynamics of classically chaotic quantum systems has a number of peculiarities (see, e.g., refs. 9, 11 and 12).