CHAOTIC IONIZATION OF HIGHLY EXCITED HYDROGEN-ATOMS - COMPARISON OF CLASSICAL AND QUANTUM-THEORY WITH EXPERIMENT

The experimental and theoretical studies of the microwave ionization of highly excited hydrogen atoms provide a unique opportunity to explore two new frontiers of modern physics simultaneously. Since the Coulomb binding fields decrease rapidly with increasing quantum number, these atoms can be easily perturbed by external fields that are comparable or even greater than the atomic fields, which permits a thorough investigation of a variety of interesting, non-perturbative processes involved in the interaction of intense electromagnetic radiation with matter. In addition, since the classical description of the Bohr atom in a strong, oscillating electric field has been shown to exhibit "chaotic" behavior, this physical system serves as an important paradigm for the study of the quantum manifestations and modifications of classical chaos. This review describes a number of surprising phenomena that have emerged from these detailed experimental and theoretical studies of this strongly perturbed quantum system throughout the entire range of the experimentally accessible parameters - from the low-frequency regime of tunnelling ionization, to the high-frequency regime of multiphoton ionization, and to the complex intermediate-frequency regime of "chaotic" ionization. Particular emphasis will be placed on both the striking quantum-classical correspondence in the intermediate-frequency regime where the experimental onset of ionization is well described by the onset of classical chaos, as well as the remarkable quantum suppression of the chaotic ionization in the high-frequency regime because of quantum interference effects closely related to the process of Anderson localization in solid-state physics.